31 Cabriolet backfires when started cold

29Phaeton

29Phaeton

Member
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?
 
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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
 
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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
 
Just,an after thought ! The difference between you and no start is your condensor is not totally blow yet
 
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.
 
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.
 
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.
 
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.
 
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.
 
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.;)
 
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.
 
Anytime you have a when cold only drivability problem it's a fuel mixture problem. My guess is your too lean.
 
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.
 
29Phaeton said:
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.
Click to expand...

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.
 
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! :mad:
 

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