little missfire help

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aaron68s

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Here's what I got, my 340 has what feels like an ignition skip at low rpm cruise speed, just off idle cruising. Its a 72 factory electronic dizzy. Car runs hard!! Just feels like a miss cruising. My cam likes 24 initial timing. Is it possible its advanced enough and miss firing inside the cap? Thanks Aaron
 
Sounds like classic plugs or wires symptom.
 
what carburetor are you running? It could be a lean miss fire within the carburetor.
 
Anyone have any pros/cons of aftermarket ignition systems? Are the HEI any good? Is the best way to go for performance and adjustability MSD pro billet? I have a couple 6al boxes but no mopar dizzy.
 
The best way to determine if it might be mis-firing inside the cap is to check the "rotor phasing" with a timing light. Drill a top hole in-line with one of the posts in an extra distributor cap, to see the rotor with the light. You can find videos on youtube. Normally not a problem with factory distributors, only when you add aftermarket pickups like Holley XR700. However, Richard Ehrenberg sells an adjustable toothed wheel for factory distributors on ebay, so perhaps there is a problem to solve.

HEI is hotter than the factory Mopar ignition, since doesn't require a ballast and you can run a hotter e-core coil. You might notice an improvement with an MSD box. The very best ignition is distributor-less, single coil per cylinder, like GM LS engines or Mopar Hemi's. However, I know of no store-bought solution for our cars.
 
Pump shot may be coming in a hair late is my guess. Low speed lean miss is what it sounds like to me.
 
If you've set the idle mixture to maximum vacuum, try richening it a good half turn to see if that helps. Just as a test.

Also, I'm running an MSD box in my car and it was a very noticeable improvement over the chrome box I was running. I'm just running an old mopar electronic distributor too, no fancy billet stuff.
 
I still need to work on my timing, the initial is good, but my all in is to early and to much. Would that cause a miss at low rpm cruise speed?
 
Did this "miss" start since you put on the Holley.. Did you set the float level,, if so, try lowering the float level a half turn,, to eliminate the chance of fuel dripping/syphoning down the carb ,, which can cause symptoms as you describe .. then re-adjust later...

Can you look down the carb at the rpm you have probs,, and see fuel occasionally dripping down carb?

I don't think it's timing.. my .02
 
aaron68s said:
I still need to work on my timing, the initial is good, but my all in is to early and to much. Would that cause a miss at low rpm cruise speed?
Click to expand...

Curious what your all in is. Too much and too early will not cause your problem.

Fuel dripping down the carb will not cause a misfire either. Rich condition YES. Misfire highly unlikely especially with how much initial timing he has not to mention the all in timing he says he has.
 
inertia said:
Did this "miss" start since you put on the Holley.. Did you set the float level,, if so, try lowering the float level a half turn,, to eliminate the chance of fuel dripping/syphoning down the carb ,, which can cause symptoms as you describe .. then re-adjust later...

Can you look down the carb at the rpm you have probs,, and see fuel occasionally dripping down carb?

I don't think it's timing.. my .02
Click to expand...

The whole motor is new, I set the float levels when I put the carb on, adjusted the a/f with tach and vac gauge. The motor runs awesome pulls hard, idles good, when I stomp it there is virtually no hesitation, just has a skip at low rpm cruising with the throttle just cracked,
 
Mad Dart said:
Pump shot may be coming in a hair late is my guess. Low speed lean miss is what it sounds like to me.
Click to expand...

Mad Dart said:
Curious what your all in is. Too much and too early will not cause your problem.

Fuel dripping down the carb will not cause a misfire either. Rich condition YES. Misfire highly unlikely especially with how much initial timing he has not to mention the all in timing he says he has.
Click to expand...

If he's at a low cruise, throttle steady, pump shot isn't used here, also at low cruise, fuel dripping will definately cause a stumble or engine irregularity,.

Our problem would be the definition of missfire,, is he speaking of a literal misfire,, or is he speaking of some sort or engine irregularity at low cruise, which could easily, and more likely be carb,,..

If it "pulls hard", when missfire would be expected at high cylinder load, then I'd be less inclined to worry about timing,, and be sceptical of carb..

I'd still lower the float half a turn,, just to see.... takes 1 minute.. my .02.
 
Sounds like you cammed the motor and left the vacuum advanced hooked up. Most make this mistake. If you are going to use the advance it is adjustable with an allen wrench in the vacuum port. And you need to use a 8.5 not and 11.0
 
Wanna thank you fellas for giving me some ideas to look at, think I have solved my problem. I started playing with the carb a little more, turned the a/f screws out about 3/4 turn on all 4 corners and seems like my stumbling/miss is gone. Guess it was a lean miss. Thanks again Aaron
 
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