black spark plug

I have a 81 seca 750 -
# 4 plug is black[dry]
all others are nice
valves just adjusted[#4 was the worst]
#4 was pretty gummed up[wet] before the adjustment
do you think it may have been carbon on the valve seat?
do i need to readjust that valve-[intake]

 

I'd put a new plug and see if it makes a difference.

 

i swapped a known good one

 

Maybe you under-adjusted these valves. What did you do exactly?

 

i measured and swapped for a 250 but when i stuck it in it needed more clearance [i didn't have anything smaller so i sanded it down with emery cloth] and got the correct clearance.i did stick a wire between the valve and the head to retrieve the shim . this may have knocked some carbon off.

 

For starters, REPLACE THAT SHIM.

You cannot "grind down" shims, they are only case-hardened. The hard surface doesn't go very deep at all. Don't destroy your motor to save $8.

When you say "it was the worst" how tight was it?

Fix the shim, and do a compression test on the motor. If the results of that come back fine and you're still blackening the #4 plug, it would tend to indicate a failing valve stem seal in that cylinder although not necessarily on the intake valve, it could be co-incidental to a degree. If the valve was uber-tight, it could have been overheated enough to cook its seal though.

A wet-with-oil plug won't be caused by carbon on the valve seat. If the rings are OK (compression test) then it indicates a leaky valve stem seal.

 

.08mm--can the stem seal be replaced without removing the head?

 

There is something i don't understand. You wrote that the measured clearance of your intake valve was 0.08 mm, and that you swapped the shim for a 250. So I assume that you had a 255 before.

If this is true, the 250 should have fitted correct and have given you the right clearance, no need to sand it down. I suspect that the shim wasn't sitting properly when you measured your clearance once you put the 250 shim in there.

Follow bigfitz advice and keep us in touch.

 

the comp test[dry] was 95psi all others were 140 ish.Would a sticky ring cause this? i've been soaking it with deep creep.

 

also would like to add that the exhaust temp is always lower on that cylinder

 

It would be since at 95psi it's barely running.

So now do a wet test, which should tell us if it's rings, or just a bad valve.

Don't use too much oil, a scant tablespoon per will do. Test all 4 and let's have those numbers too.

140-ish is good, quite decent actually so unless something bad has happened, I'd put my money on that cylinder probably being in as good a shape as the other three ring-wise with just a cooked valve.

Diagnosis before panic is always the best course of action. If you're lucky it will simply need a valve job, and replace one or two valves. Good opportunity to replace the valve stem seals, get all the valves lapped, clean everything up nicely-- it sure "freshens up" a motor if the pistons/rings/cylinders are still fine.

 

it went up to 125 wet-still a little low