I just bought an 87 250x from a guy at work. Last year, he rebuilt the engine with a 12:1 Wiseco piston and a Web cam. It has a Unifilter in the stock airbox and stock exhaust. From the time he put it together, it never ran right. It starts and idles, but sputters when you give it gas. I think it's running rich, cuz the plug is black. I took the carb apart, and moved the clip on the needle one position leaner. It ran better for like 5 minutes, but started bogging again. I moved the clip again, but it ran worse. When I put it back, I couldn't get it to run hardly at all. It would start, but only run for about 30 seconds before it cut out completely. Should I change the main jet and totally clean the carb, or should I just move up to a 400EX carb? Any other suggestions on where to begin troubleshooting?

 

How did the engine run before he installed the piston and camshaft?

How long did it sit up between the time it was torn down and the time is was put together?

I would say the problem isn't neccessarily the jetting but a dirty carb and possibly even incorrect engine timing when he put it all together........did he degree the cam when he put it in?

If it was rich, and that was the only problem, what you have already done should have corrected it.

 

I think it's all put together correctly, because when it does run, it runs good. It ran great for that five minutes. I had it pegged going down the road, and it seemed fine at full throttle, like the main jet was ok. But then it just died on me totally when I went to slow down and turn around. When I finally got it started again, it would run decent if I held it at like 3/8 throttle, but just shut off after about 30 seconds.

He had the engine rebuilt by someone else, a friend of his dad's i believe. I think it was done quickly (didn't sit around apart). I think there's something in the carb, like dirt or something. When I get a chance, I'm gonna take it totally off and apart.

What do you think about the high compression piston and cam? Can I run pump gas with it? The compression on it is nasty - I can hardly kick it over. Also, should I just start over with a complete retune?

 

The pilot or idler jet controls the air/fuel mixture mainly from closed to 1/4 throttle. Idler jet comes in sizes 35 to 60 at a 5 unit increments.

jet needle- 1/4 to 3/4 throttle opening


main jet- 3/4 to wide open throttle

it sounds to me either you need to adjust the jet needle to lean it out, or you need to clean your carb,

BTW i found a page that told me all of this stuff and im pretty sure its all right but correct me i guess

 

Well I leaned the jet needle all the way out, and it didn't help. My next try is cleaning the whole thing. IF that doesn't work, change the carb.