308 Carburetor popping issue

What I was thinking

 

Okay, the last couple of weekends since my last post were rather poor weather-wise, so I hadn't had the chance to take the car out and test it. This weekend is a long one here in Canada (our Thanksgiving) and the weather was perfect this afternoon, so it was the perfect opportunity today.

And, no real difference. Carbs still popping on #7 and occasionally on #8. What I had did this time around was I installed new plug wires, caps and rotors. I relocated the spark plug extenders to different cylinders, thinking if they were the culprit, the popping would follow, but that's not the case.

So, my next steps are going to check the following:

1 - Swap coils around. If the coil for the front bank now is weak, it should follow to the other bank when it's relocated.
2 - Check fuel pressure along the fuel line route.
3 - Compression test. The leak down test is static and tested the integrity of the sealing of the valvetrain/pistons which showed OK, but the compression test may reveal a broken valve spring(s). I'm hoping it isn't but I can't rule that out. At least with winter coming up, I'd have time to pull the engine... Ugghhh...
4 - Cracked exhaust manifold? It was suggested to me offline by a fellow F'Chatter. I've experienced cracked/broken exhaust manifolds on other vehicles in the past that caused more prominent symptoms than what I'm experiencing with this car, but I'm not ruling that out either.
5 - Vacuum leak along the carbon canister circuit? I sprayed WD40 at the little tubes on the intake manifolds, but no change in RPM. If there is a vacuum leak, it may be in the system, like the three-way valve, or the carbon can itself. Cylinders #7 and #8 are the first in line along this circuit.
6 - Bad Pertronix triggers? I've got the MR-183 kit in the rear dizzy. I would've expected these to run flawlessly, but maybe one of the triggers went glitchy? I have a dizzy set up with points (two R1s to run both banks), so can swap that in and see.

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I haven’t read this thread, but just discovered my popping carbs and back fires were due to my carb jets being for high altitude while my car and I are at low altitude. Just a thought.

 

Since you are using at Petronix that may trigger in a slightly different position, is it possible the rotor phasing is off? Taking and old cap and cutting a hole in it will allow you to verify it.

 

Or find a guy with a Distributor machine that can test it and or set it up properly if it is off.

 

The rotor is in the correct position (verified with the red mark on the dizzy body and the mark on the flywheel).

I have an Allen Syncrograph machine.

 

The rotor being aligned with the red mark is not relevant if the firing device (point cam vs electronic eye) has been modified. If the electronic eye is not phased properly with the lugs on the distributor cap, then there are issues. That is what Dal308 is referring to. Having a cap with a portion cut away allows you to run the distributor on a machine and check where it is firing and then look at the rotor to wire contact phasing. When the electronics are mounted in the housing, they have to be placed properly to allow the phasing to be correct. The Allen Syncrograph should tell you what you need to know. I am not saying that is the issue, just a little clarification on the "phasing" comment.

 

Yes, I understand what you're saying.

But, it looks like there are bigger issues at play here, which the signs were pointing to before.

I took my boroscope and looked down #7 and wasn't too happy at what I saw.

The attached photos are rather poor (taken off of the digital scope screen with my phone), but is better when viewed by eye. It explains the air rushing past into the crankcase when doing the leakdown test...

Tell-tale signs of broken rings scratching the bores.

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