I have been chasing this problem for months now. Was reluctant to post it here for fear that it is something really simple that I am overlooking, but thought I would describe it anyway since we are all friends here.
My 56 Customline dual quad motor has developed a strange, inconsistent stumble when it reaches operating temperature or above but only under a load. Idles fine and revs up fine in neutral. Dizzy has a Pertronix ignition, and I have recently changed the cap and rotor. 8mm Silicone wires, new coil also. Did long periods of winter storage in Maine, so I even ran a couple of bottles of carb cleaner through the tank. Timing is good, so now I'm thinking that maybe it's a vapor lock issue or a worn distributor shaft bushing? It's been VERY hot here...over 100 degrees, but the motor usually runs at 180 in this weather with shrouded 2,500 CFM puller fan. So, any comments or ideas would be welcome.
Thanks,
Rono

  Ron Lane,  Meridian, ID

Hi Ron,

Only does it when warmed up...  only does it under load...  new coil already installed...

Without an oscilloscope to hook up and watch while driving the car...  Charlie Brown would  suspect the spark plugs...
_______________
Hope that your knee is working good for you!

Hope my suggestion of plugs nails it for you!  


NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

Thanks Charlie...I haven't checked those in a while. I'll check that.
The knee is doing well for only 6 weeks post-op. Can ride the Harley and operate the clutch in the 56. Just need to get a little more flex so I can kneel easier. Thanks for asking!
Rono

  Ron Lane,  Meridian, ID

Ron:  I had a Ford once that idled like a top, ran fine virtually always, but had a miss when it went into overdrive with the AOD.  Spark plugs were suppose to be good 'til 100,000 miles, BUT replacing the plugs eliminated the miss!

That's what triggered the suggestion.  Hope it helps...

Glad to hear that you're doing well!  

Regards,
Charlie


NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

Ron,

I've got an OT car in Tucson. Does the same thing, even though it's port injection. I looked at the plugs first, and they were fine. Fresh fuel solved the problem. Being in Seattle, we always stored the cars with full tanks. This new ethanol stuff, even with additives, doesn't store well. Might just be something in the fuel at your high temps. The race car guys would know better, but I think at high temps the ethanol evaporates so fast ( in the intake) they go lean. If nothing else show up, maybe a little race gas in a near empty tank. Then you'll know it's the fuel, not the motor.

Glad to hear the knee is coming along. When you figure out how to get up after you kneel, let me know.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

You might want to check for the amount of ethanol in your fuel.  This means doing the test yourself and not relying on the numbers posted at the pumps.  I’ve found as much as 30% from pumps marked as not having any more than 10%.  You don’t have to be a chemist to know that ethanol will do everything it can to separate from the gasoline as it sits.  As a general rule, premium fuel has half the amount of ethanol as the 87 & 89 octane grades so that’s also something to consider.

Lorena, Texas (South of Waco)

Charlie, It looks like you were right on with the solution to the problem. #7 plug appears to be gas fouled. I'm checking the valve lash now and will also make sure that plug wire is okay. Running Champion RF11Y gapped at 0.035 with the Pertronix ignition. I've heard some recommendations as high as 0.045, but that just seems way to much gap in my opinion.

Rono

  Ron Lane,  Meridian, ID

Rono.  I'm old school.  We're on the same page on this, methinks.  .045 sounds like an excessive spark plug gap to me, too.  I've read performance tuning articles on y-blocks by Les Ritchey circa late '50 early '60s involving adjusting spark plug gaps and valve lash settings to effect elapsed times in drag racing competition.  I don't recall any spark plug settings that large being used or advocated.

The greater the plug gap, the higher the voltage the coil has to produce to fire the plug... AND the harder the electrons try to find an easier (spelled alternate) path to ground.  Even with the plug wires we've got available today, the voltage required to jump a .045 plug gap begs for ANY alternate path in the high voltage circuitry including inside the distributor cap.

Miker and Ted certainly had excellent thoughts pertaining to ethanol fuels coupled with the temperatures you're operating in.  With no moving parts and quality materials, a spark plug malfunctioning is not at the top of any trouble shooting lists.  When I ran into the slight hesitation problem with my late model Ford AOD shifting into overdrive, I thought for sure it was a valve body problem with the transmission.  While only one plug was apparently misfiring, I replaced them all!  

Let us know how you make out, Rono. 

Regards,
Charlie   

NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

I think you're making a big mistake judging sparkplug gaps based on articles from the 50s and 60s.  That was before HEI and the sophisticated electronics of today.  .045 would be too big for points.  Pertronix, maybe not and some other aftermarket ignitions, absolutely not.

Experimenting with plug gaps is free.  Try it out and see what happens.



Lawrenceville, GA

I replaced all the spark plugs with new Champion RF 11Y gapped at 0.035". Checked the valve lash and for my Comp Cam with the roller rockers the settings of 0.012" Intake and 0.014" Exhaust were right on. Then I traced #7 plug wire back to the cap and found that is was not 100% seated to the bottom. I could hear and feel the contact click when pushed down hard into the cap...so could it be that simple? The car fired-up and idled great, but there is so much smoke and ash in the air from surrounding forest fires that I haven't taken the car out for a good test run yet. HOPEFULLY the wind will shift and move this crud out of the valley. I'll keep you posted.
Rono

  Ron Lane,  Meridian, ID