21 minutes ago, Mark Maras said:

 But, as I recall, the compression was good on all cylinders. Correct me if I'm wrong @jalexquijano.

Ill have to do a compression test again. That was way back in 2018. I did replaced both valve seals in that cylinder and the problem was never fixed

1 hour ago, Mark Maras said:

 But, as I recall, the compression was good on all cylinders. Correct me if I'm wrong @jalexquijano.

True but we never got it to move from cylinder #4 no matter what we tried.

Maybe the oil ring is bad or the gaps in the compression rings aren't properly offset. Ive got no idea but there must be some cause and it seems like we tried everything else

Could be just bad oil rings.  Might be possible to just hone the bores and change rings to stop the fouling.  

It is an old California Datsun engine.  Who knows what he put in there.

Ill run a compression test first to determine the psi in each cylinder. I was also considering on closing the fuel mixture knob at least 1/2 turn and let the car idle for 15 to 20 mins, pull out the rear carb plugs (4, 5 and 6) to determine any fouling

Edited February 21, 20232 yr by jalexquijano

in the long, painful history of this engine, have you checked  the valve stem seals ?

4 isn't going to be getting extra fuel from the carbs, so it must be oil, and at idle, the valve stem seals are a good candidate.

The only way to check and replace those seals is opening the engine. We are still guessing here. 

@jalexquijano. I remember discussing changing the valve seals several times. Did they ever get replaced?

I believe he stated earlier that the valve seals on #4 had been replaced

18 hours ago, Mark Maras said:

@jalexquijano. I remember discussing changing the valve seals several times. Did they ever get replaced?

Both valve seals in that cylinder were replaced last year.

Is the black crud on #4 dry or oily and does it smell like fuel when you remove it?

Ill have to leave it fouling again as i cleaned it with a brass brush

It looks carbon fouled to me but "we are still guessing here"..

I would run Sea Foam through every so often because extended idle causes carbon fouling.