1930 Model A 4 cylinder Backfires on Immediate re-start

[thejaym]

Hello Everyone,

I have a 1930 Model A 4 cylinder all stock original coupe in 6 volt positive ground system and I'm having a weird issue with the engine.

Problem: When the engine has run, does not matter if 1 minute or 1 hour, if I stop the engine and want to immidiately do a re-start, it makes a huge shotgun style backfire in the exhaust system. If I waith a good minute before doing the re-start, it does not make the backfire. And when I say backfire, it's a huge huge shotgun bang to wake up the neighbor 3 houses down from mine!!! Other than that, the engine start on a 1/4 of a turn, does not hesitate or miss, pulls good with no backfiring when running. The only trouble is only at the re-start within 30 seconds after having shutdown the engine.

So far, I have done the following work:
- flushed gas tank, replace gas filter
- rebuilt carburetor
- new cap, rotor, points, condenser, spark plugs, ignition coil 6 Volt
- replace distributor, checked for wires inside distributor making sure they are not touching distributor housing or the advance timming plate
- took compression on all 4 cylinder ; 45 45 45 50, removed head and replace 4 exhaust valves not properly sitting, intake valves seem to be fine
- played with the timming either way: more or less advance.

Anyone had a similar problem ? Any help or inputs would be greatly appreciated.

Thanks!

Jay

[Smog Tech]

Jay, Is every thing on your engine stock?

[SeaSlugs]

Are you shutting off the engine after coming to a complete stop and low idling it for a few seconds? Sounds it may be a tad rich getting raw fuel in the muffler so is your carb set correctly?
Wheres your idle screw set? both levers fully up should produce a nice slow packita packita packita noise and can darn near follow the fan blade tips with your eyes. - if it sounds like a modern 4 banger your idle is way to high.

When you go to restart it are you retarding the spark fully (up)?

Do you use choke every time you restart it? - only need it the first start of the day if T shirt weather and above...

[Marshall V. Daut]

Get into the habit of fully retarding the timing lever right before you stop the engine. That will greatly reduce the possibility of a backfire. There is something wrong somewhere because a backfire as you describe is not normal for a Model A. But until you find the problem, stay on good terms with your neighbors and dog by retarding the timing lever fully each time you turn off the ignition key.
Marshall

[Jacksonlll]

This could be caused by a hot spot in the exhaust system waiting for some fuel to come its way from an over rich fuel system. But you say it happens right after a cold start??? That should rule out a hot spot. Things don't get hot that fast. Don't choke a warm engine.

[JBill]

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Very bizarre. Most backfiring happens when you shut the engine off. I'm certainly no expert, but I think that's why a lot of modern engines have a fuel solenoid that cuts off fuel flow immediately when the engine is shut down so as to avoid fuel being sucked into a hot exhaust system. Let us know what you find out.

[Farrell In Vancouver]

Check your exhaust system thoroughly for leaks too even if its rich when starting its getting enough air to reach the lower explosive limit of the fuel.

[thejaym]

Hello guy's

Thanks for all the inputs.

Ok so, to answer a few questions:
- engine is completely stock with no modifications
- I use the choke only on the first cold start.
- I notice that it does the canon shot mostly when the engine has run at least 5 minutes
- I tried starting with full advance or closed advance same result
- when I shut down the engine, I wait that the engine is slow idle before shuting it down
- I did not try to slow the advance down with the lever before shutting it down (will try it tomorrow)
- the carburetor side mixture screw is backed up 1 turn (tried it at 3/4 or 1 1/4 & 1 1/2) same result
- the mixture on the choke rod is open 1 turn (tried it more closed or open same result)
- was thinking maybe a blocked exhaust so I disconnected exhaust at manifold, same result

Will tried again tomorrow with advance lever down before shutting off engine.

Thanks again for all suggestions

Jay

[Marshall V. Daut]

No! Timing lever UP retards the spark!
Marshall

[1955cj5]

When shutting the engine down, try shutting off the fuel with the tank valve and allowing the engine to die by itself when it burns all the fuel in the carburetor bowl.

Remember to reopen the tank valve when you try a re-start..

[Tom Wesenberg]

Originally Posted by 1955cj5

When shutting the engine down, try shutting off the fuel with the tank valve and allowing the engine to die by itself when it burns all the fuel in the carburetor bowl.

Remember to reopen the tank valve when you try a re-start..

If you do this, don't forget to turn the key off, or you'll loose a coil or even the whole car is the coil catches on fire.

[Dollar Bill]

OP States: "I tried starting with full advance or closed advance same result"

Just a friendly reminder, starting with any advance could result in a fragged Bendix, or bent starter shaft. Should it kick back, that 60+ pounds of flywheel has a LOT of inertia.

Backfiring usually relates to a timing problem.

[Farrell In Vancouver]

Originally Posted by thejaym

Hello guy's

Thanks for all the inputs.

Ok so, to answer a few questions:
- engine is completely stock with no modifications
- I use the choke only on the first cold start.
- I notice that it does the canon shot mostly when the engine has run at least 5 minutes
- I tried starting with full advance or closed advance same result
- when I shut down the engine, I wait that the engine is slow idle before shuting it down
- I did not try to slow the advance down with the lever before shutting it down (will try it tomorrow)
- the carburetor side mixture screw is backed up 1 turn (tried it at 3/4 or 1 1/4 & 1 1/2) same result
- the mixture on the choke rod is open 1 turn (tried it more closed or open same result)
- was thinking maybe a blocked exhaust so I disconnected exhaust at manifold, same result

Will tried again tomorrow with advance lever down before shutting off engine.

Thanks again for all suggestions

Jay

Are you saying it still blows off at the exhaust manifold with the muffler disconnected?