Issue with 40 degree spark advance

Hey everyone , hope someone can help me figure this.


1993 Chevy k2500 4wd 7.4l 4l80e locker rear 4.63 gear


About 2 months ago the pickup coil packed it in.

New disturbutor (napa special)
New NGK plug wires
Champion Plugs
New tps
New IAC
Rebuilt tbi with kit
New fuel filter

Everything works on the truck as it should idle's fine , slight stutter when first cold start though

I have a scanner iac 35-40 ar idle , BLM is 133 at idle (128 when cruising at around 1500 rpms ) I do have an exhaust leak in the flange right before the O2 Intergrator is 128 90% of the time.

Map is showing around 40 at idle

Pump voltage is 13.7 (9.4 at crank)

I double checked the timing and it is set at 4 degrees with advance wire unplugged.

I do have a 25 rpm jump at idle 725-750

Spark advance is 16 degrees at idle

Tps is .45 volts at idle 5 at WOT

Knock shows 3-5 at start up and usually goes up to 7-10 driving around town.

No fuel pressure test done


The issue that I am having is when cruising on the highway, the spark advance hits 40 degrees and she bucks or feels like a lean stutter. This only happens at 40 degrees advance. Barley touch the throttle brings the advance down and all is good. Its a tow vehicle and it has zero issues towing a 8000 lbs TT


so help lol

Sorry about the novel but I know people like all the info

A simple check might be making sure the spark reference angle in software matches the angle the distributor is set to. I don't happen to remember 4 degrees as an option, although it's been a number of years since I've worked on a 7.4.

I did a data log this morning, so maybe someone can see something. Its german to me lol


The issue is bucking at light throttle on the highway only when the scanner shows 40 degrees advance

might be a problem with the new off-brand distributor, can you pick up a wrecker one to try?

last time i had a stumble only when highly advanced, it was a poorly indexed distributor rotor on a carb n points rig