300cc tuning and performance

[kilo0195]

Ok... so i figured I'd start this thread because my other thread about the coil question got off topic and as I drive my 300cc I realize that I,m going to have many more questions. I also dont know how to edit the title of the other post lol. Anyways long story short about my bike 2007 gz250 1.25" exhaust- 1.5 at the Y- and emgo shorty muffler with 1.75 tip. Ported exhaust, rejet 147.5 main and larger pilot, k&n pod filter, decked slightly, nitrogen 320mm rear shocks, 16t front, 300cc big bore kit... and I hope I didn't leave something out. after about 100 miles on it now i can say that it definitely vibrates a little more... just the nature of a thumper, but was enough it made the baffle in my muffler chirp on decel. I had to remove the baffle, which sounds amazing btw. I also changed the stock plug to an E3 but their plug seems a little cold, so I will be putting in a ngk DR8EIX iridium tomorrow. I will then read and compare the 3 plugs. My main concern now is that it seems to be running warmer maybe. The 300cc jug has better fins on it, but would that be enough? I'm not really feeling like being a test rat for an oil cooler mod. Also what operating temps should I be looking for when I measure?

[kilo0195]

Carb is about dialed in now. The main jet is still the 147.5 and the needle is shimmed pretty far. The real change was I had to change the 52.5 mid jet, or whatever it's called, to a 70 with the mix screw backed 1.25 out right now. Plug looks good after driving at this setup. Going to invest in a infrared thermometer and get some temp #s to post.
Here's the plug with about 30 miles on it... damn near perfect. I switched to the ngk iridium. The e3 was too cold and even had a more black base ring and insulator indicating I wasn't burning as well. http://imgur.com/a/oa4Bs

[kilo0195]

Ok so outside temp today was 72... I drove the bike for a while and decided to check temps when I got back. Parked it and let it sit and idle for 2-3 minutes before checking. External head temps on the exhaust side were 354°F to 362. The intake side was 316°F to 320. Of personal note I got to see just how much the intake charge cooled things as when I shut it off the intake side temp jumped from 317 to 332°F within about 3 seconds. So does anyone know what a good external head temp should be for the stock gz250?

[kilo0195]

Ok for anyone actually following this I just went up from a 16t sprocket to a 17t on my 300... so that's 2 over. Keep in mind I'm 150-155 lbs 5'6". Wish I knew the rpm of the motor... will have to install a tach at some point... but I'm very pleased with the results. I bought a generic JT sprockets suzuki, Yamaha, etc 17t 520 sprocket on eBay for $12 that had the same tooth pattern. A custom sprocket was going to be $97 and for something I didn't know was going to work... yeh... $12 is do much better. The splines are just a tad loose so I took up the space with some thread tape temporarily. Will go for a custom one or a shop said they can possibly electroplate the inside splines of the sprocket... I'm not a machinist so I have no idea what they're talking about. Back to the point, the 17t was very nice in town. I lost a little bit of that traffic light to light speed it had on the 16, but made up for everything in that my shift points are ideal for what I'm used to on larger bikes speed wise. On the highway doing 55mph in 4th is about where it was in 5th on a 15t sprocket. I still had enough torque to peg the speedometer on the flats and enough to easily maintain 75 on the hills. From experience with the 16t that will suffer drastically if I ride 2 up with one of my kids. My youngest at 50 lbs I'd guess 70 on hills would be easily maintainable. My oldest at 110lbs I would guess around 65ish... this is based on my experience 2 up on the 16t sprocket and scaling it to the 17t and how it rode. For someone much bigger I'd keep it 16t... then again if I was much bigger, a gz250 would not be the bike I'd be riding. I can already keep pace with some of the larger 750s on cruises even when they punch it to pass etc. I'd imagine the lower rpms of the 17t will at least make it feel like I'm not riding on a giant adult sex toy at 70mph.

[kilo0195]

I've got a kittle over 1800 miles on the 300cc kit and all is well. It does run very well. It has a bit more vibration mainly in the mid rpm range and in 4th seems to resonate about 45-50 mph. That said havent noticed any issues and ive road her HARD these past 1800 miles. Its never enough for me and so after the 17t sprocket I decided i hadn't messed with the bike enough. I played with the valve timing a bit and currently have it 3° advanced. This seems to give a better torque curve for me (No dyno just seat of the pants) in how I spend the majority of my time driving... highway/back road mix 55mph to 65mph. Retarding about 2° seemed to keep low end acceptable but much better suited to 75+mph on the interstate. No clearance issues even with the deck and new piston. This has led me to think about having the cam heels ground a few thousanths so that I can overtighten it thus (cheaply) giving a few thousands extra lift out of the stock cam. That may be another winter project to add to my to do list. I am currently trying to find a shop that will fab an aluminum frame for the bike, as well as swapping an aluminum tank. Weight reduction is ultimately where I'm headed now. I might possibly switch the gn250 34mm carb or buy an efi kit (about $320) and convert it to throttle body injection. I'm certain that much more power out of this bike and I will need to tear into things like stronger gears, clutch, etc. As i have it right now my bike is 318lbs wet so I'm very curious what shedding 40-50lbs could do. I just wouldn't want to drive it on a windy day as it can be a little entertaining as it is doing 75+ on a windy Illinois day.

