300cc tuning and performance

[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]

Originally Posted by 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.

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.

Originally Posted by 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.

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]

Originally Posted by 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.



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]

Originally Posted by 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.

[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.

[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?