The Stovebolt Site Forums The Shop Area The Electrical Bay Annoying Intermittent Mis-Fire Hesitation ?

1948 3100 1/2 ton, with 1956 235. Plugs look good. But recently started what feels like a mis-fire hesitation. At mid throttle. It idles fine, and will rev just fine while stopped. It doesn't feel like no gas. It feels more like ignition. Plugs look good. Points look ok. Cap and rotor replaced a year or two ago. I checked timing. The distributor doesn't lock down tight all the way. there's a little wiggle room. Any ideas what to check or replace next? It's bugging me. thanks.

How old are the plug wires? They can break down internally and still run OK until a moderate to heavy load goes on the engine.
Jerry

Plug wires aren't too old. I think replaced in the past few years.. but that might be it. Maybe a general tune up , then back to whining and complaining here. ? Plugs, points, wires, cap, rotor, condenser, timing, adjust valves. Then we'll see.

Help me out Jerry, I need to sort that out in my mind.
Can you explain the mechanics behind the ignition wires lack
of performance at moderate to heavy loads vs normal throttle
settings? The only difference that I can come up with is the
change in timing from the vacuum advance at various throttle
settings or pressure differences in the combustion chamber.
A bad plug, maybe to wide of a gap or something like that I
think I could see effecting the ignition of the mixture.
Weak coil? Cap and rotor gap opening up or oxidizing maybe?
I suppose if they are carbon wires they could build resistance
and not allow a high enough voltage conducted to the plug
which might not fire when pressures are higher than that at
small throttle settings.
You the teacher, so put me on the right track, cus I've
experienced it before myself.



Thanks,
Denny Graham
Sandwich, IL

Denny, radio-frequency suppression "wires" are actually string impregnated with graphite and sometimes a few other exotic components. They spread the resistance out over the entire length of the wire, rather than concentrating it in one place like a resistor plug does. A good resistor wire will have a maximum resistance of 1 K-ohm per inch, so a 12" wire is good if it has a resistance under 12,000 ohms. The end result is a current-limiting effect that prevents, or at least minimizes RF field generation surrounding the plug wire as current is flowing across the plug gap. It's also why the spark that jumps from a suppressor wire looks orange/yellow instead of blue- - - -lower current flow. As wires age, the graphite tends to clump up in places, resulting in areas of very high resistance inside the insulation. Under acceleration, the load on the piston and the increased pressure in the cylinder requires a higher voltage to fire the plug effectively, and a tired plug wire that has increased its resistance over time isn't up to the task of providing the extra voltage demand.

Plug wires deteriorate from the inside, just like radiator hoses do, so a bad wire or hose is not obvious from a visual inspection standpoint. Some things just need to be changed on a time or mileage basis, not outside appearance.
Jerry

Thanks for the lesson, hope Scot was following this and takes a look at his plug wires.

Denny Graham
Sandwich, IL

Thanks Jerry, and thanks Denny for asking the question. I'll be out getting new plug wires and will report back.

OK, I'll say it. Check to see if you accidently crossed two plug wires. If one has never done that, then one has either, not worked on very many engines, or one is a liar.
Carl

Never done that Carl, made a list and checked it twice.

Think it's kind of gonna miss fire all the time with crossed
wires rather than when under a load---NO?

Kind of fits into your category of "make a list and checking it twice." I wasn't able to ascertain when the OP did what and when the problem began to occur.
Carl

A recent PM conversation reminded me of a problem that's associated with 6 cylinder HEI systems. An open-circuited or high resistance plug wire in certain positions on the HEI distributor with the coil in the cap can cause the high voltage to jump through the insulation of the cap to one of the screws that attaches the coil. It creates a tiny pinhole that's almost impossible to see, and the miss only shows up initially on a hard pull such as a long uphill run, or accelerating to pass, etc. Eventually it becomes a pretty constant miss that can't be fixed with normal tune up procedures. The V8 caps don't have the problem because the screws and the plug towers aren't close enough together to cause a problem. Back when GM had the odd-firing 231 cubic inch V6 in the Buick and Oldsmobile compact cars, this was a pretty common problem.
Jerry

There were no recent changes. I noticed that it just started missing. Like something was going bad.

Starting with new plug wires this weekend!

Well, I started with changing just the wires, nothing else.. and there might have been a little improvement. But then I just did what we all know to do... all of it. Points, Plugs, Condenser, Cap, Rotor, Set Timing, and a New Coil, just in case, along with the new wires. It was something in there, or a combination of weak parts, because now it runs delightfully smooth, just like it should.

The next time, the problem-parts will most likely be something else.

However, Denny is right about figuring out which part was/is bad (by testing one part at a time). This will give you a set of spare parts to carry on long trips.