Backfire on acceleration

[dfrazz]

My 440 has developed a pretty good backfire on acceleration from a stop.  I have noticed I have an exhaust leak since pulling the drivers side headers to replace the steering box.  It idles fine, even revving the engine in park, no backfire.  But accelerating from a stop, just easy on throttle, and I get 2-3 backfires and then if I continue or increase throttle I get big backfire that sometimes make the engine cut out and I back off the throttle completely.  I installed a Firecore RTR distributor, coil, wires and a Proform 750 last fall but it did not have any issues until recently, and the steering box is the only thing I changed.

Can an exhaust leak cause backfiring on acceleration?  Or should I look at something else.  Thanks for you help guys! 

[cudabob496]

most always timing

[YellowThumper]

No on the exhaust leak.
Timing most likely. Since you changed the plug wires I would check to confirm they are all in the correct order. If two are swapped that are close in firing order engine will still run and can even idle reasonably good but would cause backfires when you start pushing it.
Been there many years ago...
Also, I suggest checking how your timing advances in new distributor. Might be good at idle but advances too soon.

Or, the dreaded burned out valve. If all else fails check your compression.

Good luck

[Chryco Psycho]

Well there is 2 types of back fire , 1 into the carb & 2 into the exhaust , exhuast leaks will allow oxygen into the exhaust & can cause a burn in the pipes .
 But you said the engine uts out which sounds more like intake this could be a lean situation , bent intake valve , or timing  I would start with adding a little more fuel from the accel pump  then check timing & wires first & maydo do a compression test

[tman]

If this happened after changing your listed items, I think firing order too.  If not, what CP says.

[dfrazz]

Thanks guys, I'll double check the wires - accel pump - timing and go from there.

[HP_Cuda]

Did you get the Firecore with or without vacuum advance?

Sounds like timing to me and maybe internally the plate slipped (two adjustment screws) giving you some more mechanical advance.

Hopefully you will figure it out quick. 

[dfrazz]

Did you get the Firecore with or without vacuum advance?

Sounds like timing to me and maybe internally the plate slipped (two adjustment screws) giving you some more mechanical advance.

With  vacuum advance and it is connected. 

[cudabob496]

remove vacuum advance and problem may go away?

[dfrazz]

remove vacuum advance and problem may go away?

It seemed to like the vacuum advance when I timed it in after install.  I will check it without vacuum when I get a light on it.  Thanks!

[cudabob496]

It seemed to like the vacuum advance when I timed it in after install.  I will check it without vacuum when I get a light on it.  Thanks!

vacuum advance can give up around 50 degrees more before TDC, on top of your total mechanical advance.  All it does is provide a little better gas milage,
and a more clean burn, but can interfer when you play race car sometimes.  I stopped using it, and car is quicker when I stab the throttle.

[HP_Cuda]

Its possible but not probable that vacuum advance is doing it because it really only kicks in at higher rpms like freeway speeds.

Frazz stated it was from a stop.

Definitely sounds timing though. I'm pretty sure I remember Frazz saying he went through firing order not long ago too.

[734406pk]

My 440 has developed a pretty good backfire on acceleration from a stop.  I have noticed I have an exhaust leak since pulling the drivers side headers to replace the steering box.  It idles fine, even revving the engine in park, no backfire.  But accelerating from a stop, just easy on throttle, and I get 2-3 backfires and then if I continue or increase throttle I get big backfire that sometimes make the engine cut out and I back off the throttle completely.  I installed a Firecore RTR distributor, coil, wires and a Proform 750 last fall but it did not have any issues until recently, and the steering box is the only thing I changed.

Can an exhaust leak cause backfiring on acceleration?  Or should I look at something else.  Thanks for you help guys!

I know these 440's are a tight fit around the drivers side with the steering box. When you removed the D.S. header to gain access to the steering box, did you have to remove the spark plugs and wires to get the header to move? I remember changing header gaskets and having to take the plugs and wires off for clearance just to do that. Check the firing  order. 3/5 can easily be reversed.

[cudabob496]

and sometimes crossed wires can cause the wrong spark plug to fire

[734406pk]

Also check your plug wire resistance with an ohm meter. Firecore wires are listed at 250 ohms per foot of length. Removing the plug wires may have broken the core in one (?) of them causing a high resistance. You can get a cross-fire (backfire) inside the distributor cap on acceleration since your spark voltage increases with engine load. The engine can run fine in neutral otherwise as you have mentioned. The other symptom with the engine almost shutting down sure sounds like high voltage is finding an easier path to ground through the RTR ignition module or mag pickup inside the distributor. (High resistance in a plug wire or sparkplug).