Backfire through exhaust when shut off

[Reserector [Veteran Member]]

1977 Dodge 440 V8 with Carter Carb. Stock cast manifolds, y-pipe to single exhaust/muffler.

New plugs, wires, carb rebuild, timing chain, but problem persists:
When I shut the engine off after driving any distance, there is a moment of silence followed by what sounds like a small cannon firing. It is violent enough to destroy mufflers once in a while.

If I let it idle for 30 seconds or so, it is less likely to fire off. It may still go "POOF". I work around this by letting it idle for half a minunte, then put it in Drive and kill the engine. No poof doing that.

It has done this for years. How can I make this problem go away?

[601cherokee]

Get a new camper.

Not a clue...maybe get a FI kit?

Tried higher octane fuel? I had a 'yota that would diesel and backfire when shutting it off after it got a good bit of age on it. I could put 89+ octane in it, and it stopped.

[khoward]

im sure you have tried this but mine does it when its ideled to high and it has done it when carbon had built up on the dist. cap

[Reserector [Veteran Member]]

(3rdAugust2010 03:01 PM)601cherokee Wrote:  Get a new camper.
Are you kidding? This thing is barely 1/3 of a century old.

Not a clue...maybe get a FI kit?
Not in the budget.

Tried higher octane fuel? I had a 'yota that would diesel and backfire when shutting it off after it got a good bit of age on it. I could put 89+ octane in it, and it stopped.

I run premimum as it is because of the 10.5:1 compression ratio.

(3rdAugust2010 03:52 PM)khoward Wrote:  im sure you have tried this but mine does it when its ideled to high and it has done it when carbon had built up on the dist. cap

I,too, figure it has something to do with the high-ish idle when it is in park (not under load). Surely there is an adjustment I can make to something.

[Rangertoy]

it could be several things, timing advanced a little too high, high idle and or the carb could be loading up with fuel/running rich

[oO_Rogue_Oo]

The carb loading up was my first thought; the idle circuit. Is it a 2 barrel or a Thermoquad? What do the plugs look like when you pull them?

[Reserector [Veteran Member]]

Thermoquad.
Can't answer about the plugs. I'll have to check them next time I shut it down.

I'm planning to go over the whole rig so I can start using it again. I haven't run it in quite a while.

[oO_Rogue_Oo]

All the smog equipment still on it? Could be an exhaust leak / too much air in the exhaust (air pump).

[Reserector [Veteran Member]]

No smog stuff on this.

I can understand it pumping fuel/air mixture into the exhause as it slows and stops, but what is causing the delayed ignition? It is usually 3 to 6 seconds after the engine stops. Heat in the cast manifolds maybe? With a little carbon thrown in for good measure.

I figure that shutting it off while it is in gear greatly reduces the amount of fuel/air mix that gets pumped into the exhaust after shut down.

[redneck23ms [Veteran Member]]

does the carb have a powervalve? if so it could be bad and would cause it to dump extra fuel in the manifold which can cause backfires like that.
edit: nevermind i see its a carter. pretty sure they don't have powervalves.