Speedmaster Distributor causes backfire under load...

[GassMann]

Hey all,

I rebuilt a 440 block and upgraded (so I think) the distributor with a Speedmaster electronic distributor. Problem is, under load, it seems to backfire into the carb implying I am not getting the spark advance I should be getting. The only thing I upgraded was the distributor. Should I be upgrading the resistor as well? Or... is there some other magic I need to know about to get this unit to perform as expected?

The carb is a new Edelbrock 750. I have the timing set to specs. It idles and revs up in the garage just fine. But on the road, if I punch it... hesitation and backfire?!?

Suggestions on where to start checking?

 

[Don Frelier]

Sounds like you have a rotor phasing problem.
(Maybe)
It's when the rotor is closer to the next terminal when the spark occurs.
To tell you need to drill a hole in the cap and shoot the timing light and see where the rotor is pointing when firing.
Mopar do have narrow spark tower terminals and rotor tips
Try your old distributor and see if it goes away.

 

[GassMann]

Sounds like you have a rotor phasing problem.
(Maybe)
It's when the rotor is closer to the next terminal when the spark occurs.
To tell you need to drill a hole in the cap and shoot the timing light and see where the rotor is pointing when firing.
Mopar do have narrow spark tower terminals and rotor tips
Try your old distributor and see if it goes away.

Thanks Don... I had never heard of rotor phasing before. The old distributor is non-functional. It was broken during transport prior to the rebuild as the block was not original to my Charger. After some research, I see what you mean. I pulled the cap and removed the rotor and noticed the rotor came off really hard. The tang inside the rotor is a bit marred though I don't think it is moving on the shaft so that's not it. However, the inside of the cap did have a fair bit of carbon... more than I would have expected for a new cap and rotor with so few miles. So I think something is going on with the spark. I will get a new cap and rotor and drill a view hole in the old one to peek in. Sounds like a fun thing to do anyway.

 

[Geoff 2]

Check the pick up air gap. Should be 0.008". Also remove/check the spark plugs to see if they are carbon fouled.

Is the cam bigger than stock? Cam specs?

 

[GassMann]

Check the pick up air gap. Should be 0.008". Also remove/check the spark plugs to see if they are carbon fouled.

Is the cam bigger than stock? Cam specs?

Thanks Geoff... It's a stock cam. But I will also check the gap and fouling. Its brand new so I didn't think to check anything like gaps and all.

 

[GassMann]

Thanks Don... I had never heard of rotor phasing before. The old distributor is non-functional. It was broken during transport prior to the rebuild as the block was not original to my Charger. After some research, I see what you mean. I pulled the cap and removed the rotor and noticed the rotor came off really hard. The tang inside the rotor is a bit marred though I don't think it is moving on the shaft so that's not it. However, the inside of the cap did have a fair bit of carbon... more than I would have expected for a new cap and rotor with so few miles. So I think something is going on with the spark. I will get a new cap and rotor and drill a view hole in the old one to peek in. Sounds like a fun thing to do anyway.

OK... some closure on this. Rechecking the timing and I found the timing to be about 18 to 20 degs... WAY out. But while fiddling with adjusting the timing, I found one of the three wires from the new Speedmaster had a failed crip. I had to disassemble the connector, solder the wire in, reassemble and finally set the timing. Way advanced as it was definitely would cause the backfire. Goes to show you how long it's been since I rebuilt an engine.

Now, I am set to 12.5 deg BTDC at 500 RPM. Now for a road test.

 

[GassMann]

OK... some closure on this. Rechecking the timing and I found the timing to be about 18 to 20 degs... WAY out. But while fiddling with adjusting the timing, I found one of the three wires from the new Speedmaster had a failed crip. I had to disassemble the connector, solder the wire in, reassemble and finally set the timing. Way advanced as it was definitely would cause the backfire. Goes to show you how long it's been since I rebuilt an engine.

Now, I am set to 12.5 deg BTDC at 500 RPM. Now for a road test.

Hey all,

So I am still missing something. I missed the part where I should have bypassed the resistor in the install of this distributor. I bypassed the resistor now and things are OK. But I have a follow-on question. The distributor I purchased does not have a vacuum advance. What is the difference between a Speedmaster with vacuum and without?

Should I be looking to replace the distributor with electronic pickup and a vacuum advance or stay where I am and figure out what I missed?