back firing

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i have a 73 mustang with a 351c. Nothing fancy about the motor..elderbrok carb,stock distributor with a pertronic igniter.newer heads ,new plugs wires. while idling I can rev it up and it sounds good but when I drive it down the road it wants to back fire and cut out some. how about a list of things to check? Seems ive checked most but maybe im missing something that's obvious but my brain is failing to remember....been awhile since tearing into a ford. thanks in advance!

 

[Don C]

Sounds like it's too lean. Could be a vacuum leak or gummed up carburetor.

I would also check timing, spark plugs and plug wires. If none of that works, I'd put points back in to make sure it's not the Pertronix.

 

[barnett468]

in addition to the above, disconnect the vacuum advance until the car runs perfectly.

try plug wires from another car if you have some that will fit.

if a plug is black or the electrode is wasted or the gap is too big, it can cause probs.

 

[rocket366]

In addition to the above...

Flat Cam Perhaps?

Paul

 

[Jeff73Mach1]

Has it ever run right since you owned it? If it ran good, and you put it away a few years and now it doesn't. it might be something as simple as a vacuum leak from a bad hose or sucking oil because of a plugged pcv valve. Check all wires for good connections at both ends. You may need to rebuild your carb and check your fuel filter, pump and fuel itself. Ethanol can do nasty stuff. That would be a generally easy and inexpensive fixs and would make sense.

Electrical is also suggested by the idle being good and load causing it to fall off- a weak spark can come from many issues, but plug wires and plugs are the primary culprits in my experience. Coils fail less than we think and caps and rotors are relatively easy to inspect.

If it has been driven without problem until the last few thousand miles, then mechanical issues like cam wear, oil control problems, physical damage are going to warrant consideration and possible investigation. etc It is far less likely this is your problem,.

 

[jbojo]

+1 on the ethanol

Just had to rebuild a carb because the needle valve for the floats were stuck in the closed position and not getting gas properly. The fuel bowl was varnished and passages blocked too. Also a electric fuel pump in the tank is seized in another. These were in ATV's that sat over the winter. This was not in a car but the same thing can happen to a cars carb.

 

[VegasFastback]

I second checking for vacuums leaks. 90% of the time that my cars are running like crap its a vacuum leak.

 

[Bill73Ragtop]

"If none of that works, I'd put points back in to make sure it's not the Pertronix."

In my experience the Pertronix either works or does not. If the air gap is not set properly, that can create problems, but that's an initial set up thing.

 

[mycoses]

my experience w/ back firing was always a timing issue. indicates ignition of gas at the wrong time. set your timing and watch the timing mark as you slowly rev up the engine. it should move evenly around the balancer. if timing mark is jumping around you have a timing problem. pertronix or maybe a worn dist gear. a valve sticking open can cause back firing.