exhaust backfire

L

larry harding

Senior Member
i,ve been working on a friends 29 4 door leather back sedan. new (modern) points and condenser, new kit in carb, checked and re checked the ign, timing.
everything checks good and it runs as good as can be expected, engine is probably a bit tired, the engine cranks and runs, but when i let off the throttle i get a lot of muffler backfire, i am running good spark advance. i have a 31 pickup and a speedster i built neither one backfires as much as 29. it can be really loud at times. any ideas?
 
Sounds like you have a smaller version of Chitty Chitty Bang Bang! You have touched on the items I would have looked at, you might want to check the GAV? Just throwing it out there to see if it sticks.

Good Luck!
 
Exhaust backfire happens when unburnt fuel inters into the exhaust system and ignites . You could have sticky exhaust valves, 4 oz of MMO added to 10 gallons of gas could help . A friend was having issues and asked on forums about this issue, he got answers all over from carburetor ignition exhaust leak , knowing his A doesn’t get driven much I suggested the MMO in the gas and his issue when away .
 
Just had this issue; timing was way off, GAV needle and idle mixture screw worn, removed air cleaner, etc. Set up the engine per original and the Service Bulletin procedures. If stock head, set points and plugs to specs.
 
The most common cause an interrupted ignition circuit that allows fuel mix into the exhaust before the intermittent ignition comes back to life. As was mentioned, a sticky valve or valves can aid in igniting any fuel mix that escapes a normal burn. Worn or leaky valves can also be a contributing cause.

I knew a kid that had an old worn out Ford Pinto that he loved to switch the ignition off and then back on to get a good blast from what was left of his exhaust system. Some folks just like to be noticed even if it puts them in a bad light.
 
I don't disagree with what everyone has said. However, I had a similar situation where the engine started, idled and drove fine. Engine was timed correctly, points set correctly, idle air screw set normally, idle set to 450 rpm. The engine would backfire through the exhaust if I quickly closed the throttle at speed. If I gradually let off the throttle it would not backfire. Examining the spark plugs revealed it was running lean so I opened up the GAV another 1/4 turn and the backfire stopped. Now the plugs look better, very light carbon around the plug base and the strap, electrode and porcelain all a light tan.
 
rotorwrench said:
I knew a kid that had an old worn out Ford Pinto that he loved to switch the ignition off and then back on to get a good blast from what was left of his exhaust system. Some folks just like to be noticed even if it puts them in a bad light.
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I used to do this with a healthy 351 Cleveland. The very last time I tried it the blast opened up the bottoms of both mufflers like cans of tuna. Very expensive lesson learned.....
 
And I frequently point out Vince Falter's thoughts on the Spark Advance.

As in the Model A has need of about HALF the advance possible through the use of the lever.

See Vince's thoughts...

https://www.fordgarage.com/pages/nurexadvance.htm

Vince has disabled the "cut/paste" option for his pages, but I suspect he won't mind a verbatim copy of his words since we're in agreement.

The stock Model A distributor was designed by Ford in 1927 to have 40 total crankshaft degrees of possible manual advance capability from initial timing position, in order that the initial spark timing could be set (retarded) at 0 crankshaft degrees (TDC) for enabling starting by hand cranking, in cold weather, and with poor fuels, all on a low speed, low compression engine. The Model A was never intended or recommended to be driven at full 40 degrees ignition advance (or at 0 degrees full retard!).
Click to expand...
With my initial pin timing, about "half way down" seems optimal for highway advance. Your start point may vary.

There is more pertinent on Vince's page above.

Joe K
 
In my high school days I had a Toyota pickup with dual side pipes on it and I would turn the ignition off Turn the ignition back on and it would make a loud backfire and shoot about a 6' flame out of both side pipes!!! My favorite place to this was while driving under overpasses!!!!! The crazy things we did in our youth!!!!
 
"...but when I let off the throttle, I get a lot of muffler backfire..." This is not normal & tweaking the GAV is a bandaid. When the accelerator closes abruptly from a hi-speed engine RPM, the Cap Jet should keep the engine running normally as the Main Jet abruptly stops. It may be that the fuel mixture is too lean because the Cap Jet is not working as it should.
A vacuum leak is a likely culprit in this case. When decelerating with the throttle going to the idle position, the manifold vacuum should abruptly go to 17-24 InHg, and the gauge needle should be steady. If the gauge needle oscillates continuously, there is a valve issue that can be as simple excessive valve stem clearance.
I have a vacuum gauge in my Model A, and I often peak at it while driving. You determine much about how your engine is performing with a vacuum gauge.
 
In 1970 driving home with my new to me A , bounced on a RR crossing it died ? Pull over and waited a bit and she started . Driving around my home town every once and a while it would die , turning the key off and then back on she would restart , sometimes with a bang ! I probably blew out an original ford muffler ( sorry fine points guys ) dad had me cut the muffler off and he gave me an obsolete straight through heavy duty truck muffler . No more joy with an occasional backfire after that for the teenager , and also the intermittent issue with the ignition switch went away ? !
 
Be sure to fully retard the spark lever before turning off the ignition. Even if there are problems with the carburetor or an incorrect GAV setting, retarding the spark will eliminate the majority of backfires. Retard the spark while the engine is running, wait a second or two and then turn off the ignition. Until you can isolate the cause of the backfiring (an abnormal condition), retarding the spark will reduce the number of embarrassing backfires that neighbors love so much.
Marshall
 
I will ask again. Larry, what type of carb are you using?

Are you using a zenith, tillotson, Marvel, Model B?
 
nkaminar, at full throttle the manifold vacuum drops to 5 InHg. Cruising at 50 MPH, the manifold vacuum is typically 15 InHg. At idle the manifold vacuum is about 23 InHg.
 

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OK, thanks. I was wondering because of the PCV system I put on my car. It it drops to zero then the PCV system would stop working. I will put my vacuum gauge on my car next time I have a chance to take it out. This is a big shop gauge and nothing I would want to keep hanging on my dash.
 
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