Backfire through carb on acceleration

[47topless]

I am new to Model A's. My new car, 1931 Vickie. I had really bad backfire through carb. I bought and installed a new distributor, cap, body, plugs etc. and a rebuilt carb. I am "fairly" confident that I timed the engine correctly. Engine still has a backfire through carb on acceleration.
As a check, I installed a new coil but left the old one installed on the firewall. Engine starts really well but still has some backfire through carb.
A crazy thing I noticed was, what appeared to be oil, dripping down out of the old coil from the coil wire contact point.
Any suggestions on solving this backfire would be greatly appreciated.
Thank you,
Phil

[1crosscut]

Check for a leak at the manifold gaskets.
Spray starting fluid around them while at idle. If rom changes then there is a leak.
Could also be a sticky / slow valve.

[Big hammer]

4 oz of MMO in a full tank of gas, go for a drive and enjoy the drive ! you probably have a sticky intake valve staying open on the combustion stroke

[nkaminar]

Backfire through the carburetor is an indication of a lean mixture or a sticky intake valve.

The lean mixture can be caused by a manifold leak, a leak at the carburetor gasket where it mounts to the intake manifold, or a leak at the throttle rod. Or it could be caused by a partially clogged jet in the carburetor.

Sticky valves can be caused by sludge or carbon buildup on the valve stem or a carbon or other particle under the valve seat causing it to be partially open. It could also be caused by a bent valve stem. Unlikely, but if there is not valve lifter to valve stem clearance, the valve will stay partly open. When the spark plug fires it will send hot gasses out into the intake manifold to ignite the air/fuel mixture there.

The Model A coil is used in the inverted orientation. If the coil was designed to be used in the upright orientation it can be filled with a fluid that will leak out when inverted. The coils that are designed to be used inverted are filled with a solid material.

Do a compression test or leak down test to see if you have a stuck valve or one that is leaking. Use the trick Crosscut talked about to find any vacuum leaks. Clean the carburetor using carburetor cleaner and compressed air. You have to take it apart but it is usually not necessary to take all the jets out.

[Thodge]

You could try richening up the GAV setting and see if it improves. That would help isolate a lean or valve problem.

[Gene F]

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I bet the Marvel Mystery Oil does the trick

[47topless]

Originally Posted by Thodge

You could try richening up the GAV setting and see if it improves. That would help isolate a lean or valve problem.

Could you please explain?
Thank you!!!!!

[47topless]

Originally Posted by Gene F

I bet the Marvel Mystery Oil does the trick

That's what I'm hoping for.
Thank you

[nkaminar]

The GAV stands for gas adjustment valve. Like a lot of things on the Model A the air/fuel mixture is adjustable by the driver. The knob that you pull to choke the engine also turns and adjusts the GAV. Turning counter clockwise will put more gasoline into the engine while turning it clockwise will put less gasoline in. It is usually turned one turn counter clockwise to start a cold engine and kept at about 1/4 open to run the engine. However, each car is different and a little experimenting is needed. Do not turn the GAV tight against the stop (clockwise) because it can damage the needle valve and seat.

Another way to add more fuel is to slightly pull out the choke. This is a temporary measure just used to see if running lean is the problem. Try pulling out the choke about 1/4 of the way, more or less, to see if the car is running lean. You can keep it out using one or more cloth pins or a needle nose Vice Grip pliers. If pulling out the choke works but not the GAV, then that means there is a clogged jet in the carburetor.

[Thodge]

If you are usually running your gas adjusting valve around 1/4 turn open, try opening it to half a turn or even more and see if the backfiring on acceleration stays the same or lessens. As stated above, the most common causes of backfiring through the carburetor are lean mixture or an intake valve stuck open. If making your mixture more rich helps, then you’re chasing a vacuum leak or fuel delivery problem and if it doesn’t then it’s probably a valve. Does the backfiring happen right as you open up the throttle on acceleration or does it start to happen during a sustained pull?

[47topless]

This is such wonderful information. Thank you !!!!!!

[1928 Sport Coupe]

My 1928 Started backfiring through the carb after running great for an hour or two. I thought it may be running out of gas. After adding gas it still backfired and had low power. I put it in the garage to replace the steering gear with a rebuilt two tooth.
I checked the timing and points and found the point gap was less than .014. I readjusted the points to .018 and it seemed to run better, but I could not get it out on the road to test. Does this sound like a possibility? Could the condenser be bad and cause this? The distributor was rebuilt about a month ago with a new condenser installed.
Thanks

[ronn]

listen to big hammer- they sometimes gum up from sitting.

[Y-Blockhead]

Originally Posted by 47topless

A crazy thing I noticed was, what appeared to be oil, dripping down out of the old coil from the coil wire contact point.
Any suggestions on solving this backfire would be greatly appreciated.
Thank you,
Phil

I don't see where anyone addressed your coil leaking. Sounds like you have an oil filled coil. I would suggest you replace it with an epoxy fill coil.

Oil filled coils should only be mounted with the secondary wire facing up.

[nkaminar]

Y-Blockhead, re read Post #4.

[Y-Blockhead]

Originally Posted by nkaminar

Y-Blockhead, re read Post #4.

Gotcha Bud, missed it.