[alexdf]

@kilo0195: what bore did you buy and where? I had not found anybody making the modification to gZ250, or did you swap the engine from gz250 to gn?
Thanks in advance!

[kilo0195]

I bought the kit off alliexpress from Sam's Motor Store, it's listed as a gn250 but a 2007 model... it's Chinese so enough said there. Further reading will show that it fits the gn250, gz250, and a few others. What I did find thru my own hours of searching is that some people had issues with having to mill their jug or case for it to fit, but in those cases they ordered one for the wrong year. Not sure how it's different or can be screwed up, again I say it's Chinese, so anything is possible. My bike is a 2007 so I made sure to order the kit that covered the model year 2007 and didn't have any issues. Everything was a direct replacement and the jug/piston swap took all of about 4 hours. The whole kit cost me $140 and was delivered via dhl from China within 11 days of ordering. For the price I can say I don't know why this isn't much more popular of a thing to do. https://ibb.co/cyT72Q

[alexdf]

Thanks @kilo0195 ... I have no problem with chinesse parts or motorbikes, here in Mexico most people runs chinesse bikes, and most mates of my MC run a chinesse bike with more than 7 years and 50K miles :)
I have found this one:
1993-2017 Mod GN250 update to 300CC GN300 Big Bore 78 MM 18 MM Pin at Sams Store , think is same as yours (the link you provided drives me here to the forum).
Its not expensive, i´m gonna try if it fit into my 2009 :-)
Thanks!

[kilo0195]

It's actually pretty inexpensive for the performance difference... it's like a new bike. If you've done any kind of engine work before then it's a pretty easy task as well. When you remove the old piston make sure to take note of which way the triangle is pointing... I think it's towards the front tire but don't remember. The new piston needs to face the same way as the valve cutouts are different on each side of the pistons. I almost made that mistake in my haste. If you have any questions feel free to ask, and I hope you don't have any issues with your kit either.

[kilo0195]

Just a side note for anyone following and interested in seriously modifying one of these bikes... now that the nights and mornings have been in the 40s here I've noticed some changes. With the cooler temps come a more dense air charge and thus more power. Well I think I've found the limit to the stock clutch. Anything more than what I've done already and the clutch isn't going to hold. The little bit of added power in the mornings is showing under flat out acceleration. Especially going from 1to 2 and 2 to 3 I can definitely feel the clutch take longer to grab even when the engine is well warmed up. Now under normal driving or even more aggressive driving it doesn't slip at all. I'm talking about max throttle shifting out of 2nd at 49mph kind of acceleration. With it only doing it with extra pep I'd say she's done being modded unless a stronger clutch is put in.

[kilo0195]

So I'm @ 2300 miles on the build now. Still ride it like I stole it. Think I'm going to be replacing a clutch this winter. It's starting to chatter a little in neutral until I pull the clutch lever in. Granted with the way I've been riding it I'm surprised that's the only problem.

[Vegas Street Rider]

Stay safe.

[kilo0195]

Will do. Just really wanted to see what I could do to and with this bike without it just giving out. I must say I'm impressed so far for a "250" thumper. I'm really considering getting an s40/ls650 thumper at this rate and tweaking it.

[kilo0195]

Quick note... I posted a question on a new thread wondering if a 90-95 dr250 cam would fit. It will and is profiled to give more power. I've found a shop that grinding a one off cam for my bike and is also going to send me some titanium retainers and double sprung valve springs. Once the head is off I may deck it a little more to bump the CR if clearance will allow. As far as my clutch, I may look into seeing if I can fit an extra disc, or if there is a better clutch pack that will fit this bike. I'm really hoping that the new cam will help my acceleration on the highway. Accelerating while I'm cruising at 75-80 can be a little slower than I like right now with a final drive of 2.41. Will update after I get the cam and install it.

[kilo0195]

Few other updates I forgot. I definitely use more oil now. I went thru a 1/2 quart in about 1000 miles. Compression is good, no oil on the plug, etc. Not sure if it's that I'm riding her harder now that she can perform or if it's a difference in piston design. I also had changed from the 17t front to aluminum 16t front a d 39t rear... effectively changing final drive from 2.41 to 2.43. I ditched the o-ring chain and am running a non o ring chain. More maintenance but I'm always jacking with it so no big deal, and I'd rather have the extra torque to the wheel. The sprockets and chain have made a decently noticeable difference on how well she takes off from a stop. I will also be changing the clutch to Kevlar plates and heavier springs. Since the build I've glazed the stock discs which is allowing it to slip under hard throttle even when not shifting... gotta love cork and rubber lol. I've sent an extra cam I had off to the shop and am waiting to hear back.

