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1930 Model A backfire and stalling on deceleration


MBittner
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Hello everyone,

 

I have a Model A that has started to stall when coming to a stop from a cruising speed. This seems to only happen after driving for several minutes with a warm engine. There is also an occasional loud backfire out of the tail pipe between shifts, or on decel. The car otherwise runs perfect. I am running a Tilotson carburetor which is now brand new. This problem also occurred with the old Tilotson carburetor which was replaced due to the housing being severely warped. Has anyone else experienced this issue before, and if so what did you find?

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So you are saying you are having this trouble with 2 different carbs..

Sounds like to much of a coincidence to me.

Ethier it is not a carb issue or your carb set up is out of wack and YOUR  duplicating the problem second time around.

Check your float level and your running mixture again for possibly to LEAN if you thinking must be carb.

 

Leaky intake gaskets can have some popping and back fire cause but not stalling.

 

 

 

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At this point I don't think it's a carb issue. The new Tillotson is brand new, and it runs a lot better than the old one. The GAV was stripped out on the old carb. What is a normal setting for the GAV? I was running about half turn open at 45mph when it stalled coming to a stop.

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My experience is that pop on deceleration is almost always ignition related. It's not really a backfire; better referred to as an exhaust explosion, when unburned fuel in the exhaust lights off due to a momentary interruption in ignition and then ignites when spark is restored. An aggravation more than anything with worst case being blowing the exhaust system apart.

 

Go thru your distributor and check for bushing wear and overall condition of both primary and secondary ignition circuits.

 

This is a good excuse to pull the distributor and have it set up on a Sun distributor machine, or to be really authentic a Ford Rotunda (Allen Electric) distributor machine. There are people who have them.

 

And I wish I was one of them!☺️

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Yes it could be other distributor related issues but, with a couple of decades of Model A Ford ownership, I can say that is usually the classic symptom of a failing condenser. The condenser is a quick easy inexpensive item to change and will likely resolve the issue.  

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If your car accelerates ok and only backfires on deceleration I would think the condenser is ok.

The gav setting differs on most A's.  Give it a 3/4 turn to the left and see what happens. 

That is easy to check. If it does not help then change the condenser.

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3 hours ago, JACK M said:

Does it have points?

I had something like this happen once and the points were almost shut.

Yes, it has points, but someone before me upgraded them to the newer style. My next steps are to check the points and timing. I actually replaced the condenser about two years ago just because I heard it was problematic. I will swap it with the old one and see if there is any change.

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I read above your going 45 mph down the road  and the engine stalls (quits) and you stop...With or with out backfiring or stumbling, but like out of gas I bet.?

Then what? .It starts right away or sits a short spell and then starts and you go again for a while?

 

If so ,I'd lean toward a large piece of shxt is in the fuel tank following the gas draw swirling down to the bottom of the tank and blocking or slowing the feed to the carb bowl..

The car sits a spell ,movement of the car fooling around, gas gets by or moves the blockage enough so gas slowly fills the carb.

You get going again for a while and the blocking bit follows the swirl(like an unseen current) again and settles down to block off the fuel outlet at the bottom of the tank again  Repeat and repeat...

 Take off the carb gas line check for flow ( note you may only get blockage and motor stall with fuel trying to flow )and blow back through gas line into the tank 

after the car comes to hault and see it the car starts right up sooner.?

 

We've seen 3 model A's in the past with old cork bits in the tank do this.

One fixed is the use of one of the tubular filter screens you put up in the bottom of the tank with the shut off valve.

Other fix..pull the cowl tank.

And make sure you install the tank filler screen/fume arrester.

 

Take note: deceleration with  closed throttle (such as going down a hill and using engine braking ),a slight poping and spitting out the carburetor is considered very  normal and generally shows(back in the day) your mixure is just about right and on the " lean" side for economy ,which is good, but only if overall performance doesn't suffer.

 

Todays fuel may require a bit of a richer mixture on some A's.

Edited by Flivverking (see edit history)
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  • 5 weeks later...

Hello all,

 

I wanted to give an update on this post in case anyone else runs into the same issue. I appear to have solved the problem. The backfiring was caused by a rich condition. Basically when the engine warmed up it was running way too rich causing fuel to build up in the exhaust and detonate on deceleration. This also caused the stalling. The way this all started was that my Tillotson carburetor had a lot of leaks due to warpage. It was severely warped between the two housing halves, and at the manifold. So it had fuel and vacuum leaks. I replaced it with a brand new Tillotson. The reason the new carb didn't fix the issue, and made it worse, is because I set the base line mixture settings on the new carburetor based on the old carburetor settings. Whoever set the mixtures on the old carburetor had set them rich to compensate for the vacuum leaks. Once I realized this, I leaned out the idle mixture on the new carburetor by about a 1 1/4 turn, and the issue cleared right up. A backfire out of the tail pipe is NOT caused by a lean condition, but a rich condition. I hope this helps.

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Hard to believe you went through all this and didn't read your plugs early along the way.

Oh well ,you got it fixed and learned something of value too.

"A"+ is your grade!

Lets stay out of trouble for at least a year.😌

 

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