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?