what causes a low fuel backfire? looking to buy a truck and owner says it had a low fuel backfire and bent the butterflies in carbs. does this sound right? thanks

 

I'm not really a mind reader but in general, back firing comes from several causes. Low pressure back firing would be from "low fuel pressusre", which is the result of a plugged fuel filter or a weak fuel pump. Neither are really a big deal.

 

Sounds fishy to me. I've never known backfire to be caused by anything other than timing issues, the spark plug firing while the valve is open. And I've never heard of backfire bending the butterflies.

Here's what Wikipedia says:

Backfiring in internal combustion engines occurs outside of the combustion chamber, and is typically the result of an improper air to fuel ratio. An overly lean air-fuel mixture (i.e. an overabundance of air) can lead to a failure to ignite in the combustion chamber, also called a "misfire". The unburnt fuel then enters the exhaust system, where hot components can cause the fuel to ignite unpredictably. Alternatively, rich air-fuel mixtures (i.e. an overabundance of fuel) can result in incomplete combustion, again causing unburnt fuel to enter the exhaust system.
Backfires may also occur before the combustion chamber. One possible cause of this is timing. If the timing is too advanced, the spark plug fires before the intake valves close, causing the combustion to propagate into the intake manifold, further igniting the air-fuel mixture there; the resulting explosion then travels out of the carburetor and air filter. On many small marine engines, a screen is placed over the intake of the carburetor as a flame arrestor, to prevent these flames from escaping the intake and potentially igniting fuel or fuel vapors in the enclosed sump or bilge of the boat, causing a fire or explosion. Alternatively, the engine timing may be retarded, in which case the combustion is not completed by the time the exhaust valves open, allowing the combustion to propagate into the exhaust system.
Additionally, improperly adjusted carburetors that create a lean condition during acceleration can cause the air fuel mixture to burn so slowly, that combustion is still taking place during the exhaust stroke, and even when the intake valve opens. The flame front can then travel up the intake and cause a backfire. In this situation it is conceivable that there is a backfire occurring in the intake manifold and exhaust manifold simultaneously.
In both cases (combustion occurring before and after the combustion chamber), the result is a sharp pop, which is colloquially referred to as a "backfire". However, for troubleshooting, engine mechanics more strictly define an ignition of fuel within the engine exhaust system as an "afterfire", while a "backfire" is this same process taking place in the induction system.

 

Might check for crossed plug wires, or a cracked dist cap.

 

Thanks for all the info, it sounds to suspicious for me and buying a truck without it running is something I don't think I am going to do.