31 Cabriolet backfires when started cold

[29Phaeton]

Newbie here so I apologize if this is basic. I start my car and it starts right up, but unless I pull the choke about 2/3 out it starts to backfire through the carb when I try to give it gas. My GAV is about 3/4 turn. After about 5 minutes I can push the choke rod all the way in, move the GAV to about 1/4 turn and it starts to accelerate better (not always without a pop here and ther). I know most cars need some choke when cold, so is this backfire normal when starting an A when cold and without choke? So far I have rebuilt the Zenith, new fuel line (and verified good flow), new distributor, cap and plugs (all gapped to specs), adjusted the timing which seems spot on and just replaced the manifold gasket. When she's warm the idle is good with a miss here and there but sounds decent. I'm not running with a thermostat and my car is still 6 volt.

I should also point out I removed the plugs and cranked the engine by hand to verify the valves are not stuck. They all opened and closed as they should.

Do you start your A partially choked until she warms up? If not, does yours backfire through the carb?

[Mitch//pa]

The first thing you should do is find out why you have a miss here and there. It should not do that and may be contributing to your cold issue.
That being said a properly tuned and correct running car does not need to be choked during warm up. The gav should be opened more for that initial Cold start and warm up. I usually open it about 1 to 1/1/4 turns from closed.
All your basics and settings need to be correct as well as the internals of the carb. I always install a set of flow tested jets from renners corner on carb rebuilds

[Steve Plucker]

I had a back fireing problem once...changed out the condenser and that solved the problem.

Pluck

[jm29henry]

I agree with Steve I had the same problem as you . I took,the carb and switched it with the one on my other car still same problem .The other carb ran great on the working car.so I was convinced that after trying everything I know about fuel and air and vacuum I had a blown head gasket !!! It wasn't .as a whim Amd alast resort before pulling head I changed condensor I was amazed it is still running perfect today !!! A couple of bucks and a few minutes what do,you have to lose

[jm29henry]

Just,an after thought ! The difference between you and no start is your condensor is not totally blow yet

[jbond]

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MY car will backfire or pop pop when cold UNLESS I richen the mixture with the GAV. After a few minutes of running I can back the GAV down and it runs fine until the next cold start. A cold start for me in Florida is an outside temp of 50 degrees or colder.

[Tom Wesenberg]

Stock, well used engine with 50 LBS. compression, I open the GAV 1 to 1 1/2 turns for cold starts, then pull the choke for a couple compressions, then release it. As soon as the engine starts I pull the spark half way down, and may also have to pull the choke half way for a second or two, then it stays running smoothly. Once warmed up I can close the GAV and don't need to use the choke for restarts. My engine runs about the same when warm, whether the GAV is 1/4 turn open or closed.

[29Phaeton]

Thanks. The new style distributor included the condenser so I'm sure that's been replaced. I haven't replaced the coil and mine looks really old so I may try that.

[Kevin in NJ]

Do not assume a new condensor is good.

The new condensers are made by low bid contracts. Over at the Mustang site there have been complaints that new name brand condensers have failed.

Your best bet is to buy the A&L burn out proof condenser.

[29Phaeton]

I have everything back together. Even after a carb rebuild including having the channels professionally blasted clean the car seems to run lean. If I have the GAV open between 1/8 to 1/4 the car idles perfectly. But, when I step on the accelerator it starts to feel like it's under powered and backfires through the carb. If I open the GAV 1 to 1.5 turns the backfire goes away and the engine revs higher but you can tell it's running way too rich at idle and almost stalls until I reduce the GAV back to 1/4 turn. I've done everything from a new head gasket to all new electrical components under the hood. Gaps are verified...just seems to run lean. Overall the engine sounds really good. I did verify there are no manifold/intake leaks.

After two months of trying to figure this out I'm thinking an ad on Craigslist may be my next step as I'm not getting anywhere.

[Synchro909]

Don't lose faith - it will be something simple. I bet when you find the gremlin, you'll give yourself a palm print on your forehead.

[mhsprecher]

You should find someone with one of the local clubs who can help you out. Do you belong to one of the club chapters? Where in So Cal? You may have someone on here who is near you who is willing to give you a hand.

[Mitch//pa]

Post#8. New style distributor...

New style does not mean the best style

GAV should be open cold 1-1.5 when cold

Then reduced to 1/8th or 1/4 range

Yes i agree you need someone to troubleshoot your issue instead of replacing parts.. troubleshooting this can take 30 seconds to 15 minutes...

Did u ever read the vehicle operators manual??


http://www.motormayhem.net/wp-upload...ion-Manual.pdf


Keep us posted ,, dont give up

[Tom Wesenberg]

Hold the coil wire near a head nut and let us know what color and how far the spark will jump.

[RonC]

Anytime you have a when cold only drivability problem it's a fuel mixture problem. My guess is your too lean.

[Farrell In Vancouver]

Please check and make sure when you turn the knob on the G.A.V. that the rod is actually turning as well. There are some repop types that are just unthreading when you turn them. Your choke rod should be able to snap the choke plate back open when released as well.

[George Miller]

Originally Posted by 29Phaeton

I have everything back together. Even after a carb rebuild including having the channels professionally blasted clean the car seems to run lean. If I have the GAV open between 1/8 to 1/4 the car idles perfectly. But, when I step on the accelerator it starts to feel like it's under powered and backfires through the carb. If I open the GAV 1 to 1.5 turns the backfire goes away and the engine revs higher but you can tell it's running way too rich at idle and almost stalls until I reduce the GAV back to 1/4 turn. I've done everything from a new head gasket to all new electrical components under the hood. Gaps are verified...just seems to run lean. Overall the engine sounds really good. I did verify there are no manifold/intake leaks.

After two months of trying to figure this out I'm thinking an ad on Craigslist may be my next step as I'm not getting anywhere.

My guess weak spark. It takes a hotter spark to fire a lean mixture. Just because we replace a part does not mean it is good. If you have a so called modern lower plate it might have a bad connection. Also a lot of the new condensers are junk.

[29Phaeton]

Thanks all. I've been to my local group meeting and most suggest the same as online and much of this I have tried. I will test the spark color and distance tomorrow and post the results. This issue is consistent when the engine is hot or cold...doesn't matter, it will backfire if the GAV isn't open at least a full turn but then it idles poorly/very rich unless you return the GAV back to 1/4 turn when warm. After that she idles beautifully.

Frustrating!

[George Miller]

You might also want to check all electrical connections. Like coil, inside dist, junction box, dash, generator, starter, battery.

[700rpm]

It's in your distributor.

[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