Low Compression and Knocking Noise 1981 GS650G

[Haon832]

On my last ride a few miles away from home I started to here a vibration around 3,000 rpm's. I drove it straight home to diagnose the problem. At an idle I can hear a knocking sound every 2 to 3 seconds from the top of the engine and any time I rev it to 3,000-4,000 rpm's there is a rather violent vibration that doesn't sound good. Previously before this occurred, I did a compression test on all of the cylinder and got a reading of 170 psi which is well within the spec. However, after the vibration and knocking started, cylinder 3 is only reading 90 psi and the others are still reading 170 psi. From what I know, this loss of compression can be caused by either a problem with the valves, or a problem with the piston rings. So my question is, where do I start. Should I start with checking the valve clearances? If they are all to spec do I start pulling the engine apart to take a look at the pistons and rings, or is there a test I can do to see if the piston ring is cracked or not seating against the cylinder wall? Or, is it possible another issue could be causing these symptoms besides the valves and ring? Any help or advise would be great. Thanks.

[Nessism]

How long since your last valve adjustment?

[Guest]

The history of the 81 650's is not a happy one, especially engine #s 122709 and earlier (from P25 of the supplementary factory manual, p/n 99501-36020-03E), where they changed the design and part numbers of the crank and rod bearings, presumably for better oil flow in that area. I have one, and it has exhibited nearly the same behaviors that yours does. Have not fixed it, but a member here, wm. groebe, has tried, and if his posts are still available for search, it's an eye-opener. If you have a decent set of tools, and a fair amount of mechanical knowledge, it IS fixable, but quite involved. Good luck!

[Haon832]

I have only had the bike for 3 years and haven't adjusted the valves. I now know this should have been one of the first things I did when I first acquired the bike. Since I got it, I haven't put more than 1500 miles on it, so hopefully I didn't let it go too long. I am a college student and bought the bike as a learning project so I am taking this as a lesson learned to keep up on my valve clearances.

[Haon832]

Well, that's not the best news. Thanks for your reply I'll check out groebe's posts about it.

[Nessism]

Sounds like you fried a valve from neglect. That sucks.

[Haon832]

Sounds like you fried a valve from neglect. That sucks.

Yeah that is what I'm afraid of. Before this project I literally new nothing about motorcycles and was't aware of how crucial valve clearances are. My dumb thought process was, "It's running good, must not be anything wrong with it." I now now this was a bad mistake. How involved is replacing a valve on these engines?

[tom203]

Pull the cam cover off.... Maybe you'll get "lucky" and find your camchain jumped. A burnt valve shouldn't make an unpleasant vibration and knocking. In any event, I hope the problem is in the top end,cuz repairing the crank end on this model is a bad experience,as Groebe in California can attest. My 81 has 53k miles on it and still runs well - engine #104xxx.