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Backfiring After Shutoff...?


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Hello - last weekend I drove my 71 mach 1 to and from a "get together" (because we all like to subtly show off). However, when I got home, and shut it down, the engine shut down pretty rough and 3 seconds after it quieted down there was a loud backfire from the exhaust. Any idea where I should start? I recently installed a new Holley 670 carb.

Vacuum leak? Timing? Other? Unfortunately i only see her on the weekends so i try to do my planning before then. Any help is much appreciated. 

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Posted (edited)

Overheating and/or timing ?  ...send your car over here to the UK and I'll check it all out for you ....for a very, very, very long time :whistling:

Edited by 1sostatic
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Enjoy's searching out 71-73 history

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Sounds like you're experiencing the classic "engine run on" or "dieseling". It could be a symptom of all the things mentioned - timing, overheating and/or a vacuum leak. You could also have your idle speed set too high, fuel percolating in the bowls, poor quality fuel, too high a heat range plug,or about a dozen other things. I'd start with the carburetor, if that's all you changed. Check for vacuum leaks around the baseplate, make sure all vacuum hoses and plugs are in place and in good shape. Set the idle to factory spec and make sure the timing is where it needs to be. 

Ford had some interesting ways of reducing emissions in 71, and one involved retarding the timing to require a larger throttle opening to allow more air into the mixture at idle. This caused engine run-on, so they equipped the carburetor with a throttle position solenoid, that would snap the throttle shut when the ignition was switched off. If you aren't able to cure the issue, then the solenoid will definitely help. 

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27 minutes ago, Hemikiller said:

Sounds like you're experiencing the classic "engine run on" or "dieseling". It could be a symptom of all the things mentioned - timing, overheating and/or a vacuum leak. You could also have your idle speed set too high, fuel percolating in the bowls, poor quality fuel, too high a heat range plug,or about a dozen other things. I'd start with the carburetor, if that's all you changed. Check for vacuum leaks around the baseplate, make sure all vacuum hoses and plugs are in place and in good shape. Set the idle to factory spec and make sure the timing is where it needs to be. 

Ford had some interesting ways of reducing emissions in 71, and one involved retarding the timing to require a larger throttle opening to allow more air into the mixture at idle. This caused engine run-on, so they equipped the carburetor with a throttle position solenoid, that would snap the throttle shut when the ignition was switched off. If you aren't able to cure the issue, then the solenoid will definitely help. 

This is great information, thank you! Fortunately, I only use Premium fuel (is there an additive i should be using to improve quality?). I will update to see if any of these have improved the issue.

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I too have a Holley 670 and had several issues with it at first. Good news is, I now have it running as close to "good" as you'll get, (never perfect). 

Q; is there a strong gas smell from it after running for a while? Although Holley "say" it will run right out of the box, it won't. 

As Hemikiller says and he knows his stuff, it could be one or more things causing the backfire. Did it do this ever before changing the carb? 

Q2; do you have an insulated spacer under the carb. I found that the primary throttle plates can hang up in the bores of the (stock) intake, so a spacer will lift the carb and after making sure the plate clear the spacer bore, will prevent the throttle sticking. 

Q3; do you have an extra spring on the throttle return or just the factory throttle cable. I am assuming there are no huge mods to this engine, I didn't check if you said anything about that, so I'm assuming it's pretty much stock. At least 1 extra spring will help snap the throttle shut. I have two on mine. I also have a 1" fiber spacer that I taper bored to allow for the carb bores and the intake. (mine is a 4V M code)

One step at a time is the only way.

Geoff.

 I learn something new every day!

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The street avenger series of carbs are notorious for being problematic. Most notably rich at idle and light cruise. I improved one for a friend by converting the fixed idle channel restrictor to a changeable one by drilling and tapping the hole and using screw in restrictors that had a smaller hole. He drove the car modestly so he wasn't interested it how rich or lean it was at WOT, his choice. Just as information, excessive return spring pressure will accelerate throttle shaft bore wear. Unless it is driven a lot that is not likely a concern. Chuck

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13 hours ago, c9zx said:

 Just as information, excessive return spring pressure will accelerate throttle shaft bore wear. Unless it is driven a lot that is not likely a concern. 

 You're quite right Chuck. Excessive return spring pressure will do that. I ought to have mentioned that in my post. Thanks for mentioning that fact.

In my case, the first spring was not sufficient and nor was the second on its own, hence using two. On the upside of that, when it wares out the shaft, I can junk that carb and buy a better one!!

Geoff.

 I learn something new every day!

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That is an excellent piece of rationalization! Chuck

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