Backfire

Lkitchens

Lkitchens

Junior Member
1930 model, timing is set great and car runs wonderful for about 1/2 mile, then the backfiring begins. Had this issue before taking to mechanic and they have installed new pistons. But car is still doing the same thing, runs great and then goes bad after a short drive.
 
Intermittent ignition circuit or coil as well. Easy thing first though. I hope it needed new pistons. This would not be a common thing to cause intermittent issues. Temperature related items are usually in the ignition circuit but sticky valves can also be intermittent.
 
I'd be looking at the electrical side of things. A dodgey aftermarket ignition switch is a common cause of trouble, as is the coil and especially the condenser. While your timing is right, you say, I'd also check for the infamous #3/4 crossfire and other places where spark might be tracking to places it shouldn't.
Having said all that, remember that half of electrical problems are in the carburettor!
 
Usually takes a little farther than a half mile but if your gas cap is plugged you might be creating a vacuum in the gas tank and starving for fuel.
Drive with the gas cap off to see if that solves it.

Rubbing block on your points may have worn down and now the point gap is too small.
Check gap and re-set if necessary.
A few swipes with some fine sandpaper across the point surfaces before setting them would be good.

Spray some starting fluid on all of the manifold gaskets while engine is idling to see if you have a change in engine speed. If you do you have a leaking gasket.

When working through this do one thing at a time and test drive. That way you will know what the fix was.
 
I had a similar problem with a club members car and it turned out to be a combination of things but primarily it was the coil, after it got warm it got very very weak. I have had a lot of people tell me that a coil works or it doesn’t. Well that ain’t true. Another problem I experienced was that over a bit of distance (say, maybe, a half mile or so LoL) the float bowl level would drop to the point that the engine wouldn’t die it would just spit and backfire (thereby using less fuel and keeping just enough fuel for the engine to continue to run). I’m sure that you know all the places to look for restrictions in the fuel line (disconnect the fuel one at the carb and open the valve (with a can under it of course) at let the fuel run out for some time. If it does that ok then check the carb inlet screen). It might help to confirm what carb you have. I assume a Zenith but about 1/3rd of the As I see have a Tillotson (and a few Marvels).
 
thanks for the feedback
we have found a lot of rust inside the fuel tank that is inhibiting fuel flow
we are working to rectify that issue
will keep you posted
 
Check the fuel line where it enters the carburetor it may be pushed in too far and restricting fuel also condenser and coil will cause this.
 
Are you running a Tilotson carb? The pot metal used in the Tillys tends to warp. Heat exacerbates the warping. You may be opening up a gap at the mating surface between the carb and intake manifold as the car heats up, leaning out the mixture. An overly lean mixture will cause a backfire through the carb.
 
Every Tillotson I ever touched was warped enough at the mating surface to cause a problem, every one. It comes in part from people over tightening the nuts/bolts. And then when it leak’s, they tighten it more. That’s why I asked what carb he had.
 
jeepguy1948 said:
Every Tillotson I ever touched was warped enough at the mating surface to cause a problem, every one. It comes in part from people over tightening the nuts/bolts. And then when it leak’s, they tighten it more. That’s why I asked what carb he had.
Click to expand...

Zenith CAN suffer from the over tightening too.
 
Paul Shinn, the model A guy, has a series of how to videos on YouTube. Great resource. These have helped me work through similar issues. What body style 1930?
 
Cleaned the fuel tank and it was running great. Drove to the gas station and filled it up, drove away and suddenly lost all fire. Took the fuel line loose at the carburetor and flowing great. I don’t think it’s a fuel issue this time, I’m not getting fire. Think the coil went out? Can’t get any fire trying to crank it.
 
I had a hard starting problem just last month. The spark was weak or intermittent. Occasionally it would back fire. I purchased a new coil, the "flame thrower" by Petronix. The spark seemed better but still had difficulty starting sometimes. I am using the new upper plate by Nurex and have the condenser mounted on the firewall. I did have 6v right up to the points with the ignition ON with a piece of paper between the points. So new coil, new condenser with cleaned points and 6V available at the points. What is the problem?? On a hunch I opened the fuse and checked the continuity across just the closed points. There was a high resistance or open circuit across the points. The points were definitely off of the cam and closed but no continuity. This was the problem but why? I found that when the block on the points wear, the points can be ever so slightly held apart by the mounting screw opposite the contact end of the points. As I was adjusting the point gap, I could see that the mounting screw was moving into the edge of the elongated hole on the end of the points which was binding up the movement of the points. I took a Dremel Tool and ground the edge of the elongated hole on the points to give a freer movement for the point adjustment. I rechecked the continuity across the points and now the resistance was zero ohms. Problem solved.

I think the tapped hole in the upper plate for securing the points is slightly off because it doesn't take to many miles of driving to wear the points block and create interference during the adjustment of the point gap. Lkitchens, have you checked the resistance across your points when they are closed? Ed
 
This slide shows the issue in Post #9.
 

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A condenser and a coil often will start to break down once internal heat builds up with use and will also cause the same misfiring failed ignition spark.
 
Hello, known electric problems are , condenser, ignition switch, both repro and original, if not rebuilt, the wire inside the cable will short out , repro points that don’t line up and block wears quickly. I had a 1937 Ford that had running problems, finally traced it to a weak coil , and over charging generator, it would run well when cold but started cutting out when coil got warm, took most of the summer to figure it out.Good luck, at least easier to change coil on Model A than a V8!
 
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