Backfire thru the Carb

[Tom in SW VA]

My 28 Model A roadster is backfiring through the carburetor. Yesterday it was so loud that my wife heard it 200 yds away. It seems to happen when I retard the spark and when I retard it to start the car. Timing or what??? Any suggestions??? Please advise. Thanks,

[Tom Wesenberg]

I haven't heard loud backfiring through the carb, have I had a lot of loud backfires through my exhaust when my ignition switch was acting up. The switch contacts didn't make a good firm contact and were hit and miss for a few weeks, until I located the problem. I would barely touch the key and the engine would turn off.

[BRENT in 10-uh-C]

Tom, you may need to check the points to see if there is any corrosion. Also make sure nothing electrical is rubbing inside the distributor.

[Mitch//pa]

backfiring through the carb or popping with the timing retarded can be attributed to a lean mixture. open the GAV some and see if that helps any. of course also make sure your base settings are all correct, points, timing etc. i also have had those wireless lower plates, bad lower plate wire, bad condenser cause that also. there may not be a single answer for this so take it one step at a time

[SJCormier]

I've seen a stuck valve cause quite a magnificent backfire through the carb on a buddy's 29 pickup. Of course it had also been sitting for nearly 15 years.

[Tom in SW VA]

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ppt

[tbirdtbird]

open the GAV more. It never hurts to play with the GAV, that is why it is there

[Mitch//pa]

good idea...
i guess the responses were not good enough it was brought back to the top

[SJCormier]

Probably would'nt hurt to check the ignition timing either. I have seen timing that was set too retarded cause very similar symptoms.

[Mitch//pa]

another good idea

[Tom Foster]

My old aircraft tech (retired AF crew chief) taught me that a true BACK-fire through the carburetor is ALWAYS caused by a lean mixture. Anything else is -- something else. A downstream explosion (out the tailpipe) can sound like a backfire but is not.
We had a fellow club member who was coming home from a parade in his '29 Special Coupe when a faulty condenser caused it to "backfire" several times -- just as he was passing a highway patrolman who was giving a ticket on the side of the road. We always teased him afterwards that he did it on purpose..
Fearless

[newshirt]

Have you pulled the top distributor plate and looked at the connecting wire? It could be loose or intermittently grounding to the case when you move the spark lever.

An intermittent distributor connection can cause a misfire which allows unspent fuel to accumulate in cylinders. That extra fuel could cause a backfire.

[JonC]

Lean fuel mixture or tight intake valve is generaly the cause of intake backfires so says the Indian and Harley manuals I was brought up on, hey they are all flatheads I would check the mixture and intake valves.

[Tom in SW VA]

I brought it to the top to gain more ideas. The more input the better, I always say!!

[tbirdtbird]

did anyone mention the GAV?

[Mitch//pa]

nobody mentioned the GAV, running lean, timing off, condensor or a dist plate wire issue...
i think along with the free box of depends we should give out reading glasses

[BRENT in 10-uh-C]

Originally Posted by Tom Foster

My old aircraft tech (retired AF crew chief) taught me that a true BACK-fire through the carburetor is ALWAYS caused by a lean mixture. Anything else is -- something else.

Hummmmmm, .....So a faulty intake valve is NEVER a possibility?



Tom, why don't you load it up and trailer it down so we can look at it together. .....oh wait, your trailer is already at my place!! . .

[newshirt]

Originally Posted by Mitch//pa

nobody mentioned the GAV, running lean, timing off, condensor or a dist plate wire issue...
i think along with the free box of depends we should give out reading glasses

Sorry, Mitch. I was one of the dummies that typed before adequetly reading your's and Brent's posts. I'm not sure why. I guess I just glossed over them. That happens to me too. Sometimes I wonder if I'm invisible. And thanks for the offer, but no Depends required.

[H. L. Chauvin]

Per the sincere, old AF Tech Crew remark in reply #11:

"Anything else is ....... something else."

Sounds like a very wise "Keep Looking" Model A comment to add to every mysterious Model A problem one can imagine.

And the "something else" appears to always be a big Model A "surprise" later found at the end .......... which causes one to stop looking ..... or something like that ...... happens very often with our Model A's. LOL

[Tom in SW VA]

[QUOTE=newshirt;1034851]Have you pulled the top distributor plate and looked at the connecting wire? It could be loose or intermittently grounding to the case when you move the spark lever.

An intermittent distributor connection can cause a misfire which allows unspent fuel to accumulate in cylinders. That extra fuel could cause a backfire.[/QUOTE

I don't have the pigtail on this car. I have the conversion upper plate. .