383 Backfiring

[ethurlow]

66 300 with a 383. Brand new edelbrock 650 with long tube headers, msd distributor & new plugs & plug wires. It starts & runs great. I can floor it from a stop or anything under 10 mph & it does great. As soon as you get to second gear & floor it, it backfires & sometimes wants to cut off. I’m testing at 30 mph.
Tried a second new carb, capped off vacuum advance & have had total timing anywhere from 28° to 38° with no change

 

[CBODY67]

No advantage ot unhooking the vac advance line as there is no vac advance at WOT.

When was the last time you replaced the fuel pump pushrod? They wear, too. Shortening the rod shortenes the stroke of the fuel pump diaphram. Result, less available fuel AND how soon the bowls get full again.

Just some thoughts,
CBODY67

 

[Dads300]

Was it running fine and then you changed something or did it act up out of the blue? I would make sure there isn’t anything wrong in the distributor, nothing seized, nothing falling apart, correct air gap on pickup coil if applicable. Double check your firing order too.
Travis..

 

[SwissMonaco]

I would check the clearance of the timing chain. Set the mark to TDC and then turn the crankshaft against the direction of rotation until the rotor in the ignition distributor turns. Then check how many degrees you can turn the crankshaft back until the rotor turns. I had a crankshaft of almost 16° on my 440. Since I changed the chain, my problems have disappeared.

 

[Big_John]

66 300 with a 383. Brand new edelbrock 650 with long tube headers, msd distributor & new plugs & plug wires. It starts & runs great. I can floor it from a stop or anything under 10 mph & it does great. As soon as you get to second gear & floor it, it backfires & sometimes wants to cut off. I’m testing at 30 mph.
Tried a second new carb, capped off vacuum advance & have had total timing anywhere from 28° to 38° with no change

You don't mention what carb you are using.

It sounds like you have a lean condition of some sort. That could be fuel delivery, or it could be that you haven't tuned the carb to your engine.

The long tube headers will make the engine run lean at high RPM or wide open throttle and the carb tune will need to compensate for that.

So, it could be an overall lean condition... Did you look at the spark plugs?

Or it could need an accelerator pump adjustment.

Edelbrock has a pretty good procedure on tuning their carbs. https://edelbrock-instructions-v1.s3.amazonaws.com/edelbrock/carb-tuning-guide.pdf

 

[ethurlow]

I would check the clearance of the timing chain. Set the mark to TDC and then turn the crankshaft against the direction of rotation until the rotor in the ignition distributor turns. Then check how many degrees you can turn the crankshaft back until the rotor turns. I had a crankshaft of almost 16° on my 440. Since I changed the chain, my problems have disappeared.

little to no play

 

[ethurlow]

You don't mention what carb you are using.

It sounds like you have a lean condition of some sort. That could be fuel delivery, or it could be that you haven't tuned the carb to your engine.

The long tube headers will make the engine run lean at high RPM or wide open throttle and the carb tune will need to compensate for that.

So, it could be an overall lean condition... Did you look at the spark plugs?

Or it could need an accelerator pump adjustment.

Edelbrock has a pretty good procedure on tuning their carbs. https://edelbrock-instructions-v1.s3.amazonaws.com/edelbrock/carb-tuning-guide.pdf

edelbrock 650

 

[ethurlow]

edelbrock 650

You don't mention what carb you are using.

It sounds like you have a lean condition of some sort. That could be fuel delivery, or it could be that you haven't tuned the carb to your engine.

The long tube headers will make the engine run lean at high RPM or wide open throttle and the carb tune will need to compensate for that.

So, it could be an overall lean condition... Did you look at the spark plugs?

Or it could need an accelerator pump adjustment.

Edelbrock has a pretty good procedure on tuning their carbs. https://edelbrock-instructions-v1.s3.amazonaws.com/edelbrock/carb-tuning-guide.pdf

and plugs look great

 

[CBODY67]

When I first read the original post, it sounded to me like a fuel supply issue at WOT, after driving a while. It might be a new fuel pump, but if the fuel pump pushrod has worn and shortened, the result is a shorter stroke for the pump and less total fuel supply to the carburetor. A new push rod or an electric pump would fix that.

NOW, after it backfires at WOT, when you return to cruise mode, does everything smooth back out as normal? In that case, look at ignition issues. Especially plug wire routings and separations from each other. But with the lessened demand on the fuel supply system, a marginal supply system can also let things smooth back out.

What happens if you might load the engine against a very firmly-applied foot brake (in an area with a good bit of vacant area in front of the car)? This will load the engine harder and if it is an ignition issue, it might show up there, too.

What about if you wire around the MSD box to run a normal ignition?

Just some thoughts and observations,
CBODY67