[Water Warrior 2]

What weight and brand of oil are you using?

[kilo0195]

Castrol Power1 synthetic 10w40.

[kilo0195]

The Kevlar clutch and stiffer springs from Barnett are a nice upgrade. I actually stalled it leaving the driveway because I had gotten used to how much the cork and rubber slipped. Only complaint would be that holding the clutch in at lights can be a little tiring on the hand now, especially because shifting to neutral is damn near impossible when it's running (always been that way). Anyways I got what I was after which was to eliminate any slipping under hard acceleration. The cam should be here in about 2-3 weeks. In talking with the shop we've gone with a profile somewhere just above a stage1 or mild cam. Anything more and I'd have to greatly increase compression, more so than just the deck job I've got. It would entail finding another shop to make a custom high compression piston that frankly I don't feel like spending that much money on. The whole bike so far has been dirt cheap to mod, and I'd be spending close to what I've put into the bike total just for the piston. The cam profile will add .070 lift and much more duration and ramp speed. Sorry I'm not allowed to share the exact specs of the cam. Anyways, sometime shortly after the new year I should have the cam installed and be able to give some details as to it's improvements.

[kilo0195]

Cam arrives tomorrow... I'm a bit excited! Over Christmas break I had her on a dyno and got a hp and air/fuel reading. They couldn't get a good rpm signal (some sort of interference) so there was no way for them to get a reliable torque reading. I was putting 21hp to the wheel which isn't too bad considering my 2.43 final drive and wide heavy tire I'm running. Besides I've seen a stock Buell blast 500cc dyno at 22.5whp. He also had dyno tuned a couple gz's in the past for customers and said they were in the 14 to 14.5whp range when done. I really would like to address the tach problem to get a good torque reading but that's a different story. Air fuel mix was ungodfully rich until I hit wide open throttle so she is way out of tune and I could possibly pick up a little more power with a good tune. I will correct that after the cam swap. For the swap I will be removing the head and the jug as well. The head I plan to clean up the port job on it and the jug will be sent to be decked. How much I can deck will have to be determined especially as the head already has .040 or .045 shaved off. I hope to be able to get at least .045 off the jug as I was advised the higher cr I can get the better the cam will perform. This will also give me a good look at the piston and cylinder walls to see how things are holding up. It sucks she'll be down for a while, but I can't wait to see the difference.

[Water Warrior 2]

Impressive numbers. Your mods will keep you fixing the weakest link as time goes by. Not a bad thing if you want some fun and can afford to keep repairing the bike.
Carry on and keep us in the loop. :tup:

[spldart]

Pictures of motor mods please so we can all drool and get the fizzies! :D

[kilo0195]

https://ibb.co/fR6G3b
https://ibb.co/kk7TGw
https://ibb.co/cmM5ww
https://ibb.co/k12dGw
https://ibb.co/nNEsbw
https://ibb.co/dtnCbw
https://ibb.co/mtiUOb
https://ibb.co/jzNZpG

[kilo0195]

Ok finally got the pics to upload properly... or links anyways. The first pic is the bike itself. There's a pic of my filter setup that I've talked about elsewhere. Just took my k&n out of the box, tossed the box, and epoxied the rubber tube from the carb to the outside of the filter. The zip ties held it in place to dry and I left them for extra strength. There's also a comparison of the 2 different jugs with the original piston and jug on top. Sorry about the crappy photos of the port jobs. Removed all casting marks originally. I went back today and smoothed the intake tube transitions and flattened the curves. The exhaust I rounded the curves as much as possible and opened the the ends to match the new pipes as the old ones had a sort of ramp inside of them that made the inside pipe diameters as the head about 1/2". Now they're about an 1" inside. The head I touched today as well, I trenched the squish areas (as they're dramatically bigger now with a 6mm larger piston) angling them towards the plug and gradually getting deeper and wider towards the chamber. The head was also domed, meaning that the ridges between the combustion chamber and valves, plug, etc were removed and rounded. The areas were left rough to promote better turbulence on intake and compression. There's a pic also of the piston still in the bottom as well as a pic of the new cam about to go in. The jug will be decked another .045 this week before I start putting it back together. As far as how she's been running since the original work.... rich... which is an understatement. I had a lot of carbon to remove from the Head and piston. The the gasket was in great shape, as well as the cylinder wall, rings, piston, rod, and pin. Nothing out of the ordinary when I inspected everything closely. All in all definitely recommend the 300cc swap from China. For $130 I think it was and about 5 hours of time it's a pretty cheap and easy mod if you're mechanically inclined. The other work isn't necessary but doesn't take long and can be done yourself as well... minus the decking unless you have a mill. The cam is the most expensive part and will run you I think they said about $200. You can get titanium springs and retainers for a 95 dr250 for another $200 online If you're inclined. While not required for the cam profile it's added insurance for me with the way I push it. If you do the exhaust openings it will probably cost you a bit to have new pipes made if you can't make those yourself. All in all I paid more for the used and neglected bike than I did for all of the additions combined.
The new shocks, drag bars, sissy bar and brackets, engine mods, sprockets, chain, paint lights... pretty much everything and the bike itself is in the $2,000 to $2200 area. The rich issue is from the starter jet... It's currently a 60 and reguardless of what one may think it effects fuel delivery throughout the rpm range even without choke... and no I don't have choke issues I checked. I'll be dropping that back to a 52.5, changing the pilot from a 17.5 to 20, and main from 147.5 to 150 while taking a couple shims out of the needle. I know that the stock pilot, starter, and a 147.5 main it will run very very lean. The different jets will hopefully correct everything or at least get me in the ballpark. I'll start there and go where it needs from there. When i can afford it I'll dyno tune and run it to get the final numbers.

