No compression after rebuild gs750/850

[Guest]

So I rebuild a non running gs750. The cylinders were fairly badly notched where the piston rings had been sitting for over 5years with some water in them, so instead of paying for all the shop boring and honing I got a set 850 jugs and pistons for $150 out of a running bike.

I have so far done:
1979 Gs850 Jug swap
New gaskets top to bottom
750 valves lapped and leak down tested
Engine put back together and torqued down

Still haven't shimmed the valves perfectly but hopefully this week I can confirm my previous measurements and get some shims in it by Saturday.

PROBLEM:
I tried turning over the engine a few times by hand on the rotor to make sure no valves were getting hit and looks good. So I put my new compression tester on the cylinders and kick started a few times as all the batteries, frame, and wiring pieces are currently in my living not close to the engine. I got zip on the tester. The thing didn't even wiggle on any cylinders. I tested the tester with 20, 40, and 80 psi off my compressor and it works just fine. So what could be my issue, I really thought I had done everything right.


Also I'm sorry if this is covered in another post but if it is I just can't find it and I have spent hours searching for it.

Thanks everyone

[Guest]

Kick starting it may not be spinning the engine fast enough to get a compression reading. Read the directions for compression testing. They usually say to hold the throttle wide open to allow as much air in as possible, but they also say to spin it fast as possible via the electric starter.

[Big T]

Valve timing is probably wrong

[Nessism]

Agree on the valve timing. It's easy to get the cams 180 degrees out of sync. Check for 1-4 TDC by looking at the 1-4 T mark on the crank and making sure the pistons are at TDC when looking in through the spark plug hole.

[Guest]

Agree on the valve timing. It's easy to get the cams 180 degrees out of sync. Check for 1-4 TDC by looking at the 1-4 T mark on the crank and making sure the pistons are at TDC when looking in through the spark plug hole.

Thank you so much for actually saying how it could be wrong! I didn't even realize that the mark could be off by 180?. I at some point after I out the head on I had rotated the crank to check nothing was seized and did recheck TDC on 1&4. I just re did the timing and have about 110psi on 2,3,4 but looks like at some point while bringing the head back to the garage I possibly bent or got some kind of debris in the intake valve on 1 so I have 0 compression there. Now I have to figure out whats the easiest thing to fix the valve issue so I can get the bike together again soon.

[Dogma]

There might some shenanigans with the shims on the #1 valves too. Why are you thinking only about the intake valve?

[Guest]

There might some shenanigans with the shims on the #1 valves too. Why are you thinking only about the intake valve?

Well at first it was because I could feel the compression coming out the intake. Then I looked in the intake port and saw a ring of light around the valve from the work light I had pointed in the spark plug hole.

[Guest]

Yup... pulled it apart and found a bent intake valve😒 now I have find another one for cheap hopefully

[Smitty86]

so i have a 77 gs 750 and i want to do the jug swap do i need a 77 gs 850 jug and pistons or what years are compatible

[chuck hahn]

I agree on rechecking cam timing. Also remove all the plugs so it spins easier AND FASTER. And remember to hold the throttle wide open when cranking... with the starter of course