Low Compression GPZ 750

I was able to start my project 1982 gpz 750 today but the compression was low in cylinders 2 and 3 at 75 and 55 psi respectively. I know the valves aren't an issue because that was the main job I finished in this go-around. But apparently it did not fix the issue. What other issues could be causing low compression, aside from valves and piston rings?

Cylinders 1 and 4 were both at 95psi which, I understand, is where they're supposed to be.

Also, not sure if this is relevant, but when I pulled the #2 spark plug, it was slick with oil.

The compression should be in the 150 psi range, with 100 being rock bottom. If the valves were not maintained they may have burned. Way too many bikes bite the dust that way. Valve neglect wont cause the plugs to get all greasy though. Maybe a leak down test is in order?

Valves are brand new, seats cut, head was completely reworked. How does a leak down test work?

A leak down test injects air pressure through the spark plug hole to pressurize the combustion chamber.

Can perform a poor man's leak down test by introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber. This may be done at TDC or any other cam positions where both valves are fully closed. Top dead center (TDC) is when the piston is at its highest position. An easy way to tell TDC is by alignment of the T mark with the case mark when viewed through the timing window.

May introduce a spurt of compressed air by using a rubber cone-tip blow gun (rubber air nozzle) held into the spark plug hole. And of course keep holding it in position to keep air from coming back out the spark plug hole while listening for escaping air at other places. The air compressor should not be running while listening because the noise will likely drown out any sound of escaping air. Or just use a portable compressed air tank.

Air heard escaping from exhaust port indicates exhaust valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from carb intake indicates intake valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from crankcase breather indicates loss of compression past rings into crankcase (perhaps worn piston rings or cylinders).

Air heard escaping from head gasket area indicates indicates loss of compression past head gasket (perhaps due to a blown head gasket).

A leaking valve may sometimes be resolved by adjusting the clearance to within specs.

A leaking head gasket may sometimes be resolved by torquing the head fasteners when the engine is stone cold (such as after sitting overnight).

Good Fortune!

freakinbike wrote: Valves are brand new, seats cut, head was completely reworked. How does a leak down test work?

make sure the valve timing isn't off.

bountyhunter wrote: make sure the valve timing isn't off.

This was the first thing I checked, and it's spot on. I guess I'll be performing a leak down test later today.

Check out this leak down tester!

I went to O'reilly to see if they had one I could rent and the dude busted all this stuff out and put it together for me. Total cost: $30

It's basically a bendy tube connected to a three way which goes to a pressure gauge and an air gun. Pretty slick!