[kilo0195]

I think I found my oil consumption problem. Part of it I suspect being riding so hard at high rpm's. The other part of it I found is that my breather filter is soaked. It's not dripping, but completely soaked. This tells me that plenty of oil is getting to it and more than likely coming out when I'm in the high rpm's. My theory is that the larger piston is causing extra pressure in the crankcase on the downstroke. This would naturally force more oil into the breather. Right now the breather is attached to the end of the short stock vent hose. I'm going to extend it and mount the breather right under the seat next to the air filter. Hopefully the extra length and elevation will solve it. Otherwise I will have to find other and probably more expensive and extensive solutions. I wonder if a small oil cooler would help eliminate some pressure? A line from the pressure check port to cooler and then to a tap put in the oil filter cover? I'm sure the bike would be thanking me dearly for one anyways with the extra stress on the current setup.

[Water Warrior 2]

Good pics and write-up. Your thread is very educational for any members who are after more performance but don't want a bigger bike. Looking to more updates.

[kilo0195]

That's ultimately what I wanted to achieve... was a cheap bike that could be made to handle more on the cheap. Around here you can pick these bikes up for $1000 and they can modified pretty cheap. I wanted to try and accomplish what others said was impossible for this bike, and show that it can be done for little money. With the 300cc kit itself I was able to easily run a 16t front and 39t rear sprocket and have plenty of torque for 70-75mph and could easily hit 80-82mph for passing. It could peg the needle at 88 but was a little slower climb past 80. This makes the bike very capable for highway speeds and reliability at those speeds. The other mods are probably more than most people's cup of tea but turns the bike into something that can easily keep pace with most 650 and 750cc cruisers. The cam I'm hoping will bring some more torque to the mid and top end to help with passing especially if I have an extra 100lbs or so of rider on the back. The sprockets I'll keep the same as 4th gear now is basically what the stock 5th gear used to be. It allows me to use 5th as basically an overdrive for the highway. The bike is most alive in 3rd and 4th and in those gears feels like I've got twice the motor under me. If anybody has questions I'm more than happy to answer or help if they're interested in building theirs up. The big thing to note for internal parts wise is that the 91-95 Suzuki dr250 is essentially the same size for many parts. The heads are different, the dr250 has more oil journals than the gz that won't match up to your jug. Otherwise it would be nice because half the porting work is already done. Inside the Head though all the rockers, springs, retainers, bearing washer etc etc are same size specs. For these parts there are plenty of aftermarket beefier items you can buy. Unfortunately the bottom end is completely different. The bottom end internals seem to be holding up fine so I don't think anyone needs to worry about rods and the like. That's also with me pushing the bike harder than the average person would. So if 300cc is all someone is wanting to do then I'd suggest that along with Kevlar clutch plates, 16t and 39t sprockets, and the obvious rejetting. One should be able to get all of that for under $300.

[Water Warrior 2]

So if 300cc is all someone is wanting to do then I'd suggest that along with Kevlar clutch plates, 16t and 39t sprockets, and the obvious rejetting. One should be able to get all of that for under $300.
I would be willing to bet most riders would really like the extra 20% in displacement and just a 16 tooth sprocket to start. The extra CC's and slightly lower RPM might be very satisfying. The GZ would then have a bit more grunt on long slight uphill grades and into headwinds. My late wife Lynda had that as her only complaint on long trips. New clutch plates would be an obvious option if there was slippage of course.

I may be wrong but I think you are the only rider to actually upgrade a GZ to this level. Your project is fantastic. :clap:

[kilo0195]

The 300cc gave me enough torque with a 16t to pass uphill while I was cruising about 70mph... I'm about 155lbs.

