engine stumbles under load

[flathead 53]

just got finished converting my 53 f-100 to 12 volts , new distributor ,pertronix , new coil ,new wires ,plugs , truck starts right up and will idle fine bring RPMS up to 1500 engine starts to run rough and back fire , distributor is in correct as the timing ,, put new 94 carb on engine ,still the same put another carb on ,still the same , lost on what to try next , truck ran fine before 12 volt conversion , Help.

[38 coupe]

I would start by putting the good distributor back in. Measure the resistance of the new coil and add a ballast resistor if needed to get the total resistance where it needs to be. Condenser and points do not need changing for 12V. I am a very big advocate of "if it ain't broke, don't fix it" (learned the hard way multiple times).

[jimvette59]

If you do not have a ballast resister install one and or check the coil to see if it has a resister in it or not. JMO

[glennpm]

For a 12 V system you'll want to target about 4 ohms total resistance, sum of the coil's primary resistance plus an external ballast resistor.


Glenn

[JayChicago]

Originally Posted by glennpm

For a 12 V system you'll want to target about 4 ohms total resistance, sum of the coil's primary resistance plus an external ballast resistor.

I think you meant to say target 4 amps. You get to that by having about 3 ohm total resistance in the primary circuit.
I = V/R
4 amp=12 Volt / 3 ohm

[deuce_roadster]

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I have never used a "pertronix" but do they even need a resistor? There are no points to burn up. I have heard the electronic distributors don't play well with generators as they like a stable voltage that doesn't vary. I will stick to points and condensers and 6v on my flatheads. When everything is "right" 6v works fine and reliable.

[glennpm]

No, looking for about 3 amps so the points will last longer This is from a recent post I made:


"I had a running and starting issue this week so I'm replacing my Petronix II with a Bob Drake rebuilt 1942 crab distributor on my 59AB. I'm planning to add a ceramic 1.5 ohm inline with the coil's primary resistance of 2.3 ohm. I have a 12v negative ground wiring system.

Running resistance = 2.3 + 1.5 = 3.8 ohm

Ohm's Law I = V/R

I = ~12v/3.8 = ~3.3 amps

For starting I want to add a push button momentary switch so the coil will see about 5.5 amps when the ceramic 1.5a resistor is bypassed."

[JayChicago]

Glennpm
All of that makes perfect sense. Good logic there. If that coil is putting out good spark at lower amps, that’s all you need.

[Ggmac]

Is your new distributor only mechanical advance ? If so read the instructions on adjusting the advance curve . Or CAREFULLY after you start it , bring it up to where it starts stumbling and wuth the distributorclamp a LITTLE loose , rotate it A LITTLE. See if the stumble improves

[glennpm]

Just took it out for a short test ride and it is working great, great power, no pings all is wonderful! Tomorrow may be a different story :-)


Thanks for the tip Ggmac and yes it is now back to stock mechanical.


Glenn

[flathead 53]

thanks, distributor has vac and mech advance , does not require A resister ,tried A mallory distributor before the pertronix, sent the mallory back to speedway same problem. Using A pertronix flame thrower 40,000 out put coil .

[drolston]

Originally Posted by flathead 53

thanks, distributor has vac and mech advance , does not require A resister ,tried A mallory distributor before the pertronix, sent the mallory back to speedway same problem. Using A pertronix flame thrower 40,000 out put coil .

Some dark night take a look under the hood while you jazz the throttle up and down. If you see sparks jumping anywhere around the coil, plugs or ignition cables, you might want to replace that flamethrower with something a little less intense. Our 7mm cables do not insulate as well as the usual 9mm cables.

[Bill OH]

The stock vaccum distributor ran rough and did for me the very thing you describe. (To much advance because of the high vaccum) Mechanical advance solved the problem. Packard 440 Ignition wires are very reliable.

[tubman]

I had a MSD "Ready to Run" distributor on an 8BA and it had too much advance (Chevy Curve). I went back to a mechanical only Mallory and have been very satisfied. Although the major car manufacturers seem to know how to make electronic ignitions, it seems the aftermarket is having problems.