Backfire

Forker

Forker

Senior Member
Took a drive the other day and went about 3 or 4 miles when I pulled in to gas up...as I shut her down she gave out a loud backfire, which surprised me as she doesn't normally backfire at all. Drove another 30 miles making a few stops along the way, yet not another backfire the whole time.

Does anyone know why it would backfired the first time I stopped but not again over the next few hours and stops and how to avoid it in the future?
Thanks
george
 
A few things-

What was the engine RPM when you shut off the ignition? If it was running fast, it can backfire upon ignition shutoff.

Where was the GAV set? Was it running really rich or lean?

You didn't mention if it was running well before first shutdown. Also didn't mention how old the gas was before adding new gas.
 
If your RPMs was a little high when you shut her down it could backfire .

Try coming to a full stop and let the idle settle and retard the spark before shut down .
 
Unfortunately, the answer to your questions that I can honestly answer is that the gas was fresh and not old, wasn't paying attention enough to make a guess. It seemed to be running just fine.
 
What about idle speed? Does your car idle smoothly or stumble around like a hot rod with a big cam in it (indicating a vacuum leak problem)?

If this helps, the normal quick "run in to store" style stop shutdown routine should be-

Park car, set parking brake.
Retard spark (lever all the way up).
Let engine idle for several seconds.
Shut off motor.

You should have no trouble with this unless your fuel mixture is way off or the engine idle speed is too fast.

My Tudor with a stock motor and original babbit idles in the 250 RPM range and is still very, very smooth. That's admittedly low compared to some other model As, and very low compared to a modern car. Most Model As (in my limited experience) seem to idle in the 500 RPM range. If you set your idle speed based on what you're used to in a modern car, then it's too high.
 
Back in from the garage; based on the suggestions I think the problem might have been GAV in that it was turned clockwise all the way, so far in fact that it was stuck and I had to spend a little time lubing it to get it loose and to be able to adjust it at all...it now turns freely. Not sure how it happened to get stuck like that, but in thinking back I didn't adjust the GAV at all last trip out. Hopefully when I take her on a long drive this weekend she'll run even a little better.
How do most run with GAV set, I've heard full clockwise and backed off a half turn is recommended? I suspect that is something you just have find the optimum for each vehicle for by trial and error and feel?
Thanks
george
 
Thanks PS, that's a nice simple check list!

Park car, set parking brake.
Retard spark (lever all the way up).
Let engine idle for several seconds.
Shut off motor.
Click to expand...
 
The GAV setting is going to vary. Some of the barners here are experts on this, and some THINK they're experts, so beware when asking an open question like that.

On my car, the GAV about 1/4 turn open is best, but your car could be different. I have learned that your spark plugs tell you what your GAV setting should be. Sooty plugs mean the mixture is too rich (GAV open too far), white, pitted plugs tell you the mixture is too lean (GAV closed). Experimentation and frequent spark plug cleaning will tell you where to set yours.

The size and flow of the carb jets and many other factors determine if your carb runs rich or lean. There's no "one size fits all" answer. If someone tells you there is, then they fall into the category of those who "think" they're experts.
 
I guess there is nothing like a muffler that close to the exhaust manifold. Nice little hot container to hold a little raw gas on shutdown. <g>
 
Character is a good thing for sure...its just that I worry about giving people a heart attack, you should have seen the elderly lady gassing up next to the pump I pulled into when it blew...she almost ended up on the ground covering her head. As she was calming herself down she suggested that "I really needed to get that fixed" :eek:
george
 
All the A's and T's seem to have a "sweet spot" on the GAV, and they're all different. Mine vary from a little over a 1/2 turn open to almost fully closed, and everything in between. PS's checklist makes sense, and is pretty much what I do. Never had a backfire in one of my cars, did in one of my brother-in-laws though - made our WWII veteran uncle dive for the ditch. He had been in Patton's 3rd Army rolling across France.
 
Whenever mine does it a $4 condenser does the trick. Very troublesome just before a 4some tees off.
 
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