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Old 09-29-2010, 11:13 PM   #1
Mig29pilot
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Default Flathead "Back fire" on engine decelaration??

I have a 34 Ford Coupe with an all original flathead and a 50 Merc coupe also all original. Both seem to back fire on deceleration of the engine and this happens when the throttle is backed off when just sitting still or driving down the road. If there anything specific I can look for to find the cause? Vacuum leak maybe?
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Old 09-30-2010, 12:03 AM   #2
andyleonard
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Default Re: Flathead "Back fire" on engine decelaration??

Exhaust leak? Backfire how? Out the tailpipe or back through the carb?
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Old 09-30-2010, 02:23 AM   #3
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Default Re: Flathead "Back fire" on engine decelaration??

loose plugs.
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Old 09-30-2010, 03:19 AM   #4
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Default Re: Flathead "Back fire" on engine decelaration??

Condenser ?
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Old 09-30-2010, 06:14 AM   #5
Vic Piano
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Default Re: Flathead "Back fire" on engine decelaration??

Is it a backfire or just rapping off?
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Old 09-30-2010, 07:23 AM   #6
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Default Re: Flathead "Back fire" on engine decelaration??

Backfire in the exhaust is from an exhaust leak letting oxygen get sucked into the exhaust pipe when you let off the throttle. The oxygen then mixes with the unburned fuel mixture and will pop. The fix is seal up the exhaust system.

If it is backfiring through the carburator, never mind my response

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Old 09-30-2010, 07:50 AM   #7
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Default Re: Flathead "Back fire" on engine decelaration??

My '52 F-7 has a cracked exhaust manifold and when you decelerate, it will go for a short distance and go off like a cannon followed by a series of minor pops. If I keep a slight pressure on the pedal, it doesn't do it. Kind of scary for any pedestrians that might be in the area.
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Old 09-30-2010, 08:49 AM   #8
Mig29pilot
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Default Re: Flathead "Back fire" on engine decelaration??

Out the tail pipe
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Old 09-30-2010, 08:51 AM   #9
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Default Re: Flathead "Back fire" on engine decelaration??

The plugs are tight.
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Old 09-30-2010, 08:53 AM   #10
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Default Re: Flathead "Back fire" on engine decelaration??

The exaust system it sealed and tight
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Old 09-30-2010, 08:54 AM   #11
Mig29pilot
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Default Re: Flathead "Back fire" on engine decelaration??

Thew condenser is new.
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Old 09-30-2010, 09:25 AM   #12
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Default Re: Flathead "Back fire" on engine decelaration??

Lean condition in carburetor idle circuits will cause popping on decelleration. Open idle mixture screws a little and test drive.
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Old 09-30-2010, 09:30 AM   #13
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Default Re: Flathead "Back fire" on engine decelaration??

Nearly all the causes have been list with the exception of a cracked distributor cap. Leaky exhaust valves will sure do it as was mentioned before.

I would suggest concentrating on one car at a time though. The two vehicles may have completly different causes.
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Old 09-30-2010, 10:00 AM   #14
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Default Re: Flathead "Back fire" on engine decelaration??

Valves not sealing. Valves sticking.
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Old 09-30-2010, 10:21 AM   #15
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Default Re: Flathead "Back fire" on engine decelaration??

You only need a very small exhaust leak to get the "backfire" .

Double check you exhaust manifolds and tubes for a small leak.
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Old 09-30-2010, 10:28 AM   #16
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Default Re: Flathead "Back fire" on engine decelaration??

Sounds like one way or the other you have too much oxygen in the exhaust system. It doesn't take much of an exhaust leak - a pinhole will do it - to make it pop. For me, it would be worth another careful look before doing much work upstairs. Adding extra gas to the idle circuit might provide some intel, too, if there's no leak.

You only have single carbs on these cars, right? Unbalanced duals will do what you've got.
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Old 09-30-2010, 11:56 AM   #17
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Default Re: Flathead "Back fire" on engine decelaration??

Check the ignition timing
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Old 09-30-2010, 12:22 PM   #18
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Default Re: Flathead "Back fire" on engine decelaration??

Vacuum leak?? When I was in my late teens, my buddy had a '48 Chev with a wolf whistle(remember them?) with the whistle disabled somehow so that when he pulled the cable to make it whistle, all it did was just let vacuum in so it would backfire.
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