I have a 1985 Dodge Ram 350 pick-up truck with a 4 speed transmission and the muffler explodes when I let up on the gas while slowing down. What is causing that to happen and how do I fix it.

What I believe may be happening is too much of a fuel-air mixture getting into the muffler, and when it gets hot enough, it ignites, causing the "exploding" your talking about.

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Originally Posted by duckydo

I have a 1985 Dodge Ram 350 pick-up truck with a 4 speed transmission and the muffler explodes when I let up on the gas while slowing down. What is causing that to happen and how do I fix it.

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To the best of my knowledge some where there is raw gas getting into the exaust system.
You either have a overflow in the carburator or the EGR valve is stuck open.
Either way unburned gas or fumes oaregetting in the exaust and when you let off and then it ignites and blows the time out of your muffler.
Like when I was a young driver I would switch the key off for a few seconds then switch it back on and make on heck of a backfire noise.
You may have a dead cylinder that is not firing and exausting raw gas into the exaust also.
Check for a miss, some are hard to detect so you will have to check the compression, should be 115 to 135 .
Check each spark plug wire GOOD, pull each wire off while running to see if you have any change in the running, careful you don`t get bit!
Run at night with no light to see if your have a wire that is arching to ground, just raise the hood and rev it up a little and just look at the wires to see if you see any blue arcs.
If you doo relace the whole set asnd get some good wires.
Hope this helps,
Joe

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Originally Posted by duckydo

I have a 1985 Dodge Ram 350 pick-up truck with a 4 speed transmission and the muffler explodes when I let up on the gas while slowing down. What is causing that to happen and how do I fix it.

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I agree with helpO5
Too much fuel is being allowed to move, unburnt, through the engine, and when it hits the next hot spot, usually the cat or muffler, KA-BOOM.
Find out why. The carb might be dumping... float level or leaky gasket...
Check the timing and plugs, etc. If not, the exh may split one day and leave you with "Hollywood Headers"