The oil cooler issue I found on alliexpress as well. $79 gets you a kit for a gn250 that includes everything you need. One hose goes into the oil pressure check port and the other goes into a ported oil filter cover that's included. I ordered that one for the insurance and peace of mind. I haven't been able to get good temps readings but I'm sure especially after the new setup I will be running a little warm. It says it will be up to 45 days to get here though... bummer. At least it's cold outside.

As far as upgrading to this level... I think I was outdone before I started. In my research before the 300cc I came across photos of a gz with a pulley attached to the crank on the stator side. They were using that to power an AMR300 roots type blower. I'm pretty sure it was making far more power than I will be haha. That said I'm confident I spent far less than they did. My NA route is probably a little more reliable as well.

Another thing I will try to do when I get my jug back tomorrow is calculate my compression ration. I shaved the Head a little already but I also countered that probably when I domed the combustion chamber. I basically removed all the sharp ridges and made everything a nice round dome instead of its stock lumpy dome form. They're taking .050 off the jug which on average would bump CR by a full point. Keep in mind though there is also now an extra 3mm wide squish area all the way around the combustion chamber on the head. If I had to guess I'd say it will probably be close to 10.5/1

[kilo0195]

Was able to get everything today to calculate my CR. Comes out to 10.42/1 roughly. To get my chamber volume I used water with dish soap (to reduce surface tension), put spark plug in, then put a flat and clear film over the chamber. I then put a small hole in the film so that I could add the water. This ensures the water is level and also reduces the circumference at which the water will want to try and raise above the surface of the film. Can't remember the names from chemistry or whatever but hopefully you get what I'm trying to say. After carefully measuring I was able to get the same numbers back to back so I called it good enough. Also looks like I'll need a single indexing washer for the plug to ensure it's facing the right way. Seems as I can only get the washers in a package for about $14 I think I will spend the $20 and just get a Brisk 360 degree plug. Then I will cut the spark plug port down inside the chamber to match the plug evenly all the way around. Yes I'm getting a bit anal with the details but it's all apart and I might as well take the extra few minutes and do what I can to better flame propagation. It also doesn't cost me a thing. Even if it doesn't remotely increase power to some millionth degree it will still help eliminate detonation problems from hot spots next to the plug. Anyways back to the CR point. If one was to buy a 300cc kit and also have the jug shaved .050... you'd have a CR somewhere around 10.4 to 10.5 fyi.

[kilo0195]

I got it all back together today. I couldn't remove my stator cap to access the crank, so I had to turn it by pulling the timing chain to line the cam and gear up. I must say that this part alone took me about an hour all because I couldn't get a wrench on the crank to keep it from moving. I stripped the slotted part of the cap in my attempt to free it so I'm not sure how I'm getting it out in the future. Anyways other than that it was smooth sailing. My starter got a work out as it took a little effort to get it to turn over. I might increase the size of the wire to see if easier juice flow helps. Filled the tank with 93 to start, but will try 89 octane down the road to see if it still can run without detonation. My jets still haven't came in so the carb is still the same. Like I said before it was way rich low and was ideal high rpm and full throttle. Now it feels right down low and up until about half throttle. Full throttle and it feels lean with it barely dropping out of power occasionally. It was passable to get the cam broke in though. First impressions? Keep in mind the carb jetting is off... There's torque a plenty down low. A good bit of this will probably be from the 1.4 point CR increase. Up until about 4500-5000 rpm the torque difference feels kind of like when I changed to the 300cc from the 250. Up top she struggles a bit, but that would be from fuel delivery I'm almost positive. The cam grind should increase power throughout the rpm range with a focus on the mid to top end, so it should be night and day difference when it's jetted properly. I'm hoping when all is tuned properly that I can run the dyno close to the 30whp mark.

Side note I shielded my plug wire and coil and still have tons of interference in the tach signal. I have just no clue at this point other than maybe the generator causing a large magnetic field.

[Water Warrior 2]

Oh my, you are really getting down to the Nitty Gritty with everything. 30 rear wheel horsepower would be awesome on a GZ. You would be able to frighten a lot of bigger bikes due to the power to weight ratio.

A GZ with a blower! It must have had a bucket full of mods to even run properly. Sounds like a short life engine if opened up on a regular basis. But what the h**l, it was likely a different way to get where you want to be with the GZ without a large cost.
I find these projects very fascinating. They are works of art in some cases and deserve to be appreciated for their effort and planning. :tup:

[kilo0195]

https://ibb.co/kKd9KR

My "shadow" couldn't wait to get out on the bike especially as he said I had to "prove to him" it was going to be faster lol.

