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ran fine but now just backfires? wont start?

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ran the newly rebuilt with 13.5-1 piston crf250 at the track this weekend.. ran great but near the last pratice was a bitch to start... so i ride very hard and clear anything possible, im in the a class pratice... I rode it hard for maybe 4 hours.. valves were good before... Now it olny backfires when kicked over after many trys... im thinking the valves? but i did not shut fuel off when i trailerd it home and i got a truck with 35s so it is a steep incline to get it out... please lmk what you think before i tear it down. :thumbsup:

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I would check the valves. Have you had them re-shimmed? if so, and the stock valves Ti valves are still in the engine, they could ave zero'd out again and need replaced.

May also want to check the timing and make sure that fuel is making it into the carb.

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Cam sprockets may of moved on the cams too, so not only confirm the timing, look at the lobes, do they look right? Somtimes there is a paint mark on the sprocket and the cam, if the alignment is not right, you know the cams moved.

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thanks guys, ill check this week and let you kno... I mean it ran fine for all 6 pratices just got harder and harder to start so hopefully the valves

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thanks guys, ill check this week and let you kno... I mean it ran fine for all 6 pratices just got harder and harder to start so hopefully the valves

It sounds like a valve issue.

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ok fellers, three valves are perfect and not tight but one intake aka the left one i cant even get my smallest feeler guage in there wich is .002mm

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Could this be enough to make the exhuast backfire and not start?? im sorry ive olny raced supermini with a cr105 and crf150r but my dad did all the work but now i gotta do it all myself.

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Could this be enough to make the exhuast backfire and not start?? im sorry ive olny raced supermini with a cr105 and crf150r but my dad did all the work but now i gotta do it all myself.

One valve .002" out of specs will cause it to not start or make it almost impossible to start. If the smallest feeler gauge won't fit, the valve is zero'd out and that's your problem. I'm sure that the valve is shot and needs to be replaced. My guess is it's one of the intake valves and most of the time the right one. As for it backfiring, it's a good possibility that the valve is causing it to backfire.

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yes it is the right one. since it is zerod out it has to be replaced? or just needs a small shim.. btw thanks a ton man i really appreciaite guys like you on this site!

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You will need new intake valves. Replace in pairs. Make sure the guides for both the exhaust and intake side are all good and in spec.

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Charlie, it was a .002MM feeler that wouldn't fit in not a .002" so definately zeroed out:thumbsup:

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Charlie, it was a .002MM feeler that wouldn't fit in not a .002" so definately zeroed out:thumbsup:

Not a problem. I was just letting you know that if any valve is .002" out of specs, it will cause a starting issue. If the engine still has the stock honda titanium valves, you can shim it, but it will zero out real fast. The coating on the valve is worn off and the more you run it with a bad valve the more you distort the valve seat. Your best solution is to not run it until the valve seats are cut and the valves are replaced. You do have the option to use stainless steel intake valves instead of titanium. The stainless valves are more durable than the titanium valves. If you race hi-level MX, you will probably notice a difference between the two types of valves. Hi-level MX guys can tell a difference beteen the 2 types, but most don't notice a difference. If you're a trail, woods or weekend rider, the SS valves would be your best choice. Regardless of which valves you decide to get, you must have the seats cut to insure a proper seal between the valves and the seats.

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Charlie, it was a .002MM feeler that wouldn't fit in not a .002" so definately zeroed out:thumbsup:

I would actually challenge ANYONE to find a .002mm feeler gauge!!!

I am sure such a thing exists, but I would hazard a guess at "only in laboratories"... :confused:

Just a typo in the original write up - let's not get tooo hung up on semantics - the valves are out. New valves required.:thumbsup:

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haha ur right i dont have a .002 feeler it was really .052

Edited by allanmathews

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you said that the valves were good before you started to ride , what were they at? have you ever shimmed or replaced the valves before? chances are based on how you say you ride your best bet is to replace the intakes , shimming them as you know is just a band aid but at some point and time replacing will become nessasary

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