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random misfire

badmix

badmix

1/2 ton status
Joined
Nov 7, 2004
Posts
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Location
West Virginia
1984 K5 305cid 700r 170k miles,, holley 570cfm. Motor stock except carb, headers,,dual,exhaust w/ catch cats and dynamic muffler.

When truck warmed up driving down road steady at 55mpm, rpm 1800. I'll get a random sputter misfire that I can hear in engine and tailpipe.

What could this be ?
 
When was the last time you looked at the plugs and wires? I would start there
 
Inspect the wires first. You may have one that keeps up at low speed but doesn’t work well under higher RPM conditions

I had one recently that I caught doing this. Idled fine, but under a load it would cut out. Found the offending wire had broken inside right at the crimp
 
Yea, ive got a new set waiting ,just waiting on weather to cooperate.

I romped on truck on way home today and it was fine, ran up to 3800 rpm, it seems to be mid range issue and very random

Would anything with carb cause this ? Choke ?
 
Always a possibility. The random part is what also makes me wonder. Swap those wires out and hopefully it’s happier
 
Yeah I wouldn't waste time measuring resistance on the plug wires. 15 years is old. Swap them with the cap and rotor.

Too bad nobody makes a smaller portable version of the old Sun 1120 machine where you could watch the firing lines of each cylinder. Easy to catch the misfire down to the cylinder then. You'd think with today's technology it would be easy.
 
Have not gotten a chance to play with one, but somewhere in the lit, it says it can be switched from analog to digital and back.
Of course, that might just mean that the display is switched and the data is still all digital, but I'm not sure. I'm going to check out any Youtube videos about it if there are any after I recover from turkey season enough to understand what I am seeing.
 
FIXED !!

Can't post pics, but found/ fixed the problem., figured it was a plug wire issue and had a set sitting on shelf , figured I just do a tune up.

So changed cap and rotor and plug wires, spark plugs got changed recently.

I found the following;

one terminal corroded on dist cap #4, I've had issue with old plug wire coming apart. So weird.

Next up , was the black/ ground wire going from dizzy to coil had most of insulation chewed away. I'm sure this was contributing to random misfire.

Got it fixed and truck drove good.

I think I got lucky, what would've been symptoms if ground had shorted or broke ? No start situation?
 
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