Hi folks,
I thought I would see if y'all could help point me in the right direction. First alittle background.

Truck: 71 F-100 with a 302 2bbl out of a 71 mustang. It has later model heads not sure of the year 80's. Pertronix unit installed new coil, cap and rotor. it has headers with 3 inch pipe and flowmaster 40's on it.

Problem: I have been fighting this problem for awhile now. was not there right after or before I put the Pertronix unit in. Truck starts and idles fine. Drive to work 20 miles when I put the pedal down it starts cutting out and sputtering. It backfires under the hood (thinking thru the carb). No smoke. Now it has started something new. It just shuts off. No warning...just dead. It will start right back up but with the same problems as above. Did find that the starter was about to fall off and I retightend that but I wouldn't think it would have anything to do with the above.

Any help would be great. I will probably try new wires, new carb and this was a odd suggestion but a new ignition switch.

Thanks
Dave

 

Put a timing light on it first chance you get, but first do a visual looking for this:

Any sign of the ignition box getting too warm. (When they overheat, they usually blow their guts out the bottom)
Any corrosion or dirt on any of the connections to or from the ignition box or coil.
Any sign of leaking around the carbuerettor bowl.
Any sign of bad vacuum lines especially the larger ones, such as to the brake booster unit.
Check the distributor shaft for side-play.
Ensure the distributor timing adjustment bolt is locked down securely.

It is possible that the carb is flooding, the timing is off, there might be a major vacuum leak, or the ignition is intermittent. Any of these will cause back-firing.

Since the most recent change is electronic (ignition) I would be inclined to look there, but since it's a carburated setup the float system can't be overlooked. Fortunately, the ignition is not that hard to verify.

As I think about it, the info that after it warms up to running temp it begins to act up sounds more like a carburettor float going out (or needle and seat) than anything, but check the electronics first. You don't want to rip into a carb until you know your ignition is right first.

 

I feel good about it not being a vacuum leak. I checked out those. It has no timing marks to be able to check with a light which sucks. No fuel leaks that I can find. Whats messing me up is it runs so good for about 20-30 min then it goes down hill. would a carb problem cause it to just die with no warning at all. that is making me think its electrical. I'm not ruling out the fact that it could be a combination of carb and ignition and I have two problems going on at the same time. I did check the voltage regulator to see if it was getting real hot but it was fine. No heat at all. I may just have to start changing out parts till it goes away.


Thanks
Dave

 

The reason I think carb/fuel is because it changes as it warms up from running.

- and it HAS to have timing marks, all engines have timing marks. On a 302 they are just to the P-side of the top of the lower pulley.

Buried under the alternator, naturally... But you should still be able to find them, mustang engines are pretty straight forward. Is this an engine with a serpentine belt, or an older one?

It might even be as simple as a choke plate/mixture foul up. Or, the timing is screwed period, and the temperature interrupt for the vacuum advance kills it when it opens up.

Hard to say without seeing it. Where are you located? Do you think it will make it to Atoka/Drummonds?

I bet I can root out the trouble if you can get it here, maybe this weekend or something.

 

Timing chain, as it warms the chain to stretch, allowing the timing to advance.
I'll tell you, 3" pipes are too large, you're killing the low end on that engine, my god, the displacement on that engine requires no more than 2 1/4".

 

It has to be more direct than that!

It's tied to the carb. There's no way the chain stretches that much during warm-up.

 

I should have been more clear on the "no timing marks" it has the mark on the balancer just no marker on the housing. Its an older motor correct year for the truck (71 mustang motor). I think I'll go ahead and change the carb out. I think its one of those cheapie ones from autozone. I was wanting to change it over to a 4bbl anyway maybe put a performer manifold on it. As far as the 3'' pipe I thought it was a bit much myself but the boys at advance muffler set it up. I have always had good outcomes with them and it seems to run out fine. I'm pretty sure it would make it to Atoka. I'm here in Bartlett. Although it was realing going down hill the last time I drove it, so much so that I just limp'ed it home and got in my other truck to finish my errands. I'll toy with it this week and see if I can narrow it down some. I'll post my results.


Thanks
Dave

 

Exhaust is far to big, I had a big block and only ran 2 1/2", I currently have a 351M with upgrades, and only went with 2 1/2", I'd like the cash to get it done with new 2 1/4". Those boys at advance mufflers are ignorant to that engines needs, if you have a stock 302 with 3" pipes as in dual, built dragsters run that. I'd definately upgrade to an sftermarket intake and 4 barrel, it'll bring that engine alive, but it that's an Autolite 2 barrel currently on it, I can't see it being the problem. If it's a Holley 2 barrel, yep that would be a problem, I've had 6 and have seen none of them function properly. Get yourself a timing pointer for the engine, they bolt on under one of the bolts to the timing cover, may need to remove the harmonic balancer to intall it.

 

Yaaaa! My truck seems to no longer be angry. I replaced the accel pump diaphram and have been driving the tar out of it for the last couple of days and NO cutting out. Amazing how a $2 part can solve a big problem.

Thanks for all the help on this one.

 

(!?!!...)

Definition: "FLAT SPOT"
A condition in which increased throttle is met with insufficient fuel to maintain engine operation. Common causes are vacuum leakage, improper float level, incorrect timing, a timing chain that is very stretched or has skipped one or more gear teeth, or a problem with the accelerator pump circuit.

(From the never yet published book: "Mechanicspeak")

I wish you'd said flat spot at the beginning - but I knew it sounded like the carb...