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Zero Compression

  • Thread starter Thread starter shryke300
  • Start date Start date
S

shryke300

Guest
I am just getting back to this project after a bit of a break. (new job and all) I was ready to get this bike started and it would not. I checked compression today and I have ZERO compression reading on the gauge. I believe the gauge is working as I tried it on my lawn mower and got a reading. This is a rebuild. I did the 850 swap on my '76 gs750. I have fresh gaskets, I have checked and rechecked the cams for correct timing and I know the cylinder walls are good and that rings are on the pistons. One curious thing is that I can feel air blowing and sucking from the intake and from the spark plug holes, yet there is ZERO reading on the gauge. Any thoughts?

My thought at this time is that the valves may be being held open a bit? I got the valves re-seated and maybe the stems are too long now? Does this sound right to you all? The shims are tighter than .03mm. Also, if this is the case, do you think I may solve it with a thin shim? or will I need to remove the heads to get to the valves to shorten the stem?

Can anyone think of another problem this could be? Why ZERO compression??
 
Did you check the valve clearances when you put the engine back together, or did you just throw it back together with the old shims on the same valves/cylinders as before? From your description, I suspect your valves aren't closing completely.
 
Re-check the valve timing.
Measure the shims and put the rights ones in to bring the gap up to spec.
 
shryke300 said:
The shims are tighter than .03mm. Also, if this is the case, do you think I may solve it with a thin shim? or will I need to remove the heads to get to the valves to shorten the stem?
Click to expand...

Sounds like the valves aren't closing. You need to change the shims to put the clearance to .03-.10mm. If you need shims thinner than about 2.30mm to get the required clearance I'd pull the head and go back to your machine shop and have them shorten the valves.
 
I rechecked the valve timing and as near as I can tell, it is right on. With the crankshaft at the "T" mark for cylinders 1/4, the #1 stamped on the exhaust cam is even/parallel with the head surface. There are 20 pins counting directly over the #2 to directly over the #3. I don't think I missed anything there. I will try the thinner shims. Is the Shim Club still available? If so, what is the members name?
 
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