Low Compression in 3 Cylinders 100 Miles After Rebuild

[crazyramen]

Hi yall,
I bought this '79 750 after it had been sitting outside for 10+ years (according to the previous owner's son). When I got it it had 30-90psi compression that would go up to 60-120psi with a teaspoon of oil in the cylinder. I got it up to 120 psi in all 4 after doing a cylinder hone, valve lapping, and valve clearance adjustment. In retrospect, it probably just needed the valves adjusted to run, but the pistons and valves were really gunked up so I just went for it. That and some carburetor fixes had it running.

Now around 100 miles later I'm back down to 60-80 psi in 3 of the cylinders. I did compression tests with oil and this time there was only 5-15psi change. I double checked the valve clearances and only found one that had too much clearance. While I had the plugs out I got in there with a borescope and the cylinder walls do look fresh. The exhaust valves seem pretty good as well, but the intake valves maybe not so much.

I think this means I probably messed up the valve job since the oil didn't help this time. This was my first time lapping so there's certainly a chance but can anyone give me a second set of eyes on these? Here are the borescope pictures: https://imgur.com/a/gs750-100-miles-later-X2wmwgG

Also, when I tear it back apart, am I gonna need to get a new headgasket even though this one is only 100 miles old? Will I also need valve stem seals if I take the valves out?

[Rich82GS750TZ]

I don’t have a suggestion for you. But to get a full picture of your first tear down, did you not also do new rings and valve stem seals at that time?

[crazyramen]

I didn't do new rings, I measured the old ones withing spec. I was hoping that if it was an issue, I'd see bad compression right after reassembly.

I did do new viton valve stem seals at the time as well. I'm wondering if those would be too damaged by removing the valves again to reuse.

[Rich82GS750TZ]

I'm wondering if those would be too damaged by removing the valves again to reuse.

When I was doing this, the concern from some was that pushing the valve stem tip with its clip groove through the oil seal more than the required one time could cause damage to the seal. Others were not as concerned. I’d say if you do it carefully, with enough lube, maybe it’ll be OK, but I’m no expert.

[crazyramen]

Maybe I won't buy them ahead of time, but keep a close eye on the valves and see if there's any plastic that comes off with it!

[Big T]

It's a bit hard to tell what's what in the photos, but are the sealing surfaces of the valves all the way to the edge of the valve?

[crazyramen]

It's a bit hard to tell what's what in the photos, but are the sealing surfaces of the valves all the way to the edge of the valve?

Not on the intake side I don't think. There a bit of a lip around the face of the valves. I'll check if that looks any different in the cylinder with good compression tonight.

[Don R]

We really aren't supposed to re-use rings, I never knew why except I was told they don't always re-seat even if nearly new.

[KiwiAlfa156]

Ditto on re-using the rings after a hone. The run-in is about the rings cutting off the peaks left on the liners left by the hone and the peaks cutting the rings to seal against any microscopic ovality, etc. The initial seating is quite aggressive and then it settles into a lapping process. The rings can really only do this once, especially if they have some miles on them. Even if the compression rings do re-seat satisfactorily, chances are the thin oil scraper rings won't and she'll be an oil burner.

[crazyramen]

OK back in the shed tonight and I had some unexpected results. I re-ran compression tests on all cylinders tonight, but the battery was getting low after the second cylinder (I forgot to plug it in after the compression testing yesterday,) so I started kicking it to test. I was getting much higher psi when kicking than with the starter. After than I hooked up the battery to my jumper pack and did another set of tests with the starter to confirm. It does make sense to me that kicking would increase the compression - it only takes one kick to start her up, but it takes a second or two of starter to get her running (not any longer than my last bike, though - it doesn't seem like the starter is shot or anything.)

The results were:

	1	2	3	4
Kicking:	110	110	140	110
Starter:	80	80	125	70

Which set of measurements should I be paying attention too? The tests I ran after the rebuild were with the starter, so it's definitely still lost compression, but if the kicked compressions readings are legit, I might just keep riding it and test it somewhat regularly to confirm it's not hitting below 100.

Ditto on re-using the rings after a hone. The run-in is about the rings cutting off the peaks left on the liners left by the hone and the peaks cutting the rings to seal against any microscopic ovality, etc. The initial seating is quite aggressive and then it settles into a lapping process. The rings can really only do this once, especially if they have some miles on them. Even if the compression rings do re-seat satisfactorily, chances are the thin oil scraper rings won't and she'll be an oil burner.

Would I be able to run some break in oil for a few hundred miles to try to get them to reseat? Or are they already hardened from the miles of use and won't ever break in again?

[Rich82GS750TZ]

Just to verify, you are holding the throttle wide open when performing your compression tests, right?

[Big T]

Your starter sounds dirty. Take it apart, polish the commutator, clean and lube, reassemble
Replace brushes if necessary

[crazyramen]

Just to verify, you are holding the throttle wide open when performing your compression tests, right?

Nope. Didn't know that's how it was supposed to be done. I'll do it the right way tomorrow

Your starter sounds dirty. Take it apart, polish the commutator, clean and lube, reassemble
Replace brushes if necessary

I wouldn't be surprised. I haven't touched it at all since it got the bike. I kick start it most of the time. I'll open it up and see what I see