I am redoing the topend of the motor in a 05 crf250. I put a new weisco piston and hotcams cam in it along with a new clutch, but never mind the clutch because it is fine. I put the piston and cam in ( I did not re-shim the valves ) and put everything back together and it won’t start. It wants to start but it won’t. Every once in awhile after a lot of kicks it will fire for a split second but it won’t run. So after I tried and tried and finally decided to pull off the valve cover again and put back in the stock cam. After I got the stock cam in, I put the bearing caps and everything back in and it still won’t run!! It still fires for a split second but it still won’t run. I would have thought that with the stock cam and the stock shims would not matter because it would start fine before I pulled it apart. The timing marks are lined up correctly as I checked them millions of times. This is getting really frustrating. :bang: What could be the problem?
Crf 250 Help!!!!!!!!
- Thread starter MOTOX888
- Start date
- Aug 3, 2004
- 1,003
- 0
Did you pull the valves to check them? Did you even measure for clearance before you tore the motor down? How much have you shimmed them (total) since they were installed in the motor? A 250f should have the valves replaced at least as often as the piston, unless you get kibblewhites. If you don't, you just asking to snap the head off of one.
- Thread starter
- #3
I have not even touched the motor until i decided to do the topend on it. I took the cyl head to a honda shop to get the valves and everything else checked out and replaced ( if it needed it ) and he asked me if it was hard starting and i said that it wasnt.....so the guy said just to clean it up real good put it back on the bike and it will be fine. So right now i am leaning towards that i need to reshim the valves as they were tight with the hotcams cam. So, i put the stock cam back in to see if it would run with that one in it, but it wouldnt, which is weird because it ran before i pulled it out. So if it ran before i pulled the stock cam out...why wont it run after i replaced the piston and put the stock cam back in?
oldfrt613 said:
Spark is produced on every revolution. It's called a lost spark ignition and it's common to most four-stroke singles.
Ol'89r
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- Jan 27, 2000
- 6,961
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MOTOX888 said:
MOTO.
Did the guy at the Honda shop just look at the head or did he actually measure for the proper clearance and take it apart and inspect the valves?
If the valves were tight, it will be hard to start. The clearance could have been tight on the stock cam. If the Hot Cam uses the same base circle as the stock cam then changing cams will not make a difference. Checking the valve clearance on the new thumpers is very important.
Also, it is very easy to get the cam timing one tooth off. When you do this it will act like it is going to start, but it won't.
Double check TDC by placing a rod through the spark plug hole and feeling the top of the piston. Rock the crank back and forth until the piston is on TDC. Then, check your timing mark. The best way to do this is to use a dial indicator and a degree wheel, but you can get pretty close by feeling the top of the piston.
One last thing. Make sure the shims are properly seated under the spring cups.
Ok, one more last thing. A good way to check that your valves are seating is when you have the head off, pour gasoline in the intake and exhaust ports one at a time. If the valves are not seating the gas will leak past the valves.
Just my $ .02
