[My GMC Truck 1]

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.....
[/QUOTE

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?

This morning, I am working on making sure the timing is correct. No crossed wires ,etc

 

[My GMC Truck 1]

This morning, I am working on making sure the timing is correct. No crossed wires ,etc

Holly 580 or 570 avenger vacum secondaries. Brand new rebuilt from holly. Check the gas no water

 

[427L88]

I have found Holleys sensitive to fuel level. Be sure to check it - HOT.

 

[My GMC Truck 1]

Ok, set timing at 34 btdc , adjusted float levels,95% better . tommorow I pull every plug and wire to eliminate that. Still some what of a problem in over drive 60-65 mph chugs ? Drop it into 3 rd gear , not problem .
but I’m getting there thanks to you all

 

[69SS454]

Try two more degrees of timing. May need to install larger jets depending on how your plugs look.

 

[My GMC Truck 1]

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.

I generally only use shell premium , some times Holiday lead free . But most times it’s brand name .

 

[OLDED]

FWIW, if all else mentioned above hasn't helped or cured the problem - replace the Ignition Control Module, esp. if it's the GM HEI distributor. They can last forever or fail at most any point, I always carry a spare as it can be changed in 5 minutes. If it wasn't the problem at least you will have a spare for the inevitable.

 

[jrkrace]

Ok, set timing at 34 btdc , adjusted float levels,95% better . tommorow I pull every plug and wire to eliminate that. Still some what of a problem in over drive 60-65 mph chugs ? Drop it into 3 rd gear , not problem .
but I’m getting there thanks to you all

Do you have a vacuum advance can hooked up? Sometimes they dump a ton of advance when your cruising and it can give you a bucking or surging feeling…

 

[427L88]

some HEIs spit out like 20+ . NOT GOOD unless its an 1975 8.5:1 engine. !

 

[69-CHVL]

Put a timing light on each wire and look for consistent spark. If possible have someone sit in the car with it in gear while you check to load the motor a bit.

 

[My GMC Truck 1]

FWIW, if all else mentioned above hasn't helped or cured the problem - replace the Ignition Control Module, esp. if it's the GM HEI distributor. They can last forever or fail at most any point, I always carry a spare as it can be changed in 5 minutes. If it wasn't the problem at least you will have a spare for the inevitable.

Do you have a vacuum advance can hooked up? Sometimes they dump a ton of advance when your cruising and it can give you a bucking or surging feeling…

Nop straight to carb port

 

[My GMC Truck 1]

FWIW, if all else mentioned above hasn't helped or cured the problem - replace the Ignition Control Module, esp. if it's the GM HEI distributor. They can last forever or fail at most any point, I always carry a spare as it can be changed in 5 minutes. If it wasn't the problem at least you will have a spare for the inevitable.

That’s something a friend of mine mentioned

 

[My GMC Truck 1]

Put a timing light on each wire and look for consistent spark. If possible have someone sit in the car with it in gear while you check to load the motor a bit.

Good idea , at least I can eliminate them that way

 

[68C427GO!]

I know you got new wires but wouldn’t hurt to check at night to see if any ignition wires are taking a shortcut to ground.

 

[My GMC Truck 1]

I on my way out to do that now, thanks everything looks good, no start sparks.

So based on everything I have checked and completed. One of the members here mentioned an ignition module. I did some online searches and that seems it could be part of the overall problem.
So I ordered one today, not bad price $35 but after all the crap I have dealt with, if this fixes it great. There’s nothing left.

 

[Johnny Bravo]

This was mentioned before and I didn't see any follow up, but I had a similar issue with my 70. It would have some intermittent surges, or misses, if you would, at certain speeds. I chased it for a few months, changing unnecessary things, until I delved into a book of all things. Not saying it'll fix your issue, but it fixed mine. It was the accelerator pump spring. I had to play with the tolerance, but mine worked out to operate flawlessly with about .005 clearance. Try .010 and see if it helps. I had to play with it a bit, once I figured that was the problem. It turned out to be my own fault (tinkering when I shouldn't have been), but it made sense once I got it back on track. It still makes no sense that it happened all of a sudden, with no adjustments to anything. Good luck brother

 

[My GMC Truck 1]

This was mentioned before and I didn't see any follow up, but I had a similar issue with my 70. It would have some intermittent surges, or misses, if you would, at certain speeds. I chased it for a few months, changing unnecessary things, until I delved into a book of all things. Not saying it'll fix your issue, but it fixed mine. It was the accelerator pump spring. I had to play with the tolerance, but mine worked out to operate flawlessly with about .005 clearance. Try .010 and see if it helps. I had to play with it a bit, once I figured that was the problem. It turned out to be my own fault (tinkering when I shouldn't have been), but it made sense once I got it back on track. It still makes no sense that it happened all of a sudden, with no adjustments to anything. Good luck brother View attachment 745888

Your correct, I haven’t messed with that yet. Ok that's something to look at. Thanks

 

[lookback]

Just a shot in the dark. You had mention that in 3 gear all problems are gone right truck runs fine no miss gets up to speed without hick ups from engine? Just curious you have overdrive tans auto right? Check your cable downshift cable position. Especially the OD 200r4 or even 700. What the torque converts stall speed? A tight converter may not give you that little bit of slip needed to pick up speed under load. Sounds like your bucking under heavy load, like try to start in 3 gear all the time. Check spring on carb like Johnny mentions makes a big difference. Lots of folks think that the pump shot is just off line from dead start not so its needed all the way up to full throttle. Do a little test with a mirror look into carb run it to 3k then give a quick jab up you will see pump shot come out at any speed. I don't know everything but I always mange to make suggestions to look into. Carb may be ok, you say if you drop down into third gear clean and fast response right? Verify shot then look into trans downshift or detent cable. Low stall no cure have to replace converter. Remember just a thought.

 

[My GMC Truck 1]

This was mentioned before and I didn't see any follow up, but I had a similar issue with my 70. It would have some intermittent surges, or misses, if you would, at certain speeds. I chased it for a few months, changing unnecessary things, until I delved into a book of all things. Not saying it'll fix your issue, but it fixed mine. It was the accelerator pump spring. I had to play with the tolerance, but mine worked out to operate flawlessly with about .005 clearance. Try .010 and see if it helps. I had to play with it a bit, once I figured that was the problem. It turned out to be my own fault (tinkering when I shouldn't have been), but it made sense once I got it back on track. It still makes no sense that it happened all of a sudden, with no adjustments to anything. Good luck brother View attachment 745888

I know you got new wires but wouldn’t hurt to check at night to see if any ignition wires are taking a shortcut to ground.

Checked the clearance @ idle there more than .035 clearance. It’s down right sloppy Check it at wide open throttle no clearance non.
Looking at the adjustment pin, it looks like if I adjust it closer to the arm at idle, it will be even tighter at wide open throttle?

 

[My GMC Truck 1]

Your correct, I haven’t messed with that yet. Ok that's something to look at. Thanks

Ok, checked the clearance , at idle more than .035 clearance, actually down right sloppy, at wide open throttle no clearance, it’s tight.