Clone Compression Issue

[Stratogeezer]

I'm having compression issues with my 6.5 Clone engine. It wouldn't start, so I checked the compression. It was about 20psi. I pulled the head and checked the vales - they are sealing just fine and neither is bent and the cylinder is clean (not broken rings, etc.) When I set the valve lash to specs (.002" intake, .003" Ex at TDC compression), I get almost no compression. When I back them way off, like to .050", the compression goes up to about 70 psi and the engine runs, but sporadically.

Any idea what might be the cause and what I can try to correct this?

Thanks.

 

[Doc Sprocket]

Potentially stupid question, but I gotta ask- Valve timing correct?

Also, make sure you test compression rolling the engine over backwards- it defeats the compression release mechanism that is in place for easier starting...

 

[Russ2251]

How did you check compression?
How did you check valves for seal?
How many hours on engine?

 

[karl]

Its a Chinese clone? Usually the problem.

 

[Doc Sprocket]

Its a Chinese clone? Usually the problem.

Funny, sure- but none too helpful...

 

[Stratogeezer]

Potentially stupid question, but I gotta ask- Valve timing correct?

Good question actually. I installed a modified cam - and when I did so, I lined up the two dots correctly. So - unless it's possible that somehow the cam got out of sync - it should be correct. However - I've stopped short of looking inside the case so far, but that would probably be the next step.

 

[Stratogeezer]

How did you check compression?
How did you check valves for seal?
How many hours on engine?

Checked the compression with a compression meter - pulling the recoil starter in the forward position. It's also an obvious difference in compression with the valves adjusted in the spec vs wide open settings.
Checked the valve seal by pulling off the head and pouring gas into the intake and exhaust ports with no leakage visible.
Engine has probably 10 hours on it.

 

[jamilcobra]

Also, make sure you test compression rolling the engine over backwards- it defeats the compression release mechanism that is in place for easier starting...[/QUOTE]

+1 , and I would check the flywheel to be sure it hasn't slipped or sheared a key or something.

 

[Doc Sprocket]

+1 , and I would check the flywheel to be sure it hasn't slipped or sheared a key or something.

While a sheared key would cause it to run like crap or not at all, the key affects ignition timing, not valve timing. Would have no effect on compression.

Meanwhile, I'm very intrigued by the fact that the compression increases by slacking off the valve lash- which in my mind is somewhat akin to retarding the valve timing. After all, lots of lash would cause valves to open and close later than they should. This REALLY makes me think you oughta re-check compression while rolling engine over backwards.

Anything's possible, but I really would open 'er up and re-check the timing.

 

[crazyman]

If you tear it apart and all appears normal, try retesting using the stock camshaft.

 

[DustinWolfe]

 

[jamilcobra]

I dont really think there is an issue with compression. The valves, rings, etc appearto be good from what he is saying. When he backs off the valves it runs sporadically, which in my mind makes me start thinking ignition timing, but it would be entirely possible to miss a tooth on the cam/crank and play with it until it would crank.

 

[Stratogeezer]

Okay - thank everyone. I think I need to pop the case and take a look at the cam/crank alignment. I've got the stock crankshaft - so that can go right back in.

I'm just surprised that the crank and cam gear could get out of sync with each other. I guess stranger things have happened.

Stay tuned - appreciate all the help!

 

[CobraII351]

This may be a stupid question, but are you using the correct specs for the valve lash? English vs. Metric I glanced at the specs and had issues until I realized I was reading metric out of the book and using English on the feelers.

 

[Russ2251]

I'm just surprised that the crank and cam gear could get out of sync with each other.

They can't.