[Water Warrior 2]

https://ibb.co/kKd9KR

My "shadow" couldn't wait to get out on the bike especially as he said I had to "prove to him" it was going to be faster lol.
Another generation of riders. Take care with him and always answer his questions. Your answers will determine his future decisions in life. :tup:

[kilo0195]

So the bike has been neglected for a couple weeks as I've been busy working. Figured I'd try and get the carb sorted and still not done with that. Put the stock 52.5 starter jet in, swapped the 17.5 pilot with a 20, and changed my 147.5 main to a 150... Kept needle the same. Seems good until around 2/3 to 3/4 throttle. Full throttle she stops cutting out but still underpowered. Going to try and change my needle with one that has more taper. I can't shim the thing anymore. I actually had to cut about 1/8" off the top of the needle already to get it to fit with the shim it has lol. If the taper doesn't do it then I'll try a 152.5 main. Plug was light grey driving around with big under 1/2 throttle so I'm confident I'm good on the pilot at least. I really would love to have one of those ecotron efi deals.

[kilo0195]

Anyone still curious I've got the carb dialed in finally. 22.5 pilot with 1.5 turn out on mix screw. 52.5 stock starter jet, and 150 main jet. The needle was the fun part. I boought another stock needle for security then took my original to the wire wheel on my bench grinder. Slowly removing material and increasing the taper. This took some patience and nerve (The last part being why I bought another stock needle). I'm glad I went this route because with as much as I took off I'd have spent a fortune in different size needles finally getting to where I was. The bike is finally running where it should and has a nice smooth transition between all throttle ranges. I'm sure it's not perfect and will still need dyno tuned, but at least I'm not frying a piston or looking like I'm rolling coal in a diesel lol. :cheers:

[kilo0195]

Another note of interest... you can use the stock 250cc head gasket with the 300 as long as you have the piston clearance. The gasket I used after the last mods failed which left me with only the stock gz gaskets or a custom copper one. The stock gasket doesn't have a fire ring so I figured I'd give it a shot. I put the jug on and spun the piston to TDC and I had about .012 sticking above the deck. As this would mean the piston would be striking the head gasket I decided to add another .033 thick paper gasket between the jug and crankcase. This in essence raised the jug enough to give me the deck clearance for the stock gasket. This should bump CR to almost 11/1 now. It's running great for now. My only concern is the gasket material that is now in the combustion chamber. I don't imagine it breaking up as it's stacked steel. I could though foresee it causing detonation if the steel gets hot enough and can't dissipate that heat. So far so good, yet its still cool temps here. I will update if anything changes. PS I'm still slightly pissed about the gasket failing in less than 1000 miles.

[kilo0195]

Everything is still running well. I've been playing with the cam timing a lot. Right now I'm at 6° advance and might keep it there. Definitely has a little more balls around town and slower speeds on the highway. The needle only goes to 88mph so shifting the power band up another thousand rpm doesn't do much good... especially as the goal was to keep it reliable. Downside is the advance means more heat, which is rather noticeable, so the oil cooler is a must before summer temps hit. I also replaced the rear shocks again, this time with a combined spring weight of 1400 pounds. This stiffens the rear up some, but makes it able to actually ride 2up without bottoming on bumps. Think I'll probably end up keeping this one as shes pretty dependable, and light enough I can throw it on the hitch carrier when we go on trips. It's nice being able to take a bike with us when we go to the mountains.

[kilo0195]

Well she does better than I thought. I had someone get on the back with me... they weigh about 230# and I weigh about 160#. So we had around 390# of rider on the bike. The shocks kept it from bottoming and also still maintain a smooth ride. The real shock was I was able to maintain 65mph. Granted she had no torque left, but I'm also running a 16t front and 39t rear sprocket. If I were to switch the front or rear back to the stock 15t or 41t then I imagine she wouldn't be maxed at 65mph. Now the only time I'd ride 2up with that much weight would be around town maybe, but still is nice to know that she has the ability now to do this.

[kilo0195]

So aparently I spoke too soon. Was riding today when all of a sudden the bike just stopped while I was idling at a light. Motor wouldn't budge. Fearing I dropped a valve I pushed it back and tore it apart. What I found was that an intake valve guide had broken apart at the bottom and fell into the cylinder. Thankfully the walls look fine, just the piston top and head have a nice little dimple which should be fixable. I'll inspect the rod, pin, etc later to make sure the sudden stop didn't crack anything. The valve itself looks ok... won't know for sure till I take them out. I've never had a valve guide catastophically fail like that before. Good news is motor is ok... bad news is I'm not sure what caused the failure. If it was exhaust I'd say excessive heat, but it's the intake side.

[kilo0195]

Upon close inspection I found the culprit. The cam was advanced too far for the build and at high rpm was allowing the intake valve to contact the piston crown. I found some slight scuffing in the valve reliefs on the crown. The repeated contact would have caused enough shock over time to crack the guide. Valve is still straight somehow and retainer looks fine as well. Bad news is I think the piston has a hairline crack at the base of the pin... and the top ring journal is compacted down pinching the top ring on one side. Looks like I'll be needing a new piston. So the moral of the story kids is dont deck it too much or advance the cam too far. Found another 78mm piston on Ebay and will be ordering that along with some new guides and all should be good.

