Good luck with your R&R.

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:

https://ibb.co/ckMXFy

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

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.

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:

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.

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

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.

300cc kit install
 

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

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.

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.

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.

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.

Pics of my 300
 

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

https://images.craigslist.org/00d0d_...2_1200x900.jpg

Do it right
 

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_...2_1200x900.jpg

She does look quite nice!

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