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Discussion Starter · #1 ·
I have a 85 305 H.O. Chevy. When it is cold or has been sitting overnight or a long period of time the engine backfires down out the exhaust. Took the exhaust off. Now running open headers and I see blue flames every now and then coming out the headers so far just the drivers side. I Also have light glowing on 7&8 exhaust tubes. I put a laser heat reading gun on the tubes (Drive side 1&7 run about 320 Degrees 3&5 run about 410 Degrees) (Pass side 2&8 run about 410 Degrees and 4&6 run about 320 Degrees) Not sure if this is normal or not. But I have done the timing and its good. Fires right up but backfires real back, now after it warms up its seems to go away, but with a little flutter happing. Now can anyone tell me what maybe is going wrong? Is it Burnt Valves or is the Carb out and need rebuilt or somthing else? Thanks!

P.S. Just put new spark plugs and wires on it. All new MSD set up.
 

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Technical Support Barry Grant
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1,500 Posts
Backfire out of exhaust

As a rule of thumb a backfire out of the exhaust would be a rich condition. Especailly if you're seeing flames out of the headers. If this only happens after the vehicle sits for a while there is a good chance the carburetor is leaking down into the engine causing this situation. A lean backfire is normally going to back up through the intake. An easy way to check to see what it is would be to shut the vehicle off, and drain the fuel out of your carburetor's float bowls. Then let the car sit overnight. If it is a leaking down problem there shouldn't be a problem since there wasn't any fuel in the carburetor to leak into the engine.
 

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85Montess,

Since the exhaust backfire occurs cold and goes away for the most part when warmed up it sounds like the exhaust valves are sticky. They stay open a bit in the guide. Seen a number of vehicles do this over the years.
 

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machinest & A&P Mechanic
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363 Posts
the correct terms

well in A&P school i was always taught this:

a backfire is through the intake....
an afterfire is out the exhaust...

makes sense don't it? if people used the right terms we would not need to say out of the exhaust, or intake :thumbup:

also an afterfire is the result of a rich mixture, and a backfire a lean mixture...

-Leo-
 
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