OT intermittent exhaust side backfire V8

[greg g]

So on my Studie, after it starts and runs for a few minutes (fairly quick start then decent idle) it begins to intemittently backfire out the exhaust. this seem to come from both cylinder banks, in a random cycle. It will idle nice and smooth, the spit once, once or twice or two or three time, then settle into a smooth idle again.

Changing the timing by rotating the dist has no effect. I am thinking it may be exhaust valves hanging up, but it would seem tis would be a cold symptom and go away as the engine warms, but its seems to be the opposite.

Not there when cold, appearing when warmed. It runs smoothly off idle and does not seem to missfire at mid level rpms.

When I last did a compression check, all cylinders were at 110 lb. + or - 5 lbs

dry. Thought this was good so I did not do a wet test.

Thought maybe spark plug wires crossing or leaking but at night running shows no telltail sparks.

Can't find my vacuum gauge but there doesn't seem to be any audible vacuums leaks, and all ports are accouonted for.

So is this one of those electrical problems that will be fixed by fiddling with the carb, or one of those fuel problems that will require electical parts??

Just replaced the electric fuel pump with a 7lb max pressure job.

[james curl]

Do you have an automatic choke on the carburetor and is it operating properly? If it starts after it warms up I might suspect that it is too lean at idle. Once you get the accelerator pump in action and the transfer ports flowing then it is rich enough to not back fire. I was taught that most backfiring is caused by a too lean condition which might be wrong, but usually a little restriction in the air flow, choke, stops the problem. Your idle passage might have some build up of gum or varnish reducing the amount of fuel delivered or you might have a vacuum leak that opens with heat. I once owned an Audi 100LS with a Solex Carburetor that had a problem similar to what you are describing and I finally cleaned the parts real good in Carburetor cleaner then put them in the oven at 200 degrees and discovered a series of hairline cracks that ran through the transfer port passages causing the siphon to stop when they opened to atmosphere. I covered the whole area with Devcon epoxy and solved the problem long enough to sell the car.

Edited July 30, 2012 by james curl

[Plymouthy Adams]

dig around for the vacuum gauge..if sticking valves it ought to show on the gauge..(gauge is probably in the trunk of the bz cp) do you have any exhaust leaks near and just ahead of the muffler..have mums hold a rag up to the pipes when running so to see if back pressure will indicate a leak/hole..

[greg g]

carb (carter AFB) was cleaned and rebuilt last year before I installed it. I did put an aluminum spacer atop the intake as the butterlies on this carb, (swap meet special)would hit the manifold. Both sides of the plate have gaskets. I guess I will need to pop the air cleaner next time I fiddle with it. Tomorrow is supposed to be cooler and more conducive to garage work than today.

And Tim was nearly correct, the vacuum gauge was in my traveling kit behind the front seat of the coupe.

[Don Coatney]

Do you have any exhaust leaks?

[Niel Hoback]

Take a look at the spark plugs. It may just be an intermittant miss. If you have Champions, toss them anyway.

[greg g]

new AC's. No exhausts leaks.

Edited July 31, 2012 by greg g

[james49ply]

intermittent missfire, compression test good, I had same problem with my flathead, worn valve guides, ran good, but had the miss at idle, but compression was all good.