Hey everyone,

Long time no type! I've got a 76 F250 2wd Supercab, 460 with a C6 auto trans...

I bought it in WA and had the carb rebuilt (had all kinds of sediment in the bottom)... now I'm in AZ and having a few problems with it. First it started to backfire on me a little. Now it's backfiring just about everytime I step on the peddle a little too much for the truck's liking (not that I have a lead foot, though)... I mean, it's really sensitive now... I have to baby my way up to highway speed. Then even if I'm going good, when I reach an incline that requires a little more peddle to maintain speed, it backfires.

I've also noted that, lately, it feels like it's lurching... like it's getting gas, then not (but not so much to make it stall), etc. A mechanic here said the timing might be off, and that the difference in elevation (600' to 7000') may have effected the rebuilt carb. He suggested changing the fuel filter.

I changed the inline fuel filter on top of the engine, but I realized I've got another one on the bottom. Since the first change didn't make any difference, I wanted to get your opinions and/or expertise on whether it's worth changing out the other fuel filter and/or what else I might need to do.

Thanks for the help!
Jeanne

 

Sounds like it could be running too lean.

 

Oh, that's another thing the mechanic was nice enough to adjust for me (no charge)... he reset the air and fuel screws on the carb. What he did was close them both down, then turned them both two full rotations. It didn't change a thing with the backfiring.

By the way, for the ametuer mechanic, please explain why running lean would cause it to backfire. I thought it might be getting too much gas (periodically), hence overcombusting into a backfire.??? That's why I changed the fuel filter... I figured it was plugged with junk and not being able to get the correct fuel mixture into the carb. Am I way off base?

Thanks!

 

I would look at the vacuum advance. Seems to me that could cause it to backfire and run crappy when you step on it. You can do the check with a timing light by watching the beam as you step on the gas.

 

Thanks! I'll check out the vacuum advance.

 

I am having the exact same problem. I did replace both fuel filters
and it did not help at all. I thought that maybe the fuel pump might be going bad so I took the rubber line off the filter at the carb and put it in a empty pop bottle and turned the engine over.
The pump was definately pumping gas but when I looked at the gas it had a bunch o black debris and what appeared to be oil in it. I checked the oil and it was about a quart and a half low. Now I know fuel pumps of that type can put gas in the oil but I wandering it could put oil in the gas. I will probably replace the pump to see if that is the problem. If you come up with a solution please post it and I will do the same.

 

TRY CHECKING COMPRESSION,

 

If timing is correct, I'd look for a broken valve spring or a camshaft going flat.




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Bobby

 

Could just have a bad spark plug in it. Mine did that. Been tuned up lately?
Jimmy

 

Okay, this is what I've found out...

It's the timing. However, I didn't figure that out with a timing light... since I don't have access to one right now. I took a trip to a lower elevation for a week.... NOT A SINGLE backfire the whole time! Don't want to mess with the timing right now, since I'm going to be spending more time at lower elevations this winter.

Thanks for all your help and advice!