Topic: '71 Honda CB500F cylinder compression loss

[Rain]

Greetings more-knowledgable-than-I motorcycle owners,

I have been having a significant issue with my 71 Honda CB500 since summer that I have somewhat diagnosed and now am seeking some help delving into it.

During the summer the bike suddenly started to seemingly have its power get cut while riding it. It wouldn't put out like it was getting fuel cut off but all power would just suddenly leave the bike. After taking it to the mechanic, the ignition switch got replaced and an issue with compression in the cylinders was diagnosed. There is a 15% loss in cylinder one and a 55% loss in cylinder four while the rest are fine. It currently wont start and I'm seeking some guidance as to a starting point on the work. My current guess is to check out gaskets and piston rings but that's as far as I've gotten conceptually.

My apologies if I missed a similar thread. If you know of any info already on the topic please send me a reference.
Thanks for any help!

[MoMo]

Buy yourself a compression tester and verify that what you were told is accurate.  Simple to do but if you need help there will be plenty of guidance here.   Welcome to the forum, shop manual should be downloadable from this site...Larry

[calj737]

You dfintielydo need to verify the compression. Plus, write down the compression reading on each cylinder. The comparable compression between each cylinder is what's most important. What your mechanic describes is a leakdown test, not a compression test. 15% and 55% of what? Typically you would see a compression rating of 105 to 155 psi per cylinder. If:

1 - 80 psi
2 - 125
3 -120
4 - 60

Then you'd know the problem. See the difference? What you are looking for is a 10% spread between and among all 4 cylinders. You can also record the "dry" number, and then drip a few drops of oil directly into the spark plug hole and retest. If the numbers rise, that tells you something extra.

Of course, a compression test requires that your valves are properly set and that your throttle be wide open during testing. Use a battery charger during testing to insure a good, high spin rate on the motor.

[Deltarider]

I always found it difficult to do a compression test on a CB500. The Honda dealer did it for me in a jiffy. No need to buy stuff and then ask yourself if you did it right or not.

[Bodi]

difficult?
1- remove plugs
2- screw adapter into a plug hole
3- connect gauge, crank engine, record "dry" reading
4- remove gauge, squirt oil into adapter, repeat #3 for "wet" reading
5- repeat #2-#4 for the other 3 cylinders

My experience with shop mechanics is ... poor at best. If I do it myself at least I know what went wrong. I've had shops cross-thread sparkplugs, strip cover screw threads, crack tail light lenses, and even light a fire under a car hood and melting a bunch of wires... all without telling me - just to let me find them the next time I looked so it couldn't be definitely blamed on them.

[calj737]

difficult?
1- remove plugs
2- screw adapter into a plug hole
3- connect gauge, crank engine, record "dry" reading
4- remove gauge, squirt oil into adapter, repeat #3 for "wet" reading
5- repeat #2-#4 for the other 3 cylinders

My experience with shop mechanics is ... poor at best. If I do it myself at least I know what went wrong. I've had shops cross-thread sparkplugs, strip cover screw threads, crack tail light lenses, and even light a fire under a car hood and melting a bunch of wires... all without telling me - just to let me find them the next time I looked so it couldn't be definitely blamed on them.

+1 And, since the info provided was relative "loss" as a compression reading, it calls into question what test was performed, or at least the relevance of the test performed.

Who knows, Bodi, maybe any job more than 2 or 3 steps requires a certified degree in some countries??? Must be how they keep unemployment so low....