Backfires when turned off.

1959 - 1978, M151, M151A1, M151A2, Technical questions and discussions, regarding anything related to the M151.
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Gunbunny198
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Backfires when turned off.

Post by Gunbunny198 » Thu May 13, 2021 2:03 pm

Hello all. Recently after taking my 151 out of it's winter slumber there seems to be a back fire that has developed. Over the winter the only maintenance done to it is a new wiring harness and alt conversion. I have read a couple different threads on here about other back fire situations but it appears mine only occurs once the engine is turned off. It has a slight sputter back fire if I down shift and allow the transmission to slow the vehicle down but a small tap of the gas and that one goes away. The one that concerns me is when I turn the engine off. Ran fine all last year with the exception of a small vacuum leak. Have not moved the distributor any and made sure all intake and exhaust bolts were tight from the beginning.


Rickf
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Re: Backfires when turned off.

Post by Rickf » Thu May 13, 2021 5:19 pm

A backfire at shutdown is 99% of the time raw gas igniting in the hot exhaust due to a leak in the exhaust, most often at the donut between the manifold and pipe.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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W. Winget
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Re: Backfires when turned off.

Post by W. Winget » Thu May 13, 2021 5:38 pm

I was going to offer that there may be some sticking in the valves allowing gas mix to cross flow out into the exhaust, and perhaps some Marvel Mystery Oil added to the gas tank to add a little valve lube since she's likely running some older gasoline as well.
Just a thought.
V/R W Winget
PS hand with rag over the exhaust tailpipe while idling held by a friend would help you hear an exhaust leak if you had.
Looking for 1918 Standard B 'Liberty' truck parts


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