I finally got me 75 F-150 back together and running fairly well except for the fact that if you really get on the gas from idle it will backfire through the carb. now the only scenario i can think of where u might do this is in a panic trying to move the vehicle out of the way of something but it made me wonder, is this normal behavior of a carburated vehicle to have a lean condition if the throttle is opened completely rather quickly from idle.

the motor is a 1975 390, all stock except for a rv style cam and a performer intake, the carb is a autolite 4100 pulled off of a 67 tbird and rebuilt by a friend. 59 jets in front, 64s in back, 6.5" power valve. the motor is warming up to 180+ measured at top of rad after driving for 20 minutes or so.

other than this one "issue" it runs and drives fine and doesn't stutter under normal driving conditions.

the accelerator pump in carb is functioning and is delivering the largest shot at this point, no play in linkage. and the float level has also been adjusted. motor pulls a steady 15 pounds vacuum. no leaks.


thx for any help or info.

 

Backfiring or a stumble points either to a vacuum leak or a timing issue. Reset the timing to stock spec and re-test. But your cam might be a tooth off and that means pulling it apart.

Backfiring through the carb means that an intake valve is open when the air-fuel charge is ignited.

 

Wiped exhaust cam lobe or lobes are something else that will cause a popping back through the carb as throttle is opened and held open.

 

A lean condition from a vacuum leak ended up being the cause on mine. I was pulling air through where the throttle shafts enter the carb body....just something that happens on old carbs where the bushings wear out.


Get a can of carb cleaner and start targeted spraying around vac fittings, the carb base, and the throttle shafts while idling, and listen for changes in idle.

 

well, the condition is only if you stab it off idle, and considering the motor is newly rebuilt and doesn't have issues under any other circumstance i highly doubt it being a cam or timing chain issue. maybe timing related though. i currently have base set at 10degrees and im running whatever the replacement duraspark distributor was set at as far as mechanical and vacuum advance. vacuum advance is on ported vacuum on side of carb

 

that was the exact condition my truck was falling on its face and sputtering out the carb. See my "Stumble" Thread on this page.

 

Lean condition will do it, ignition timing will do it, and cam can do it, I have seen one case where tight valves did something like this, intake valve not seating, firing order will do it, a bad dist. cap will do it, probably some other things too.

 

I just lived this dream a few weeks ago after dropping 390 back in 76 F250. Do yourself a favor and start with the little easy things first, I was thinking doom and gloom also with mine.... Heres where I would start.....


Coil/ignition and Coil wire ? Good juice, fresh wire, connection ?


Dizzy Cap, is it 30 years old, replace ?


Wires, if your running $11.99 Pepboys specials, you may never get the plug boot to fit correctly on the cap or the plugs.......


Dizzy, is it the old one, did you replace with a crap cheapy during your build ? put the old one back in and see if that works.....


And of coarse all the timing checks, with your dizzy marked up with your marks on the harmonic balancer etc......


Honestly seems like everyone jumps on the carb for the answers and it usually ends up being something simple and stupid........


Good luck, BTW my fire thru the carb was a combination of junk cheap plug wires and the coil ........ Replaced both and it fired up like purring kitten......I ended putting a quality cap on the old Dizzy and she ran like a dream even before I did the final timing............Good luck

 

I saw a statistic recently; 90% of carb problems are in the distributor!!

 

My FE at 12° BTDC had a stumble upon hard acceleration and it was popping out the exhaust on downshifts and deceleration..... I reset the timing to 6° BTDC (stock spec) and the issues went away.

 

Mine ran like that. I fixed exhaust manifold leaks, fixed vacuum advance in distributor, replaced all vacuum hoses including one to master, put a new carb manifold gasket in, and finally rebuilt the carb. Every step made it run a little better, but it took all of those to finally make it run good. Sometimes it's a combination of things and it's a PITA to figure out!

 

Well, finally got the issue sorted out. It was a issue with a bad thermostat not letting the engine get warm. Put in a stant 195 and no issues.


Sometimes it's the dumb issues.

 

Thermostat? Wow, I would not have thought of that... learn something new every day!

 

Yeah, wouldn't have expected that. Just an observation, 15 pounds of vac isn't very good for a "newly rebuilt" motor. You could probably use some carb/timing tuning.

 

While i tend to agree, even with different carbs and adjusting base timing and tuning i have never been able to break 15". The engine is a late model 390 and I'm also not sure if the timing set is retarded or straight up. Regardless the vacuum that it does pull is steady and the vehicle has no driveability issues whatsoever so i can't imagine why it would be so low.