Compression

[angus]

Does anybody know what readings I should be getting from compression test.I'm getting 120 on 3 cylinders and 30 on last.Its a 1982 GS 850 gl.Conan

[Nessism]

Did you adjust the valves, run the engine until hot, and hold the throttle wide open?

Also, if it's been a while since the bike has seen some mileage you need to give the bike an italian tune up before testing. About 100 miles or so should do it.

[Steve]

Yep, what he said. 120 is not bad on a cold engine, might be about right is the throttle was not opened.
30 could be because of mis-adjusted valves or stuck rings. Did you put some oil in that cylinder and try it again?

.

[hannibal]

If you get a higher reading after adding oil, does that suggest the rings aren't seated properly?

[angus]

No mechanic has it and got these readings he says possibly valve adjustment or chipped/bent valve just trying to figure out if its worth doing.Conan

[Nessism]

No mechanic has it and got these readings he says possibly valve adjustment or chipped/bent valve just trying to figure out if its worth doing.Conan

The valves tighten with mileage and with enough neglect they won't seal anymore and then there will be compression loss. It's very unlikely for the valves to be chipped or bent, but if the idiot owner keeps riding the bike after the valves have tightened to the point of compression loss the valves may burn.

It's a small gamble on your part but you should adjust the valves now and then move forward on getting the engine running. There could be an internal problem causing the compression loss but you need to make sure it's not the valve lash settings alone at this point. Many times the valves can recover from some carbon build up too since GS engines are so tough. Worst case, you are out the cost of the valve adjustment. You should just learn how to do this work yourself anyway so time to start now.

[angus]

Cheers for advice bike was running good plugs are burning brown.He is surprised by compression reading,so investigating it more he's convinced its not pistons and rings just valve in that cylinder. I'm sure I could of done valve adjustment myself but fought it was a carb problem which I'm not great with.Checking for gasket for rocker cover and bikebamdit is showing that 1980 gl gasket will fit would anybody know as I've located 1 cheap.Conan

[angus]

All sorted he replaced a shim and reset clearances running better now.Cheers