Hi folks,

I just picked up an '85 F250 4x4 with the 460 engine, it's carbureted and appears 100% stock. 96k on the clock to my knowledge (slight possibility it could be 196k I suppose, though the truck doesn't really show it) and 4 speed T19 transmission.

Overall the truck runs satisfactory. Has decent power, though I have no parallels to compare it to, as this is my first carbureted big block. Doesn't have the power of fuel injected 460's I remember driving, but it pulls this big boat around pretty hastily. Has no driveability issues, when I floor it, it responds with no hesitation, and part-throttle cruising is as you would expect.

The only real issue I have with it, is an exhaust backfire that seems to happen every time I shut it down after reaching operating temp. It won't do it cold. Couple times have been gunfire-bang loud, but for the most part it's a rather soft "pfffooot" for lack of a better descriptor. Once the engine dieseled for a second or two (sounded like it got the engine to spin backwards), but 95% of the time, the backfire is what I get.

Any ideas what could be causing this? I don't know the truck's maintenance history, but I intend to change out all fluids and normal routine maintenance stuff right off. The truck was bought & remained in Colorado Springs nearly its entire life, so I surmise it's jetted for high altitude (~ 6,500 feet). I brought the truck to a lower elevation, around 4,800 feet, thought I'd mention that if it might contribute to the problem.

Thanks in advance for any help or suggestions.

 

Sounds like the idle speed is too high. That will cause run-on or dieseling, and it can also make the engine take too long to stop when the ignition is turned off. This makes the engine draw fuel through the carb, but it's not burning it. This raw fuel enters the hot exhaust manifold and creates the bang you hear when the raw fuel lights off in the exhaust.

People who want to be obnoxious and don't care if they blow their mufflers off, will be driving down the road in gear, turn the key off right in the middle of everything, and then turn the key back on. That will give you a loud bang for the same reasons.