Low Compression Troubleshooting

phillgripper

phillgripper

79 CX500
So...my 79 CX500 JUST started having compression problems. Its 60psi on the right only 30psi on the left. The bike is only running off 1 of the 2 cylinders. Any suggestions before I overhaul the top end?
 
Check your valve clearances.

If that no fixy, do a cylinder leakdown test. That will tell you where your compression is being lost.

Tearing an engine down is a terrible way to diagnose a problem. Find out what the problem is, first.
 
Check your valve adjustment. (easy to do and free)

Loosing compression can be caused by the piston rings going south, excessive wear in cylinder wall, valves not closing completely, valves not opening completely, valves not operating at the correct timing with regards to piston, cracked/leaking head or catastrophic spark plug failure. (yes, I have seen such a plug failure. damn strange, too)

Most of these mean a top end rebuild at least, some a full overhaul. But first, double check that the valves are in the proper adjustment...

Best of luck!
 
Bear in mind that CX500's are notorious for being a one cylinder starter even after a day or two and they can take a minute to behave. Make sure this isn't causing some sort of a misdiagnosis. I've also been using Iridium plugs primarily in my CX's as they help overcome plug fouling and start up issues which are also common in these bikes.

Also, after doing a leakdown test if your rings are good pull the heads off. The heads on these bikes are real easy to do and you can fit new head gaskets, valve stem seals, clean up the intake and then adjust the valves once the heads are back on. That means you know the works been done and you sould get another 20-30,000 before you may need to take them off again.

I know others would disagree but I feel it's such an easy job to do and considering a gasket blowout can drop compression, why not.

Actually, while I think of it, have you checked the coolant for milky residue and the oil for frother water contamination? That'd be a dead givaway.
 
Don't forget to have throttle wide open when you do compression test
 
what happens if you don't have a leakdown tester,,before they were readily avaliable(cheap to buy) a wet dry test was the thing to do.You know the dry,so add some oil into the barrel and test again,,,,no change,valves head.etc....compression increases suspect the pistons and or bore,,,,its old school....good luck
 
No need for a good leak down tester. Just find a way to add pressure and listen for leaks. If you have a detachable hose for your compression tester, attach an air compressor and listen for air leaking out.
 
teazer said:
No need for a good leak down tester. Just find a way to add pressure and listen for leaks. If you have a detachable hose for your compression tester, attach an air compressor and listen for air leaking out.
Click to expand...


...........................Make sure it's at top dead center COMPRESSION for the cylinder your checking or all the air will leak out the open valves
 
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