backfire?

[old Tom]

After motor is running and you shut it off it makes a loud pop or backfire. Have not had the car long and am working on getting things right. Was wondering what was causing this. Thank you

[steve s]

Probably due to a leak at the connection between exhaust pipe and manifold. Fresh air drawn into exhaust system on cool-down mixes with unburned fuel in muffler forming an explosive mixture if conditions are right. Lots of possible contributing factors, including out-of-tune motor--especially ignition timing--and incorrect carb setup can result in too much unburned fuel and too hot exhaust system.

[cjkohnast]

I had a 1931 that did that. It is a hot piece of carbon in one of the cylinders. It sets off some of the remaining gas. Was never a problem.

[WHN]

I have seen that happen in a number of cars over the years.

Not every time they were shut off but offen enough that you notice.

If the car is otherwise running good, I would not be overly concerned. Could be carbon buildup. Also check your timing. If you shut off engine with the spark fully retarted, does it still back fire? If not, than timing has something to do with it.

You will normally hear a leaking manifold.

[100IH]

Running too rich creates un-burned fuel in exhaust track, port, exh. manifold and/or muffler & tailpipe. Let in fresh air and it can ignite if hot enough. Other factor is lean misfire. Engine can not burning fuel efficiently in a given cylinder and the un-burned fuel is available to ignite under the right conditions as above.

[100IH]

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Also, running too lean can cause a misfire on a single cylinder while the rest fire, giving a dose of fuel to the exhaust.

[antiquepa]

Go with Steve, post #2, had the problem with my roadster, you can get a sleeve from snyder that go in between the man/exhaust pipe, took care of my problem and was a cheap fix.

[steve s]

Originally Posted by antiquepa

Go with Steve, post #2, had the problem with my roadster, you can get a sleeve from snyder that go in between the man/exhaust pipe, took care of my problem and was a cheap fix.

Actually, what I'd recommend is, first, go with a new Aries muffler so you're sure you're dealing with a correct fitting flange on the exhaust pipe, and, second, use a bead of muffler sealant around the flange/manifold contact area, and, third, use a good repro exhaust clamp (not an earlier, inferior fitting clamp).

Some have alleged that the sleeves are bad because they reduce the flow diameter for the hot exhaust, but I have no personal knowledge or experience to support this. However, being that iron is not cheap, it would be surprising if Ford had made those parts larger than good engineering indicated was necessary.

[old Tom]

Many thanks to all who replied. I ran it Sunday and it popped again. It aso blew the manifold gasket up by the front. and the exhaust pipe gets red hot just below the clamp to manifold. We are going to check manifold for straightness and get new gaskets.

[steve s]

Something's wrong, and it sounds like the timing is retarded. That's the number one way to make the exhaust system glow (and to warp the manifold). Do you advance the timing (pull lever down) before driving? Make sure you have correct timing pin cover for Model A; the later models fit and look okay, but the timing pin hole is incorrectly located for Model A.

[Benson]

Since you say the car is new to you.

1. Question are you retarding the spark on startup? If not starter can be damaged if it backfires while cranking with starter. With advanced spark it is much more likely to backfire.

2. After engine starts are you moving spark lever (left one) to 50% while driving? If not engine will over heat big time. Causing exhaust manifold to get very hot and glow red.

3. Since you have to remove exhaust manifold to fix blown gasket; look at the joint where muffler fastens to manifold since you say that it glows red hot in this area below joint.

Remove insert inside the pipe if installed. These inserts restrict exhaust gases and cause very high temperatures in this area of the pipe.

This restriction makes manifold run much hotter than it should and if spark is retarded it will run even hotter. As mentioned above the muffler manifold joint can leak if a cheap muffler is used.

The flanges on these repros many times are the wrong shape and will not fit.

On the clamps some times folks install them upside down and they do not fit.

It is possible to install the clamps with one half upside down again it will not fit correctly.

While you have manifold removed look at the four studs that hold it on. It is a common practice use vise grips on studs to install them.

This damages the threads and when you try to tighten the nuts they bind up on the damaged threads and the manifold NEVER gets tight enough to hold the gasket tight.

Since it is not tight the gasket can blow out. If damaged threads then a die is needed to restore threads or replace studs. If you are lucky the studs will not break when removing them.

[100IH]

If the gasket blew out it might not have been tightly sandwiched between the block and head because of exhaust manifold being warped. This could also be the air leak into the manifold and causing a lean burning #1 cylinder.

[Benson]

Cheap REPRO mufflers are not the same as Ford mufflers.

