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Discussion Starter · #1 ·
Hi everyone I am new to the forum and I have a question right off the get go I purchased a 350/350 hp crate motor from summit.
The first 1800 or so miles it ran very well. But now I have an intermittent miss. . I will be cruising around 65 mph nice and smooth and then there’s a miss for a bit and then it goes away and will do it again with in 10 minutes
I have changed, spark plugs, wires, HEI cap, & rotor. Yet there is that miss. The distributor is not a GM product, at least I don’t think it is.
Anyhow, I am at a loss as to where the problem is. I want to blame the dizzy but can’t start changing parts before I know what it is. It has a bit of a miss at idle to.
Here’s a pic of my truck. The Mustang is the wife's
Tire Sky Vehicle Wheel Car

Wheel Tire Vehicle Truck Car

Tire Wheel Cloud Sky Vehicle
 

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What kind of a miss and whats the duration of the miss? Does it stumble, pop, breaking up, etc? Or is it just a slight hiccup? What is your timing? Definitely sounds electrical to me.

Very nice truck
 

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1970 El Camino SS396, VortecPro 467, ATI TH400, Moser 12 bolt /TruTrac 3:42, Hedman headers
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Probably electrical in nature BUT don’t rule out fuel supply, partially clogged fuel filter or strainer, ? vapor lock ? Dave Ray has also mentioned poss faulty coils in the past…FWIW.
 

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Discussion Starter · #4 ·
When it misses it’s fairly steady, if at 50-60 mph and I hit the gas it will pop back fire what were we call it. As for the timing, we have tried it in several modes, maybe 10-15 degree’s advanced and have also retained it. Seems not to change at all.
Thanks on the truck. It’s a 68 GMC K15. Just had it restored after 34 yrs of being torn apart.
 

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Bill
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Sweet Mustang! Put the vacuum advance on full manifold vacuum. Set the timing to 34 degrees advanced at 3000 rpm.
 

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Discussion Starter · #6 ·
Probably electrical in nature BUT don’t rule out fuel supply, partially clogged fuel filter or strainer, ? vapor lock ? Dave Ray has also mentioned poss faulty coils in the past…FWIW.
Vapor lock I think is out, especially when it misses before it gets up to operating temp. Brand new carb, the fuel filter is clear. But that's worth another look. I checked the fuel pump and it’s working great. I am really leaning on the dizzy or the coil
 

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1970 El Camino SS396, VortecPro 467, ATI TH400, Moser 12 bolt /TruTrac 3:42, Hedman headers
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Initial problem indicated at cruise, this would rule out any issues with the accelerator pump or it’s circuit as the carb is on the “cruise” circuit if problem is properly stated.
 

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What kind of carb? Have you pulled plugs to see how there burning
 

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Dont rule out some crappy fuel. Seems to be happening more and more lately. All it takes is a little water in the tank to get hiccups like that. I would run a couple of tanks of a good name brand fuel through it before going crazy trying to find a problem that doesnt exist.
 

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1964 Chevrolet Chevelle Malibu 4 door
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What does your electrical system look like? Are all the wires old or new? Any splices or connections that may be a problem?
 

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Trouble with these old rides, cant hook up a code reader! From Bad gas to, ancient wiring, to a faulty ground strap, you have to start checking boxes off systematically. I would have guessed ignition wires, frankly. PS I measure the resistance of every wire I touch, as I've busted a few during install. They are delicate. I miss solid core wires.

So when you check spark, it is blue or yellow? Yellow = weak = coil. Coils usually fail 100%, but sometimes.....
 

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Initial problem indicated at cruise, this would rule out any issues with the accelerator pump or it’s circuit as the carb is on the “cruise” circuit if problem is properly stated.
I agree with this. Chase the cruise problem first. Sounds like a lean miss.

OP should state his idle and total timing specs (I dont see this anywhere), and how much vac adv is being added.

Also previously asked but not answered,, what kind of carb?

Also does he have an AFR meter?
 
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