F4 YF Backfiring through carb

Vacuum leak?

Otherwise, I'd say bad/dirty points, possibly, although you have checked the dwell. Did you clean the new ones of factory residue? That has caught me before.

I might say an accelerator pump issue, (e.g. leaky discharge valve will pump less fuel) but it seems to persist longer than that would...

Correct metering rod? (There are several variants as I'm sure you already know.)

Water contamination in the carb?

 

"New distributor/points/condensor/cap/coil/wires/plugs."

One of my rules of thumb is to suspect the newest parts first...

 

My thought is the mechanical advance weights are not coming in with proper amount at the proper time.

 

Too much advance too soon? That might fit the symptoms.

 

Is it backfire or is it really spitting fuel. I can't see any flame in the video (but could be). Maybe fuel pressure too high. Just another thought...

 

"when I finished the engine rebuild about 5 years ago"

Five years.... have you checked for squirrels?

 

That carburetor cough when you first open the throttle is likely the accelerator pump. Some crud in the line is getting into the pump area and restricting the flow maybe. Try running some SeaFoam through the carb. I do it by disconnecting the intake line to the fuel pump and putting a flexible line into a can of Seafoam and running the engine, sucking up the can full.

 

O.K., if there is flame involved, something is igniting the fuel mixture in the intake manifold, and it is usually an indication that somehow a combustion event is getting back into the intake. Only a couple ways to do this AFAIK------stuck/chipped/leaking intake valve, or a cylinder is firing while the valve is still open during its normal motion.

I have seen a burned through heat riser in the intake manifold do this--------but that was on a Chevy V8-------not sure if your manifold even has the heat riser passage.

Have you done a compression check ? Or better yet, a leak down check??

 

Howard, I have to agree that it should be happening all the time, but something has to be igniting the mixture in the manifold.

Perhaps the ignition source is there continuously, but you are only giving it a air/fuel ratio that will light off when you open the throttle????

 

Old trick I learned many years ago, take off the air cleaner, run the engine at 1500 or so RPM, and slowly pour a cup or two of water into the carb---------dont let the engine die, pour the water slowly.

This does a good job of cleaning out the combustion chamber of excess carbon buildup.

 

That should completely shut it down. Sounds like a gasket is blocking a hole some-where but you said you swapped carbs with nearly the same results?

Ref: post #12.

 

Get a '0' gauge needle, like maybe from an epipen (bee sting kit for people allergic to nuts or bee-stings), mount it in a vacuum line with the point towards the carb/intake manifold. the other end of the line goes into a small jug of water. The needle will act like a sprayer for a very fine stream of water, which will steam clean the valves and combustion chamber. Busts up the carbon build-up. Might sound weird, but military aircraft used something similar during WWII for power boosting during combat. And in F-head engines, with the carb mounted directly to the head the way it is, that system acts like a heat riser in a chevy or dodge engine.

 

Didn't / doesn't GM make a product called "top engine cleaner" which you fed into the intake similarly to choke out the engine and let it sit, then when fired back up, the soot would exit the building so to speak...

I used it before as a dump in tank efi injector cleaner on my 90 ranger 4L v6 when it started running rough once, worked like a charm.

ive seen reference it's the same as seafoam, can't say inter web rumors are true. See other comments about Lucas fuel treatment, have no experience there...