I've scrounged this forum for a few weeks, but this is my 1st post here.
I have a 79 K10, 230k miles, 350 with 4sp (3 + granny low). All stock. Bought the truck from my uncle, who had it from new. He rebuilt the motor at 80k, due to soft factory cam wearing out.
My job takes me all over the country from project to project, and 90% of the 40 thousand miles I've put on the truck in the last 5 years have been on the interstate with a small (under 3000 lb) trailer on the hitch.
About 2 years ago, cutting across Wyoming in the dead of summer, I developed a slight irregular miss at highway speeds. It only popped once or twice, but I had to drop my speed to about 55 mph to keep it from happening. Got to the job, checked into the hotel, and promptly changed cap, rotor, wires, and plugs. It seemed to have fixed it, but I wasn't pulling the trailer for around-town driving, so I knew it wasn't simulating the load conditions exactly.
Over the last year, this miss became *slightly* more apparent, but still, only when pulling a load up a grade at highway speeds, and only when it was pretty warm outside. If it was nightime driving, or from October through about April, it didn't seem to do it. Idle, 1st and 2nd run great, with no miss no matter where in the rpm range I push it, or for how long I have to keep the hammer down (definitely needed on some of those Colorado mountains). Sometimes, she won't do it at all no matter what the conditions. For example, drove all 2100 miles from Utah back home to Pennsy in October, she purred like a kitten the whole way without any issues.
Well, Thursday I got the call to return to work, so I left last night from Pennsy for Indiana, trailer on the hitch. About 150 miles into the drive, she started missing, much worse than anytime before. On flat ground, if I tried to push her past double nickel, she'd start sputtering and I'd have to back off. On a grade, about all I could do was let my speed bleed off to about 40-45, and she'd mostly be fine. From stopped to 50 mph, no issues at all. 1st gear and 2nd, full throttle range without any misfired. But try to give her more gas on a hill to get to 55? Forget it, she just spits and sputters and I can't build up any more momentum. 45-50 mph, seemed about all I could manage without constant sputtering, and that was only on the few flat areas. Of course, then I was getting buzzed by idiots in rice grinders on icy I-80 in hilly central PA ... not a good place to be.
Anyhow, only so much I could do on the side of road. I dumped a fuel cleaner/octane boast in the tank (93 oct is what always ran best), but no change. Before rattling the motor apart from the misfires, I opted to play it safe and nurse her back home where I could fix the issue. Took me twice as long, but 2 lane roads at 45 mph let me nurse her home, not misfire-free but drivable.
So, my intial thought is distributor putting out uneven/weak spark at high rpm and moderate to heavy loading. This would also explain why the truck just felt like it couldn't handle the hills as well as it used to, even when it didn't actively misfire, plus my ever-worsening fuel "economy" (I use that term very loosely, lol.)
My only concern is what if its not the distributor itself, but the cam bearings? I honestly wouldn't have even considered the cam, except for knowing that my uncle had a soft factory cam when he got this truck from Detroit, which started presenting problems under load and led to his rebuild of this motor at 80k. How can I test if it's the distributor itself causing my misfire, or if the weak distributor is only a symptom of a cam with worn bearings warping under high rpm and load? Or maybe it's not distributor at all, but valve float ... which would again point to a worn cam?
Any thoughts would be *greatly* appreciated. Been laid off all winter, it'd be nice to hit that highway again and starting making some cash again! Thanks