Single cylinder fouling dilemma
 

So here's my dilemma, my number one plug keeps fouling and is firing erratically. The 400 motor was rebuilt less than a year ago and started out running fine. Then it started sputtering off the line and backfiring. The idle was rough and kept wanting to die. I pulled all the plugs and only #1 was covered in black soot, the other 7 were fine. I changed it and it fired normally but after a while it happened again. I tried swapping plug wires but #1 still fired erratically and fouled. I changed the ignition coil, rotor and cap but that didn't solve it either. #1 kept fouling like it was running rich. I put in another new plug this morning and drove it about 10 miles and it looks ok, but I know it's gonna go bad again because at the end I could feel it missing again. It seems to have lost power too, but maybe that's because it's running on 7 cylinders. Could it be the Q-jet? Intake? Any suggestions???

Maybe look at the valve train area. Seal and guides in good shape? Both rockers on number one opening all the way?

Or maybe a simple compression test first...

Is it dry carbon or wet carbon/oil?

Suspect mechanical failure. Do a compression test, and leakdown test to confirm cylinder sealing. Check the valvetrain adjustment and cam lobes. Cut the oil filter open and check the pleats for debris. Worst case, it's eating the lobes on #1 and the valves not opening is fouling the plug.

The heads were rebuilt at the same time as the motor. Paul Carter built the heads and did the machining.

I'll check the rockers to see if one came loose.

It's black carbon. Not wet oil.

Maybe the rings didn't seat in the one cylinder?

If it's ONE cylinder, it's certain to NOT be the coil. You're looking for something unique to that cylinder.

That terminal of the distributor cap, plug wire, plug.
That intake or exhaust valve, or valve motion, or valve seal
That runner on the intake manifold, the gasket seal at the head, or that intake port
The cylinder wall or ring seal on that cylinder.
The head gasket seal for that cylinder

Stuck or bent exhaust valve do a compression test

Replace the distributor cap before you dig in any further.

X 2. Black sooting of a single spark plug is indicative of an ignition problem.

Try checking timing using #1 plug, then check timing on #6 plug.
Should be the same. If not?

:)

Could do it by ear, but I'm not that good.

:)

What is the heat range of your plugs?

:confused:

I had an oddball situation like that with a dual plane intake. It was gas fouling the #4 plug after about 100 miles and drop that cylinder, and the #1 plug was rich, but not gas fouled, while the other 6 plugs looked perfect.

Turned out the idle mixture on the passenger side was screwed out further than the driver side, and it just so happened the front passenger side of that dual plane intake runner fed #1 and #4. It fouled #4 because of the short runner length. #1 was rich but never fouled because of the much longer runner length on that side.

Once that idle mixture screw was set correctly, problem solved and #1 and #4 now evenly color like the other 6. Dual plane intakes are sometimes funky like that which is why stagger jetting works well on some combinations.

I stated in my original post that this is something that I did already and there was no change. Ignition was my first guess so I replaced the rotor, cap, coil, plugs and wires.

1 & 6 are the same.

Can't remember the heat range but I remember Looking for the plug specs to make sure I got the right ones.

I checked the compression on all 8 cylinders. 7 is 170, 2, 4, 6, 8 are all 165, 3 & 5 are 160 and 1 is 155. I squirted some oil in #1 and the compression came up. Paul suggested that the rings aren't seated yet. I rebuilt the motor last year but haven't really driven it a lot so it could need some more miles to settle in. I drove it around today and got on it a few times with no sputtering. So far the plug I put in yesterday hasn't fouled.

Formulajones - I thought the same thing about the mixture and turned both adjusting screws in 1/4 turn to see how that works.

I am wondering if rings not seating would cause a backfire? I guess that once the plug gets fouled, unburned fuel could ignite in the exhaust? If that isn't possible, then could a valve be sticking open intermittently? Or a lifter get suck in the lifter bore? Your top and bottom compression tests are withing 15 lbs.

Maybe try indexing the plug, see if that helps any?

Then it's high time to get drive the car, get it up to temp ( 15 minutes drive time) and give the motor 3 or 4 full throttle 5000 rpm rev up sessions to bed the rings in !

That's the plan! Gonna get it out again today.

So I got out and drove it for a few days straight and everything seemed to be going good. Today I took it out again, after it sat for about 4 - 5 days, and i got the same fouled plug misfiring after about a mile. :mad: