I have a 69 250 cs with a 390. I have recently fixed a few problems on the fuel system but now seem to have ran into an ignition problem. At idle my truck runs good, but at a slow acceleration it surges, hesitates, and under hard acceleration backfires. The timming is set to 6 and the carb seems to be adjusted correctly. New pertronix ignitor, coil, plugs. The only thing that I have not replaced is the plug wires, gonna do that tommorow. This thing feels like a vacume leek, I have checked all around the carb and every stinking hose with brake cleaner, but no leek! Is there any obscure locations that I may have a leek? Any advise is greatly appreciated.

 

IT could be that your timing is out, i noticed that you mentioned it was set at 6 but maybe the engine is still to far advanced for the type of fuel (dishwater) that you are burning just my 0.02
Steve

 

Ok man what you need to do is calm down and accept the situation and realize that you can turn your exhaust system into a flame-thrower I had this problem at one point and you know what the problem was the alternator was hanging on my ONE BOLT!! Well that was my fix, not sure if that it would help you…if it’s the top bolt like mine was then its hard to notice unless you actually look at it. I hope you get your problem fixed. Good luck

Signed,
The Don

 

I recommend using an unlit propane torch rather than brake cleaner. Some brake cleaners and not flammable. The torch is safer. If you have a backflash, the only thing it does is light the torch, not set your truck on fire. The torch is also easier to get into tight places.
Try blowing propane onto the base of the carb, along the heads where the intake manifold joins, and the distributor vaccum advance.

 

Long Shot. My son had the same problem with his 390 while back. When he set the timing he did so without removing and plugging the vacuum advance hose. Good luck.
jor

 

I have had this problem in the past too and its not uasy to find. Mine happend to be the vacuum tree, it was tight but it needed teflon tape and that cured it. I'd also check your head gaskets and the power brake hose at the vacuum tree and at the booster and the hose itself for craks.

jeff

 

i had this happen on a dune buggy and it was just i had the plug wires on the wrong fireing order. idk if this helps

 

Backfiring out the tailpipe, means unburned fuel in the muffler. Ignition is a biggie, check voltage at coil +. It sounds like you have a weak spark all the time, idle on up. Plug wires can cause this also. Timing is also critical here, make sure mechanical and vacuum advance are working, and be careful of the Pertronix, they can move on the breaker plate if not installed correctly and will cause all kinds of issues.

JWR

 

When I installed my MSD ignition I tried 8* of advance and it sputtered and hiccupped. I think 6* is too retarded. 10 seems to be the magic number for 390's. Try advancing it in increments and see if that helps.

Dennis

 

i used to have a flamethrower non wheel. i still do but that motor is no more. it ran good when you were cruising around 80 then let of it would shoot a 3 foot flame out of the sidepipes. then i made it true duals all the way out the back and it still shot them 3 foot out on each pipe. so i see people walking i would cut it off pump the gas turn the key and boom. it sounded like 100 12 gauge shotguns going off at once to scare the crap out of people then one night i was cruising and it started smoking real bad it would not go any more and caught on fire. the pistons melted and cracked.

 

Well, I'd check the Carb, that sounds like a accelerator pump or that flat little red check flapper that stops your Accel pump shoot from going back in to your float blow, some model carbs use a ball bearing. 1970 360's also had a vaum. type dash pot on the top of the carb. and when those leaked it would do the same thing. that yours is doing....
And old ford man for 40 plus yrs
Orich

 

i switched carbs and it still did it i don't worry now the motor blew up and caught on fire

 

Hmm sorry you never found the real troubles. Well so it must have been poping back uo through the carb all a long and the tailpipe. YOU never said if it just stared after you fooled with it or after adding your pointless set-up and hot coil. IF that was the case then it may have been a leaking dist rotor cap, lose coil wire. Plug wires cross firing. Like if it really run like crape when it rained or damp weather. But if this trouble came on and got worst poping back through the carb. then, I would say you burned the EXHAUST SEATS OUT By running unleaded gas with out an additive.or enough. Some engines will keep running after turning them off. Because the seat acts like a glow plus ther so HOT..........
Orich

 

it was an 83 ext. cab with a 302. everything stock. ran perfect but used gas bad but i don't remember if it smoked black. i don't think so. i could cruise fast let off and boom. the car behind me got toasted. at night if you go maybe 10 to 20 mph you gould see a steady blue flame out of the exhaust. it was my around town truck. i got it for 375, i had to replace the flywheel which i have 3 extras. c6 auto.