One plug fouling - KZ900

I re-did the top end including boring to 1015cc. Pistons are off a KZ1000 and I ordered new aftermarket rings. Removed all valves and changed seals.

At first the bike ran great. Then I let it sit for a couple of months. Next time I started it up it smoked quite a bit. The smoke fouled a spark plug. Smoke went away after some hard riding and the bike was running great. I let the bike sit for a few more weeks, went for a ride and plug fouled again. I THINK it's the same cylinder (#1) but not certain.

I vaguely remember having some trouble installing a valve seal, either on #1 or #4. I think I might have messed up the valve seal and should change both seals on #1 cylinder.

Current riding, no smoke, it's just that the plug fouls. I haven't checked the compression but when I took it on a fast ride the bike had plenty of power.

Piston rings were difficult to install. I slid the cylinder block off the pistons a few times to make sure I had them on right. So I think the rings are okay, with gaps properly staggered.

Any thoughts, would you agree it "might" be the valve seals for cylinder #1?

I'd like to try changing the valve seals without pulling the head, however I think I'll just pull the head and look at #1 cylinder to be certain.

That would be best.
Steve

Do you have a bore scope? Let it sit then peek into the cylinders. If there's oil you have bad valve stem seals.

I don't have a bore scope. Thinking about it overnight, it seems likely the valve seal or seals maybe the problem. When the bike is parked for long periods it's on the kickstand and the oil will gather around the #1 cylinder tappets.

Somebody wants to buy the KZ and I just discovered this problem yesterday. I'm ordering the gasket kit and want to get it taken care of as soon as possible. The bike is going to another island and I want to make sure it's trouble free when the next person gets it.

I commend you on your commitment to get it right prior to sale. Always the right answer.