If you take time to read Aires muffler info you will see the difference.

http://www.amuffler.com/

From website, this only part of the info available:

1. The mounting flange to the exhaust manifold requires two operations, as the size of the flange is such that the material must be annealed after the first pass to avoid rupturing the metal. Other mufflers on the market only make one pass and the resulting flange is not large enough to form an effective seal. It also vibrates loose and leaks profusely. There are many fixes to this problem, but the real solution is to make the flange to specifications.

2. Our tubing is "mandrill bent." When tubing is bent, it must be collapsed slightly on the inside of the bend, in order to be forced around the bend. This greatly reduces the cross-sectional area and restricts the gas flow. Ford mandrill-bent all its muffler tail and inlet pipes; we do the same. It requires taking the conventionally bent tubing sections and placing them into a die of the correct diameter and radius. While restricted in the die on all sides, round steel balls are forced through the inside of the tubing, bringing it back to a full round shape by brute force. This is a very slow process and greatly adds to the cost of making these pipes. Today this process is only used for precision tubing applications such as aircraft and medical requirements.

[Benson]

Back firing:

If there are any rust holes in muffler as muffler cools then vacuum inside muffler will suck air inside and backfire if any unburned gasoline is present and a glowing piece of carbon or ??.

[steve s]

Originally Posted by Benson

Back firing:

If there are any rust holes in muffler as muffler cools then vacuum inside muffler will suck air inside and backfire if any unburned gasoline is present and a glowing piece of carbon or ??.

I'm not questioning that this is true, but I'm always struck by how I wouldn't have thought it would work that way. Since the muffler is open to the atmosphere at the tail pipe, I wouldn't have thought it possible to have enough of a vacuum in there to matter. I guess it must not take very much fresh air coming in at the hot end to do the job!

I still would bet that this guy's problem would go away if he just corrected the overheated exhaust pipe issue, which I also would still bet involves just the simple fix of setting the timing and using the spark advance lever correctly.

[Benson]

[QUOTE=steve s;1610794]I'm not questioning that this is true, but I'm always struck by how I wouldn't have thought it would work that way. Since the muffler is open to the atmosphere at the tail pipe, I wouldn't have thought it possible to have enough of a vacuum in there to matter. I guess it must not take very much fresh air coming in at the hot end to do the job![/

Never mind ...

Too much coffee !!! I see what you mean.

Keurig was out of Decaf !!!

A guy that I knew in the Army always said there was no such thing as a vacuum ...

The reason that he was sure of this is: Vietnam sucks!

[steve s]

[QUOTE=Benson;1610798]

Originally Posted by steve s

I'm not questioning that this is true, but I'm always struck by how I wouldn't have thought it would work that way. Since the muffler is open to the atmosphere at the tail pipe, I wouldn't have thought it possible to have enough of a vacuum in there to matter. I guess it must not take very much fresh air coming in at the hot end to do the job![/

Never mind ...

Too much coffee !!! I see what you mean.

Keurig was out of Decaf !!!

A guy that I knew in the Army always said there was no such thing as a vacuum ...

The reason that he was sure of this is: Vietnam sucks!

No, I'm serious. I do believe you .. and the people who say a leaky connection at the clamp can result in backfire. In the small amount of time it would take for the cool-down vacuum to relax through the entire system, a slight but real partial vacuum can exist at the top of the system, which must be sufficient to pull in only very little air, which will suffice to set off a flame right at the leak site, which then can spread explosively.

[Benson]

Steve,

I wonder if it is related to the difference in temperature?

Seems that the part closest to engine would be higher temperature and air would rapidly cool towards the end of tailpipe?

Also richer mixture in muffler might be closer to engine? So if rust hole was close to engine it might introduce enough air.

Like my 95 year old father used to say the last few years ... Too G.D. much technology!!

[steve s]

Originally Posted by Benson

Steve,

I wonder if it is related to the difference in temperature?

Seems that the part closest to engine would be higher temperature and air would rapidly cool towards the end of tailpipe?

Also richer mixture in muffler might be closer to engine? So if rust hole was close to engine it might introduce enough air.

Like my 95 year old father used to say the last few years ... Too G.D. much technology!!

The tapered shape of the original muffler is supposed to compensate for the decreased volume and pressure as the exhaust gas cools. I suppose if one were using a repro straight cylindrical muffler, there would be less pressure toward its outlet. But that's all for steady-state conditions with the motor running; the original question was about after shutdown.

I don't know it for a fact, but my guess is that the backfire process is initiated up near the hottest, front end of the system, and then travels down. I had a backfire blow my muffler apart once, so I have no doubt that at least a good part of the action occurs there.