[Vegas Street Rider]

Good luck with your R&R.

[Water Warrior 2]

WOW, that could have been a lot worse. We are learning by your experience how far a GZ can be pushed until parts fail or try to live in a slightly different position. Carry on. :tup:

[kilo0195]

https://ibb.co/ckMXFy

Back together and running well. New piston, rings, wrist pin, and valve guide.

[kilo0195]

Took it apart after 200 miles to do a good inspection and all is well and no contact. Put the timing back to 0° to be safe. A custom high comp piston would be the best route for proper valve relief and able to achieve 12/1 CR, but that's more money than it's worth. This was supposed to be a cheap build, so I'm done with everything that isn't a lot of $$$$. If anyone is buying a 300cc kit I would definitely have the jug decked .040-.045 before you install it. The one point bump in CR is well worth it and very noticeably improved low end torque around town and back highways. You can get someone to do it for $50 or less. If you go with the cam as well be sure to keep the cam timing at 0° 😁. Keep in mind I'm still running a 2.43 final drive (16t front and 39t rear) and can easily cruise 70-75mph with my wife and I both (320#). Do note if you want to carry that much weight you need to replace the rear shocks. I bought mine off Amazon for $60 the pair and as a pair have a spring rate of 1400#. Plenty of strength and has enough give even riding solo to still have a smooth ride. Definitely helps it handle better as well. With the 2.43 final drive 5th is ultimately an overdrive. 4th is good to reliably get you to 70-75 and then 5th for cruising. 5th gear cuts so much torque though that you're not accelerating very fast at any speed. Ultimately fits my riding style perfect. The bike is also light enough we can through it on the hitch carrier and bring it with us on trips. Anyways... the 300cc kit and deck job should be rather cheap, reliable, and turn the bike from 14.5 whp to 22-23 whp. Which is pretty impressive for such a cheap mod, and definitely helps on a 320# bike.

[Water Warrior 2]

Sounds like you have managed to make a GZ a lot more useable without sacrificing reliability. My only suggestion would be to use a very good oil to help deal with added heat and degradation. :tup: :clap:

[kilo0195]

Alas my GZ is no longer mine. I traded it for a 07 KLR 650 with 5k miles. Going to turn it into a clean looking scrambler. If anyone has any questions about a 300cc build i will pop on periodically to check though I'm pretty sure its straight forward if you just follow the advice in the thread.

[Rowdyrolla]

Kilo0195 - so I have read this post in its entirety. I do have a question for you. Just wanting to make sure I’m clear Bc I don’t want to blow my valves or piston. The 90-95 dr250 cam will fit and work on a stock piston and head?

I have a 2005 GZ250, current mods are as follows. Slant cut the factory exhaust behind the baffle near mounting bracket, removed air filter inlet cover on filter, jetted main and pilot jets by using a jet drill kit moving up one size from bit that would slide thru the factory hole in jet, swapped bars to coupons, adjustable levers and brake master, custom made hard tail struts and rear fender with sissy bar, 130/90 front tire to match rears width, side mount plate and taillight assy, changed front sprocket to 16t from the factory 15t and purchased a 37t sprocket for the rear from the up for a Suzuki rg250 gamma.

Parts now needed due to the way I ride. Going to install a set of Barnett’s Kevlar clutches and springs with steels, also going to perform dr250 cam swap. The biggest and most expensive part of this build is I’m going to install a gt15 turbo and 42mm flatside carb with external oil pump and oil tank.

Like you I have a fascination with the smaller bikes and love to just go balls out when it comes to proving a point when I’m told I can’t do something. Lol

[kilo0195]

Sorry it's been a while... Yes a 90-95 Dr 250 has the same cam profile just make sure it's one for use without a mechanical tach.

[JBaptista]

So I know this thread is 3 years old and I’m about to do this myself. So hopefully I don’t run into any issues. But thank you for your informative information because this has given me a lot to think about for my build

[GZ300]

With all due respect there's really not much instruction here on how to do this. Anyone can say "I built a 300 and it's faster now". No shit man. If anything this thread is a warning of what not to do because the objective is not to blow the motor up and have to do it twice. Your jetting approach is haphazard at best and there's NO WAY it's putting out max HP with anything near a 150 main jet. You must be running a CV carb with no airbox or something which is a sure way to lose power.

[Vegas Street Rider]

Old thread, but trying to upgrade the GZ250 to a 300cc bike makes no practical sense.

If you want a bigger bike, buy a bigger bike. If you don't know what you are doing, you are building a small bomb. Just my humble opinion.

