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Old 07-26-2017, 03:39 PM   #1
kilo0195
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300cc tuning and performance

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?



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Old 07-27-2017, 11:31 PM   #2
kilo0195
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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

Last edited by kilo0195; 07-29-2017 at 03:59 PM.
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Old 08-04-2017, 10:37 PM   #3
kilo0195
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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 354F to 362. The intake side was 316F 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 332F within about 3 seconds. So does anyone know what a good external head temp should be for the stock gz250?
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Old 08-07-2017, 03:12 PM   #4
kilo0195
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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.
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Old 08-17-2017, 11:38 PM   #5
kilo0195
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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.
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Old 08-29-2017, 12:51 AM   #6
alexdf
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@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!
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Old 08-30-2017, 01:18 AM   #7
kilo0195
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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
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Old 08-30-2017, 09:32 AM   #8
alexdf
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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, im gonna try if it fit into my 2009 :-)
Thanks!
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Old 08-30-2017, 11:47 PM   #9
kilo0195
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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.
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Old 09-13-2017, 09:30 PM   #10
kilo0195
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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.
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