Hello all,
I have an 86 ATC 250R I just bought. When I got it, the motor seemed to sputter, especially when really getting on it. I had the motor rebuilt and the guy (who built it) said it ran great, but felt like it didn't pull hard enough on the top end. He said that it was probably a jetting issue, although he ran it in sub 30degree weather.
I've got new jets, whole top end of engine (bored .20 over), and new reeds (not boyesen, but some other semi-racing type) all installed with rebuild. Any suggestions? Does it sound like a jetting issue or an ignition issue? Thanks guys.

 

First do a compression test to make sure your bore is fine. The gauges are cheap to buy. Should probably be around 125 - 150 to be healthy (just a guess) Check the condition of your reed valves, just because there newer doesnt mean there not causing a problem. that could be a problem for top end power. Proper non-leaking exhaust is important on 2 stroke engines. Order a manual for you ATC, it will help you. If your machine is bone stock, start with the jetting the manual tells you and richen it from there...keep checking your spark plug until you see its to rich then back off one main jet. If its not stock, start with one or two size main jets above stock and go from there. The main jet is responsible for top end fuel delivery. Check your fuel filter, I believe there may be some kind of screen somewhere in the fuel line, maybe in the fuel tank shut off/reserve switch. (not sure on that) Your manual will help you. You can access your main jet by taking out the float bowl drain plug and its right up in there.

 

Dominator,

Thanks for all the info. I really appreciate it.
First thing I'll check is the compression. How do I check the reed valves?? What am I looking for? could a clogged/partially clogged exhaust or muffler also cause this problem? Does this at all sound like an ignition issue?
In any case, thanks a ton. Big help!!

 

When you changed the jets did you go bigger or smaller. You might have went to big on the main jet and its running to rich on top end.

84 ATC 300r, 86 ATC 200x

 

Don't forget the intake.Do you have a good air filter?Is it clean?When i rebuilt my 85 I had topend problems i was running a UNI filter at the time, it would run great with the lid off.Then i tossed the UNI for a K&N and that woke it right up.But i am running a aftermarket pipe and silencer also so the motor will take all the air it can get.Buy the way did you get that email i sent?