Need help: backfiring, black sooty #3 intake port (pic)

[Dick Weiss]

It shouldn't, but viewing the cylinder ends and their seats in the heads, they look like their sealing may be the
problem or from weak rings, oil ring gap not @ the top, and head torque--they should be torqued alternately @ 7-lb increments to final 28 lb; If not, the 1st head being fully torqued won't be correct after the 2nd head is torqued.
This was causing some case stress found by RIP Ray Litz back in the early days!

[Martin Benade]

Dick, what cylinder ends and ring gaps are you viewing here? The only picture here shows a pair of intake ports.

[Ron LaDow]

How does a carb make exhaust come up the intake port?

Cliff, suffice to say I can see it as (complexly) possible and also easily ruled-out, so why not try an easy test to confirm it is not that?
I have spent way too much time examining what must *clearly* be the cause of the problem, only to find out it was that thing which it 'can't be', and often in concert with another issue.
I now try to remove every single possible cause as soon as early as I can, especially when it is an easy 'fix'.

[John Berners]

Hello,
Thanks again for all your replies. I really appreciate your help. I want to express the symptoms better than I did in my first post. My impression is that ALL the cylinders are backfiring. When it is happening, it is really wild looking into the engine compartment. These are not normal deceleration pops, it's like loud popcorn everywhere with huge bangs mixed in, and sometimes flames come up into an air cleaner. The bangs are on both sides and seem like all 4 cylinders. When this first started two or three years ago, it would come on only after driving a few miles. More recently, it begins as soon as the engine starts. Which at this point I only do for diagnosis--the car can propel itself, but I don't consider it driveable--it could hurt itself or someone else.

This weekend when I found the soot in #3 intake, I seized onto it. I know the picture I posted is dramatic, and makes the impression that #3 is THE backfiring cylinder. But please forgive me--I think I created a red herring with that photo. I know you must be weary of amateurs like me who can't describe their problems accurately. I'm guilty I guess. I really don't think #3 is making all that noise, or that I would have failed to notice it was all coming from one cylinder. (Of course the soot needs to be addressed too.)

For a couple of years I've been trying to find what could make all the cylinders backfire at once. Any "in-common" ignition components: distributor, coil, plug wires. or ??? I heard low float level can cause backfiring--so maybe fuel pump, dirty gas tank, or partly clogged lines. But at onset of this problem, the car ran perfectly well until warmed up.

Every running problem this car has had in the 29 years I've owned it has been ignition. So I still expect an ignition answer. My 022 is pretty worn but I've swapped it for an 050 I had around and that didn't fix it. I swapped coils w/no luck. I have replaced the plug wires in the last 8 years, but maybe they didn't last--should try it again. I replaced the fuel filter and the fabric hoses. I just got a fuel pressure gauge to check the pump, although the car has run so well at times--could the pump be intermittent?

So I'll check the fuel pressure, maybe rebuild pump
observe float levels
watch the new clear fuel filter
maybe replace plug leads again
get the 022 rebuilt
finish switching to Zeniths

What else could cause the backfiring? Erratic sparking, lack of fuel? I'd be grateful to know anything that comes to mind.
Thanks--
John

[David Jones]

John, I suspect the Solex carbs have the normal Solex problem in that the bases are warped and you have a big air leak. Switch to the Zeniths and I am sure the problem will go away. Get the solex carbs rebuilt.

[C J Murray]

What else could cause the backfiring?

Badly worn camshaft.

[Paul Ahnell]

John: I don't see any mention of a leak-down test. Do it. Last year I had similar symptoms with good compression readings but popcorn noises and backfiring from the exhaust. I was convince it was a burned exhaust valve on #1. Did the leak-down and immediately ID'd the leaking intake valve on #2 when the compressed air blew out the carb. Pulled the head and found years of carbon build up on the pistons and valves. See Pix. Took heads to my local race-prep machine shop where they found seats and guides good but a few valves out of spec and a few close, along with tons of crud. I replaced all the valves along with the old, cracked pushrod tubes, gaskets, etc. and cleaned up the pistons. Buttoned it up and all is good now. Recommend the simple leak down test before you fiddle with fuel pump, etc. Good luck.

[Norm Miller]

Here's a long shot I've seen only once.
The combustion is leaking around the plug heli-coil and due to a crack has entered the intake port.
Possibly a valve seat also.

[Craig Richter]

Leak-down test was always my go-to. Not very sophisticated, just hooked each cylinder up to the old air compressor and listened for air leaks. But when drag racing or hill climbing, certain problems are not very likely (like flat camshafts). I just wanted to verify if any exhaust valves were bent or pistons broken from over-revving. Still a very easy and informative test for street motor problems.

[John Berners]

Hello, Thanks for all these ideas--I really appreciate all the experience and generosity with your knowledge.
I borrowed a compressor from my neighbor and am getting the adapter I need. The Zeniths are on for now. I'll keep you posted!
John

[C J Murray]

John, are you saying that the backfires happen with both Solex and Zenith carbs?

[John Berners]

Hi C J,
No, only with the Solexes. The Zeniths haven't been run since 1991! My current project is to get the Zeniths driving. I was hoping that would fix the backfiring, but there are so many other possible causes that I am not counting on it now.

Today I found it's not so easy to get a supply of compressed air quiet enough to hear to do the leakdown test! The setup I have here makes so much noise leaking from its own fittings I can't hear if there is leakage from the valves...
Thanks!
John

[John Berners]

That is to say, the Zeniths are now sitting on top of the manifolds for the first time in years, but they haven't been run and the car is nowhere near ready to try starting. If it were to "pass" the leakdown test, can I run it for a bit with all the shrouding off?
John

[Wes Bender]

Only for a bit...... Your cooling air isn't doing anything good for you.

[C J Murray]

You can run it for awhile without cooling if it is stationary and under no load. You can use a house fan to cool a little if you want. If the engine was in the airflow instead of in an engine compartment it wouldn't need a fan and sheetmetal at all.