Any way to see if the cam gear spun?

I am re-assembleing my crf250r after a new top end. I had it all together and it turned over by hand and it turned over smoothly. Then i needed to re-shim it and when i put it together again it would lock up when turning over by hand. All three timing marks are right where they are supposed to be. it locks up when the ex springs are compressed.

Also when it is TDC the compression release is resting against the rocker arm contact point. I never noticed that before.

Any help would be appriciated

I just went over everything again it is timed correctly as i understand it. I also checked to make sure the piston is at top dead center and it is. I assume it is something simple hopefully one of you guys know the awnser.

When the cam rotates and compresses the exhaoust valves all the way that is when it stops. I seems like the valves hit the piston when I crank it by hand. I have had this appart several times before and never had this happen when all the timing is lined up.

Unless the cam gear moved alot it would probably be difficult to visually see that, but if you have another cam you could try to compare the two. Best way would be to actually degree the cam if you can.

What about the valve clearances, and did any shims get mixed up? Also if the piston is in wrong there may be contact between the valves and the piston top. Hope that helps.

I am re-assembleing my crf250r after a new top end. I had it all together and it turned over by hand and it turned over smoothly. Then i needed to re-shim it and when i put it together again it would lock up when turning over by hand. All three timing marks are right where they are supposed to be. it locks up when the ex springs are compressed.

Also when it is TDC the compression release is resting against the rocker arm contact point. I never noticed that before.

Any help would be appriciated

When you assembled the top-end did you check the bottom end? Did it spin freely? Check the rod for damage? bent or deformed? When you assembled the ACCT did you disengage it while tightening up the 8mm bolts? When you did assemble the ACCT did you let it self engage or manually twist it back into engagement?

I'm working on a bike right now and the crankcase bearings went. The flywheel will only spin from TDC-BDC (1/2-stroke).

I would check your cam chain tension. Go on from there.

When you assembled the top-end did you check the bottom end? Did it spin freely? Check the rod for damage? bent or deformed? When you assembled the ACCT did you disengage it while tightening up the 8mm bolts? When you did assemble the ACCT did you let it self engage or manually twist it back into engagement?

I'm working on a bike right now and the crankcase bearings went. The flywheel will only spin from TDC-BDC (1/2-stroke).

I would check your cam chain tension. Go on from there.

The bottom end seemed OK, also the bike ran prior to this fine. With the ACCT I left it on the cylinder and used a screw driver to unwind it then held that with vise grips while working. Then release it to unwind when I was done. tomorrow I will try much smaller shims on the exhaust side as a last chance before I remove the head to start from scrath. And also I an certain I have the piston in facing the right way but if I have to I will take it all apart the re-check everyting.

Also with the cam out it will turn over smoothly all the way.

????

Unless the cam gear moved alot it would probably be difficult to visually see that, but if you have another cam you could try to compare the two. Best way would be to actually degree the cam if you can.

What about the valve clearances, and did any shims get mixed up? Also if the piston is in wrong there may be contact between the valves and the piston top. Hope that helps.

thanks for the reply. I don't know how to degree the cam. I am starting to think it is shimmed to much. I will keep at it!!

Let us know how it goes. The cam bearings are operating correctly as well right?

I'd love to see some pictures to get a better idea on pin-pointing the failure here. I love trouble shooting these bikes.

is it at all possible that you intalled the piston in backwards...If you do this the Valve reliefs will not line up with the valves. That is the only thing i can think of....I doubt it is shims...If your clearances are good then they shouldnt hit...even if they are not good most of the time they do not hit...

I wonder if he's feeling the decompression pin hitting, and thinking that it's binding the engine. If the timing is correct, the bottom end is ok, the piston is the only other thing that may bind it up. Valve clearance has nothing to do with what he's describing. Now that I'm thinking about it, even if the piston is in ther backwards, when the piston is at the top of the stroke, the valves are closed or they would slam into each other. I don't know exactly what the tolerance is, but I don't think they would make it that tight to where it would hit even if it was in backwards. I never put a piston in backwards, so I'm not 100% sure, but it makes sense.

Just my 2 cents.

I wonder if he's feeling the decompression pin hitting, and thinking that it's binding the engine. If the timing is correct, the bottom end is ok, the piston is the only other thing that may bind it up. Valve clearance has nothing to do with what he's describing. Now that I'm thinking about it, even if the piston is in ther backwards, when the piston is at the top of the stroke, the valves are closed or they would slam into each other. I don't know exactly what the tolerance is, but I don't think they would make it that tight to where it would hit even if it was in backwards. I never put a piston in backwards, so I'm not 100% sure, but it makes sense.

Just my 2 cents.

It's not the decompression pin hitting, that is opposite the contact point of the rocker arm when this is occuring. When it is all in time the piston is top dead center, the cam lobes are facing the back of the bike (mine don't seem to be pointing up nesssesarily but the lines on the cam gear are lined up?). When i start to crank it over the piston goes down as the cam starts to rotate and when the longest part of the lobe is against the rocker arm compressing the exhaust springs and valves dowwn all the way, that is when it stops. I am goint to check the piston today even though I am pretty sure it is in correct. It is not likley but i will put smaller shims in to see if it has any effect first.

Because of valve overlap it might be difficult to tell if the exh or the intake valves are making contact with the top of the piston. Do check the valve clearances with all the marks lined up, did you change any shims? If the clearances are good I would check for the piston being backwards.

Now it is getting interesting. I didn't have any small shims to put in the EX side so I took the shims out all together just to know if the clearence had any real effect. So with the shims out it turns over perfectly. Now I am going to get smaller shims and see what happens. I have to say though it is disturbung to think that going from a 150 to a 130 shim can make the difference of hitting or not. I will post after I re-shim. I didn't check the piston yet I wanted to get the valve clearences correct beffore deciding to take the head off.

With the info supplied i would say piston in backwards..

Just checked, the piston is in correctly. Also I can see where it is the EX valves hitting it. even with the 1.20 shims in it it will just barely tap the piston when turning it over. I even just pulled the valves to make sure they are the same length as the ones that came out of it. ????????????????????

I dont have an old cam here to compare. when I have the head assembled and all the timing is lined up, the decompresser pin in the cam is resting against the rocker arm. Is this correct? I am just running out of stuff to check.

I would replace the cam with a new one or a known good one. What kind of cam is installed in it?

It is a new OEM 08/09' cam.

I turned the cam 2 teeth counter clockwise and it turns over smoothly while the other 2 marks are lined up and the shim clearences are correct. I think i got a cam that was miss-aligned.

It isnt an exact way but when the piston is at top dead center there should be a spot where the lobes over lap(both valves held slightly open). One will be on its way closed the other on the way open. rotate the crank 360 degrees back to top dead center and the valves will both be completly closed. I hope this helps....If you want you can rotate it over by hand...If it doesnt bind up at all go ahead and try and start it.

It isnt an exact way but when the piston is at top dead center there should be a spot where the lobes over lap(both valves held slightly open). One will be on its way closed the other on the way open. rotate the crank 360 degrees back to top dead center and the valves will both be completly closed. I hope this helps....If you want you can rotate it over by hand...If it doesnt bind up at all go ahead and try and start it.

That is where I am today. I will see if it will start.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now