Just bought an 85 atc250r and about 30 mins into riding the thing i blew the top end. The guy i bought it from said that the top end only had about 4 hours on it, ive never seen a top end that fresh go like that. The cylinder is scored pretty good on the exhaust side. The piston is scratched pretty good on the exhaust side and is missing a small chunk right about the top ring. The piston also has an indentation on the top where it looks like a hole may have been starting. Took the cylinder into the shop and they said that it looked like the last time the top end was rebuilt the cylinder wasn't bored or honed but they dont think that is what caused the problem. It looks like it may have been running lean. After i pulled the engine apart i went ahead and rebuilt the carb, while doing that i noticed that it was completely missing the air/fuel mixture screw, could this have done that much damage? Anyone have any other ideas on why this happened? Its already bored .20 over and they suggest going .40 over with this bore. Is that still gonna be decently dependable? I've heard much over .30 over and it starts to run to hot. I also want to have the head shaved down alittle bit to get the compression alittle higher. How far would you guys go with the shaving? Thanks.

 

Well, I knew right after I read about the scoring andburning that it is running way lean. Run much larger jets when you have it running. As for shaving the head, take it to a shop...let them do it.

 

What size jets would you suggest running? It has an FMF pipe DG silencer Uni Airfilter and will be running .40 over on the bore soon. Any info helps. Thanks

 

before you start slaping new jets in it right away... Get the engine put back together and get a air screw in that carb (i'm sure that is your main prob) The jet it by doing your plug checks. Sounds like you have a pretty fun setup going there! ......What are your current jet sizes?

 

mine is shaved a 100000 thousands and its rechambered,i also got a mild port job and between the both of those it boosted the compression to 205 and its bored .40 and its very dependable.

 

just wondering.....I'm new to 2 strokes but did you really get the head milled .001 of an inch, and why?(or did you type 1 too many zeros?) My YZ has a warp limit of .0012 .

 

Im not really sure of the stock jet size, i was hoping that someone on here would know. I have that a rebuild kit on the carb now and it came with a new air/fuel mixture screw so i have that taken care of. How should i go about the spark plug test? What colors should i look for? The plug was black when i pulled it apart. Thanks for the help.

 

I've heard of the 0.010 head shaving lots of times. Ups the compression, heat, and usually requires race fuel.

He meant 0.010 inches (10 thousandths of an inch).

 

ya sorry bout that I typed one too many zeros its 10000 and it does require race fuel.

 

hmmmm, if you run a 89 cr250 head gasket you don't need to shave the head to get higher compression,,, FYI,,,