If you pursue this project, please be careful but follow your dream. Done preaching.

[GZ300]

Old thread, but trying to upgrade the GZ250 to a 300cc bike makes no practical sense.

If you want a bigger bike, buy a bigger bike. If you don't know what you are doing, you are building a small bomb. Just my humble opinion.

If you pursue this project, please be careful but follow your dream. Done preaching.
Honestly it's a straight forward upgrade. I don't know why this thread is such a huge production but I can tell you that it is without a doubt the #1 search result when someone googles "gz250 big bore" and it would be nice if there was an actual build thread. This is rambling and guessing at tuning, for no reason. Threads like this tend to get people hyped up and bikes end up in pieces because they don't have a clue. I mean ffs the guy has a 250 head gasket hanging out in the middle of the combustion chamber because he's too lazy to get the right one. Do not listen to people like that when they tell you to upgrade shit. If someone went ahead and did an actual build thread they'd have something to follow. This thread is a disaster.

I really wanna buy this forum if anyone knows who can sell it to me.

[EddieSC]

Honestly, I wouldn't mind running a 300cc upgrade on the spare motor I have kicking around just to see if I can do it but the lack of actual info is one of the things stopping me. It would definitely be nice to have an actual guide to reference so I don't grenade a perfectly good engine.

Old thread, but trying to upgrade the GZ250 to a 300cc bike makes no practical sense.

If you want a bigger bike, buy a bigger bike. If you don't know what you are doing, you are building a small bomb. Just my humble opinion.

If you pursue this project, please be careful but follow your dream. Done preaching.

Practical sense or not, it's fun! Then again I'm the type that turned a telecaster into a death metal shred machine just because.

Sometimes practicality can be overrated.

[GZ300]

Honestly, I wouldn't mind running a 300cc upgrade on the spare motor I have kicking around just to see if I can do it but the lack of actual info is one of the things stopping me. It would definitely be nice to have an actual guide to reference so I don't grenade a perfectly good engine.



Practical sense or not, it's fun! Then again I'm the type that turned a telecaster into a death metal shred machine just because.

Sometimes practicality can be overrated.

It's literally a jug and piston swap and it's absolutely practical. The jetting will be nearly the same. The o.p. of this thread way overcomplicated shit and made a lot of guesses and mistakes including the "while I'm in here I might as well" disease. Swap the piston and jug and get on with life.

[GZ300]

It's literally a jug and piston swap and it's absolutely practical. The jetting will be nearly the same. The o.p. of this thread way overcomplicated shit and made a lot of guesses and mistakes including the "while I'm in here I might as well" disease. Swap the piston and jug and get on with life.

I still can't load a pic on this site. It'd be nice.

I just did a big bore with a used Chinese ATV jug and piston I found on ebay for 45 dollars. The jug was over 1mm taller and the sleeve was too large for the case. They make kits on AliExpress which fit the case but I haven't bought one so I can't speak on the deck height of cylinder that comes with the kits.

What I did was knocked the sleeve out, had my machinist turn the bottom which enters the case down to 83.1mm, then had him take half the extra jug height off the bottom of the jug, reinstalled the sleeve, then took the other half off the top of the jug with sleeve installed for a fresh, square cylinder. Piston ended up at essentially zero deck.

I used new rings and gaskets for a 78mm piston. I used the Suzuki piston pin because it fit the ATV piston better than the pin it came with. All in all I spent $155 for a US machined setup that puts out a LOT more power than the stock setup did. It's literally a no brainer as far as cost vs. reward. The power gain is not from the CC's alone but from the increased compression ratio the extra CC's in the cylinder while combustion chamber volume stays the same. The increase is over a point of compression.

My bike now has 840 original miles. Since a 78mm setup can never be bored more, (might get away with a ring and hone at 30-40k) I'll collect, machine and store the parts to replace it in case it ever wears out. It will live the rest of its life as a GZ300.

[GZ300]

The nicest GZ250 you'll ever see in your life.

https://images.craigslist.org/00d0d_7zivPBE4W0oz_0CI0t2_1200x900.jpg

[GZ300]

If you machine the jug correctly you'll have high compression which is where you get real power. You'll need an oil cooler for the heat to run 91 octane and a Barnett Clutch for the added power if you do it right. My clutch started slipping in 3rd and 5th immediately and it's essentially a brand new bike. Do not run a big bore with high compression lean, it gets hot really fast.

The stock carb tunes easily. There's a better carb for more power but nothing is as easy to tune as the stock carb. I use a gs500 filter element in the stock airbox.

https://images.craigslist.org/00e0e_c0w6VN1A3XXz_0CI0t2_1200x900.jpg

[jonathan180iq]

She does look quite nice!

[Vegas Street Rider]

The bobbed fenders give it a nice look. What did you use to cut them and finish off the edges?