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#51 |
Senior Member
Join Date: May 2015
Location: Henderson, NV
Posts: 1,098
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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. Login or Register to Remove Ads |
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#52 | |
Junior Member
Join Date: Jun 2021
Posts: 25
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Quote:
I really wanna buy this forum if anyone knows who can sell it to me. Last edited by GZ300; 06-18-2021 at 01:05 AM. |
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#53 | |
Member
Join Date: May 2010
Location: Birmingham, AL
Posts: 37
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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.
Quote:
Sometimes practicality can be overrated. Login or Register to Remove Ads |
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#54 | |
Junior Member
Join Date: Jun 2021
Posts: 25
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Quote:
Last edited by GZ300; 06-18-2021 at 07:53 PM. |
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#55 | |
Junior Member
Join Date: Jun 2021
Posts: 25
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Quote:
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. Login or Register to Remove Ads |
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#56 |
Junior Member
Join Date: Jun 2021
Posts: 25
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Pics of my 300
The nicest GZ250 you'll ever see in your life.
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#57 |
Junior Member
Join Date: Jun 2021
Posts: 25
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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. ![]() Last edited by GZ300; 12-14-2021 at 10:10 PM. |
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#58 |
Super Moderator
Join Date: Jan 2007
Location: Dalton, GA
Posts: 3,996
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She does look quite nice!
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#59 |
Senior Member
Join Date: May 2015
Location: Henderson, NV
Posts: 1,098
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The bobbed fenders give it a nice look. What did you use to cut them and finish off the edges?
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Tags |
300cc, big bore, performance, power, torque |
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