big bore

[ldub128]

Ive got a 1990 yz250 with a ohlins 360 big bore on it. I was wondering how much does having the big bore effect the jetting. Im sure theres no way that you can run the stock stuff. I just bought the bike its my first and i didnt even know it had the big bore till i lookied it up to see why it had a ohlins jug. It lacks power like crazy. It wont even pull a wheelie in 2nd gear. And it prolly tops out at like 60mph. I dont know if the guy before me even changed the jetting. I bought it for $400 though.

 

[dirt bike dave]

Some of the big bore kits remove/disable the power valve, which can have a big impact on power, and often limit your usable rpm range. If you set it up for low end power (likely on a 250/360 conversion) it might actually make less peak power, but should have a nice strong low end.

The lack of power could be from a worn out piston and rings. If you don't have any compression, it will kill your low end power.

As for jetting, keep in mind the bigger displacement sucks in harder. That means more fuel comes through the same jet per intake stroke than it would take with the stock bore. This can kill your fuel mileage, but the stock jetting might just be in the ball park. Only way to tell is test and tune.

Does your spark plug show any signs of running lean/over heating? Is it black and overly rich?

Usually, the best way to get a big bore to live up to its potential is to have the cylinder ported by a good builder, to get the kind of power you want. A mismatch in components (low rpm set up for carb and cylinder bore on a bike with a high rpm port job, power valve open and high rpm pipe) can result in the worst of all worlds. No low end and no high end.

 

[ldub128]

yea i think i do have the worst of both worlds. This one actually has a power valve but when i got it the linkage was all screwed. I dont think i can get the linkage anymore and im not even sure what the kit came with or if it used the stock linkage. Ive got the valve pinned wide open right now but im trying to figure this thing out little by little. It seems like 1st and 2nd gear are real weak and then once it gets goin its alright but it still doesnt have the power it should. Im gonna tear it down this winter and go through everything. I just figured out that noleen still makes a top end kit for it so atleast i can get the piston and everything.