31 Cabriolet backfires when started cold

[katy]

it idles poorly/very rich unless you return the GAV back to 1/4 turn when warm

Have you tried adjusting the idle jet?

[Marshall V. Daut]

Don't discount the possibility of an air leak at the intake manifold. This would create a lean situation because more air is being sucked inside the ports than can be compensated for by opening up the GAV to enrichen the fuel mixture. Often, a whistling sound can be heard if an intake is leaking, or the engine will not run smoothly until the manifold has heated up and expanded just enough to close the gap between the block, gasket and intake. I should think that a leaking intake would exhibit all the symptoms you describe.
Are you hearing a popping sound or an actual "BANG!" backfire? I'd recommend removing your manifolds and placing a METAL straight edge across the face of the ports to ensure that both manifolds are on the same plane. Use a feeler gauge between the straight edge and the manifolds to check each port. If your manifold pair was put together from two separate cars, it's possible that one or the other had been trued in the past, which will make it difficult to seal with an orphan manifold attached. Since you are changing parts anyway and presumably ordering new parts, order the good copper manifold gaskets and the gland rings. I know Ford dropped these gland rings in 1929, but only for about three months before correcting this mistake and re-installing them at the factory. I don't understand why some guys are so reluctant to use these very inexpensive and easily-installed parts that will effectively eliminate leakage. If your exhaust manifold is not drooping in the back - as many do - and you can insert the gland rings so that the manifold bolt up evenly to the block, you will have eliminated a source of air intake and exhaust leaks.
If the test carbs you are using check out o.k. on other cars, but not yours, you can probably rule out carburetion. If the new short-proof condenser doesn't solve the backfiring, the problem is probably not in the distributor. If the coil checks out o.k with a hot spark jumping from the coil wire to a head stud nut, the problem is not with the coil. After these obvious culprits have been eliminated from the list of suspects, I think you're left with an intake manifold leak. This is, of course, assuming the parts you installed for testing are functioning properly and/or rebuilt correctly. As one poster wrote, there is a reason for this condition and it will usually turn out to be something simple.
By the way, have you run a compression check to see that no valves are leaking or burned?
Marshall

[29Phaeton]

Today I was able to borrow a "known good" caburetor and sure enough....the backfire was gone and I had MUCH more power! I was so sure my regular carb was clean so this really puzzled me. So, I opened both carbs to compare everything and could see a difference in the jet sizes immediately. Even though I had used a fresh rebuild kit on my carb, the jets included with the kit were about half the flow of what was needed. To test this even further I switched the jets and sure enough my carb ran great. Lesson learned, don't trust carb kits even when they come from a known parts supplier. I found a place online called Renner's Corner who sells jet kits and they flow test every jet they sell. What a lesson this has been...lots of money spent on other parts, rebuilding the carb was the first thing I did. But, it runs beautifully now!

Thank you all for the helpful advice!

[700rpm]

Originally Posted by 700rpm

It's in your distributor.

Haha. I Got that one wrong!

[Mitch//pa]

Renners corner as a source for quality flow tested jets is discussed here often...

http://fordbarn.com/forum/search.php?searchid=8475778