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Intermittent afterfire from exhaust

472 Views 5 Replies 5 Participants Last post by  JeepinGeo
A month or so ago, I posted a thread about the cylinder #4 intake pushrod popping out of the rocker arm. We ruled that to be from a slightly twisted bridge, so I twisted it back and retorqued the pivot bolts and haven't had that issue since. I guess a few days after I began to notice an intermittent pop/afterfire from the exhaust. Being dual exhaust, I determined it to be from the side with cylinders 1, 2, and 6. I then pulled individual plug wires, one at a time, and started the engine. After pulling plug wire #1 and #2, the pop remained. I pulled plug wire #6 and the pop went away despite being down a cylinder. I have switched spark plugs out with 2 other new ones of the same type (Autolite 985). I also switched #6 plug wire with a plug wire off a V8. Fired engine up and same noise. It's inconsistent and intermittent, but super loud and about every 5-10 seconds ONLY at idle. I then changed the cap, rotor, and coil to a blue streak HEI. The old rotor tip appeared short or bent upwards. That should rule out the cap, rotor, coil, plug, and plug wire. Noise is still there. Tonight I did a compression check. Cylinder #1 was 130, #2 was 142, #3 was 138, #4 was 142, #5 was 140, and #6 was 140. Also performed a leak down test and did not hear any air leaking past the valves/rings. Posted is a picture of the plugs. You may notice #6 is whiter, but that's because it is new. The old plug looks identical to the remaining 5. I can't pinpoint the issue so I am going to check into a potential exhaust/intake gasket leak. My only other ideas are maybe a problem with the HEI module or raw fuel running down to #6? Timing is also set at 8° and 750rpm. No white smoke from the exhaust, no overheating. Coolant level and oil is consistent and not discolored. Any ideas?
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Just slinging this out there!


Willy started "tapping" many years back ('46 Willys L-134 [flathead])

I had rebuilt him in 1996, and he had less than 20,000 miles on him.

After tearing into the EZer stuff 1st and then digging in further.

I pulled the head, I found that #4 had a 1/2" - 3/4" long piece of ring

sitting on top of the piston, you could see where is slapped around prior

to sticking.

When it "stuck" it was barely contacting the head, hence the tapping sound (the head was not damaged, magnafluxing

revealed no cracks inside or out.)

It kinda sounded like a pushrod or rocker----But a flathead has neither!

It was really different looking in the head's chamber and on top of the piston from the others.

My friend, who does production engine rebuilds, came over and said I had a

Teeny-Weeny coolant leak into that cylinder, which was causing detonation,

which also broke the end of the ring off------Said He!

IF, there was any kind of popping----I did not hear it happening????

Well, I had already pulled the pan to see if it was rod issue.

The cylinder wall did not even have a scratch!

I was going to just replace the piston and rings----but I had to buy a full set of each to get one.

Same for the rod bearings.

I would just replace all 3 in all 4 cylinders.

I had the head shaved and I borrowed a ball hone from my friend, I honed a 4 a little bit, and put it

all back together with a new head gasket.

All has been fine ever since!

Playing it safe---I have rechecked the torque several times!


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The ring segment pushed straight up and did not hurt the cylinder wall.
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After surfacing---#4 is a different color----The chamber (below) and the top of the piston appeared like they

been sandblasted or had a peppered like surface----My friend said each "micro-crater" had been

a "lightning strike" in the combustion chamber.
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Does it do it when cold or only when warmed up?

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Discussion Starter · #6 ·
For those still interested, I finally had the time to look deeper into that afterfire problem. The header flange was warped, probably from being overtightened. The gasket didnt seal as a result so the header leaked at 5 of the 6 ports. The headers weren't in the best shape to begin with, so I bought a new identical set of Hedmans. Now I'm just trying to decide what dual exhaust setup I want to run. Picture attached.
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