Topic: No compression after top end rebuild 1980 CB650C

[NeuroFunk]

 I bought a 1980 cb650c last August wasn't running at the time so I took out the carbs and rebuild them, after that it fire up used it for a couple of weeks then I realized that it was leaking oil from the head gasket so I bought a gasket kit and put them in. Put the engine back together and now cylinder 1 & 4 have low compression (110psi) and cyl 2&3 have no compression (0psi) I think there is something wrong with the cam timing. I follow the instructions of the manual but I think something is wrong
Can somebody point me in the right direction

[bryanj]

Crank mark spot on the T mark, middle of notch on cam end horizontal and cam timing should be ok BUT if you bolted the cover on without heeding the warnings you could have bent the valves

[NeuroFunk]

I took the head out the check if the valves were bent and they are not bent

[kakon864]

Did you double check your valve tappet clearance? If not I would reset/check all of them as per the manual instructions.
Also as far as the low compression in 1 and 4, are you using an automotive compression tester? I have heard that they are not very accurate for motorcycles. Also when testing compression it should be at WOT. 

[Tomshep]

If you have the cam timing so wrong that you cannot get any compression, the valves are bent so it isn't going to be that because you poured kerosene into all the combustion chambers when you took the head off again to check the seal and it all stayed there, didn't you?
So it has to be something else.  Try removing the tappet covers and then slackening the tappet nuts  and adjusters fully so that the lash is massive on all eight valves. If the engine turns freely with the plugs out, you should now be able to test for some compression with a cork in the plug hole and the electric start. Spin it over and recover the cork from where it lands. If you now have compressionon all four, there is a shop manual on this site that will tell you exactly how to set the engine up to factory specification. Follow it to the letter and if there is anything that you do not understand fully, ask us.
If you don't, you have not seated the head properly over the gasket or you have either damaged or forgotten to reinstall the piston rings.

[NeuroFunk]

Did you double check your valve tappet clearance? If not I would reset/check all of them as per the manual instructions.
Also as far as the low compression in 1 and 4, are you using an automotive compression tester? I have heard that they are not very accurate for motorcycles. Also when testing compression it should be at WOT.

I did the test without the carbs with a automotive compression tester

[dave500]

I doubt the rings wernt installed tom,give the man some credit?as far as the head not seated properly you cant really get it wrong?so long as your compression tester has a valve at the plug end itll be ok,despite what some might say about too long hose.

[Tomshep]

No credit due when somebody changes a head gasket with compression and then doesn't have any. Sensible reasons based on half a century's experience have been given in this and the other two posts you have trolled so I'll butt out now as you clearly have no need of any contribution I could make and I have found little here that I didn't know already so have fun.

[robvangulik]

No credit due when somebody changes a head gasket with compression and then doesn't have any. Sensible reasons based on half a century's experience have been given in this and the other two posts you have trolled so I'll butt out now as you clearly have no need of any contribution I could make and I have found little here that I didn't know already so have fun.

At least he returns after asking a question, and doesn't bale out  after 1 post as you did on the UK forums.....

[dave500]

well that was easy.

[MRieck]

well that was easy.

Maybe.

[scottly]

Put the engine back together and now cylinder 1 & 4 have low compression (110psi) and cyl 2&3 have no compression (0psi) I think there is something wrong with the cam timing.

If the cam timing was wrong, it would affect all 4 cylinders the same. Sorry to say, but the most likely cause of the lost compression on 2&3 is bent valve stems, caused when the valve cover is installed incorrectly, without holding the rocker arms up with rubber bands. The rubber band thing may not be mentioned in the manual you used?

[dave500]

back to your first post im not clear,you say low compression?but was the engine running ok more or less?why did you check compression?ignore that tomboy or whoever he is.

[NeuroFunk]

the intake valves on 2& 3 cylinders werent sealing so i took them to my local machine shop to check if they were bent, but no answer from them yet 

If the cam timing was wrong, it would affect all 4 cylinders the same. Sorry to say, but the most likely cause of the lost compression on 2&3 is bent valve stems, caused when the valve cover is installed incorrectly, without holding the rocker arms up with rubber bands. The rubber band thing may not be mentioned in the manual you used?

[NeuroFunk]

back to your first post im not clear,you say low compression?but was the engine running ok more or less?why did you check compression?ignore that tomboy or whoever he is.

Routine check after i close the engine

[NeuroFunk]

No credit due when somebody changes a head gasket with compression and then doesn't have any. Sensible reasons based on half a century's experience have been given in this and the other two posts you have trolled so I'll butt out now as you clearly have no need of any contribution I could make and I have found little here that I didn't know already so have fun.

i just took the head out, not the pistons

[bryanj]

THE ONLY REASON for loss of compression after fitting head if it was ok before IS BENT VALVES

[NeuroFunk]

THE ONLY REASON for loss of compression after fitting head if it was ok before IS BENT VALVES

took the head out once again, took it to a machine shop and they check everything valves, valve guides, they pressure check the head for cracks and everything is ok