i have a 84 mustang gt with a carb'd 1993 mustang 302 motor and just installed a 750 holley with mechanical secondarys just installed a new professional products distributor. anyways when i rev it up even kind of hard it back fires through the carb and spits out of the front bowls around 2000 rpm's, also in low rpm's in runs like it has a miss in it till i step into it.. should i put the vacume on the distributor or re jet it.?. the spark plugs turn black with in a few miles of driving it, and first install with the carburator it did not do this. till a few miles and putting a new distributor in.. please let me know if you have any ideas.. need to get this thing going im out of ideas.. thanks if you want call or text me 785-418-1028 my name is jesse.....
i advanced it 15 degrees and it still backfired the same exact way. i pulled a plug and its charcoal black and this is the 2nd set i put in in the past week.. when i jet it down it runs rough when i jet it up it runs good but fowls all the plugs quick.... Very confused here. shouldnt be this hard.
I would check the power valves in the carb. With all that backfiring, I would imagine they are blown and that would cause the rich condition that is fouling your plugs. Also be sure and check the primary accelerator pump. If it has a bad diaphram or is not adjusted properly, it will cause the engine to lean out and possibly backfire under acceleration.
the carburator is brand new from star performance and i still took it all apart to check everything is correct as fas as i can see.. it did the backfiring with the old distributor and coil in so i put new stuff in and it still did it up to today. it does not backfire when the plugs are clean and breand new like first start up. but after that is when it starts up.. I took the power valves out and put the block kits in and jetted it up and still ran really rough in low rpm's and did the backfiring stuff.
You should never remove the primary power valve and richen the jets. This works fine on the secondary side, but not on the primary side. It will run way too rich at idle. Have you put a vacuum guage on the car? When you said it runs ok when you richen it to the point it fouls the plugs and runs rough when you lean it down, it made me think of a vacuum leak.
Did you changed ti spark plug wires because i had a 94 mustang doing the same thing and found out that i had some bad plug wires and the wires looked good just looking at them but was burn on the inside of the wires, and if that is not the problem then i would be checking for a vacuum leaks
i changed literally every thing besides the plug wires. they are TAYLORS 8mm pretty decent ones.. ill do that and let u know..
it all started when i was having charging issues so i got a 1 wire alternator installed and got that fixed and the next time i satrted it was doin the backfireing.. i thought maybe bad gasoline but i emptyed the tank and re filled up with 90 octane. even know the old gas looked ok in a glass jar.. ill try the new plugs and wires and see what happends
Are you running an aftermarket tachometer? If yes, try unhooking it and see what happens. I've seen this issue arise on rare occasions with a tach that goes scewball and causes misfire under load. Good luck.