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Compression

mcycle-nut

mcycle-nut

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What's the compression supposed to be on a 2 valve GS750? My bike has only about 100 PSI, and is only making 10 inches of vacumn, making it impossible to tune. My carb guru figures it should be between 150-170 PSI and should make 20 inches of vacumn. Is this correct? :-k
 
comp

comp

yea 100psi isnt very good, 120 is about min. to get a good tune has the test been done right? valves checked. when my bike drops to 120 I will be doing the top end for now its at 145psi
 
According to the factory service manual, for a '77-'79 GS750 the limit is 100 psi and the maximum difference between cylinders is 28 psi.

For low compression they list the following possible causes:

- excessively worn cylinder walls
- worn down piston or piston rings
- piston rings stuck in the grooves
- poor seating of the valves
- ruptured or defective cylinder head gasket.

For a new (but broken in engine) I think it should be in the 150-170 psi range.

To do the test

- the engine should be warm (the manual doesn't mention this, but I'm pretty sure it should be warm)
- remove all the spark plugs
- put the gauge in tightly so there is no leak past its seal
- twist the throttle fully open
- crank the engine

If you get low readings, a common way to tell if it is the rings or valves, is to squirt a little oil through the spark plug hole before testing each cylinder again. If the readings improve then it is probably the rings (which are now sealed by the oil). If it doesn't change, then it is probably the valves.
 
Sounds like good advice to me! :) Let me add a possibility regarding the low vacuum levels. The VM carbs can be so poorly bench synched that you'll see low levels at start up. The slides can be adjusted so far off that the bike idles high without the idle adjuster tip even contacting the throttle pulley.
If the bench synch is done correctly, and you have no other mechanical problems, the levels you see at start up will be fine. Getting the levels uniform is what matters.
 
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