72 K10 350 skips after warmup

[clyde72]

new member, although Ive been lurking and searching here and following advice but Im officially stumped. Im no motor expert but I have done a couple 350 "freshen up" rebuilds in the past
1972 K10, 350, quadrajet ,edelbrock manifold, th 350 trans.
The engine had a skip when I bought the truck in May, i found 0 compression in #1 cylinder, 150 ish in all others, I took off the valve cover and noticed one of the #1 rockers was tightened down a good deal more than the rest, I backed it off and viola, 150 lbs compression and no skip. I used the truck as a daily all summer for 5000 miles, even towed the wifes restored 18 ft 74 Shasta camper around no problem.
Then it started skipping again so I put in in the garage for some attention, did all the brakes,lines,calipers, wheel cylinders, shocks, 2.5 inch lift...bla bla
Back to the skip, I replaced the carb gaskets and checked the valve adjustments and it got worse...I use a laser temp gun on the headers and #1 and #4 cylinders run a good deal cooler than the rest, like 280 -300 degrees compared to 550 ish on the rest.
So heres what Ive done
Rechecked compression, all 145, 150
readjusted valves, was told to leave them kind loose, 1\4 turn ...?
Replaced HEI ignition with MSD distributor and coil, 12volts to coil, timing at 14 degrees
new plugs and wires
disassembled carb and cleaned with spray can, (didnt get into measurments) replaced filters
all of this has made 0 difference in the way it runs, wont skip for the first 5 minutes, after that it starts, worse under a load, forget towing the trailer
I`m lost...

[Dashman]

Have you removed the #1 and #4 plugs to check for spark? I've experienced where the spark plug wire terminal tensioner was loose and that created a miss, but it was always present. Did you hear a snap when the spark plug boots went on? Did it smoke white at all? What if you remove the cap from the radiator? ...will it then go longer than 5 minutes without missing? I would check for a blown head gasket.

[HO455]

Welcome aboard! Sorry to hear about your difficulties. It sounds as if you've checked the ignition and fuel delivery pretty thoroughly. Like Dashman says you may have a head gasket problem although #1 & #4 aren't on the same bank. Generally you see problems in adjacent cylinders. Doing a leak down test with the engine hot or warm should answer that question.

You didn't mention whether there is any valve train noise.. You may have some bad or really worn lifters that are bleeding too much oil when the engine is hot. The valve adjustment you found cranked down may have been someone trying to fix the problem. I suggest checking the amount of rocker movement on #1 & #4 cylinders compared with one of the cylinders that are at temperature. You may find a valve with low lift.

Another quick test would be to try adding some STP or putting some heaver weight oil in and see if the problem changes.

Good luck and keep us updated on your progress. And and you should know from lurking here we love pictures!

[Steeveedee]

A couple of thoughts. If it starts skipping after warmup, that can be lean mixture. If it is doing it at idle, try closing the choke plate a little bit and see if that changes anything. Also, #1 and #4 cylinder usually get their air/fuel mixture from the same carburetor port. #6 and #8 do, as well. Maybe look at the plugs on these four plugs Vs what the other four look like. It's rare, but I've seen parts loose in the float bowl (a venturi cluster splash shield had fallen off) and half the cylinders running leaner. They won't always exhibit the same amount of misfire, either. Changing the idle mixture screws may not make a difference in this case.

[clyde72]

thanks for the replies, I didnt mention the rockers appear to be moving the same, i guess its time to look at them closer, there is valve train (lifter?) noise because, as I mentioned I have them pretty loose, a local old motorhead said to try that, I have had them tighter.
Interesting that you mention the 1 and four are on the same carb port, the problem did get worse after I removed the carb to replace the gasket because there was a vacuum leak, one of the many times I was sure I found the problem, it only had a slight skip before that. As mentioned Ive taken the carb apart and spray cleaned it but honestly I dont know enough about them to spot anything wrong. I, had my hands on new edelbrocks at Oreillys a couple times but hesitant because nothing else has worked and theres a 5.3 ls sitting in the corner waiting to go in but Im not ready for that project.
You have a good point also about the lifters, hadnt thought about the heavier oil thing
Havent quite figured out the picture posting thing...

[clyde72]

We have a 57 long bed 4wd at work with a 400 smallblock that I put a Holley on a few years ago that runs pretty good, maybe I should grab that carb and see if it makes a difference

[tommy3horse]

Hello, had a simular issue with the 350 in my 72. Turned out to be the intake gaskets. They slipped down and leaked just enough to run poorly when warmed. The intake bolts were loose as well….got it that way… Good luck!

[clyde72]

Thanks, intake seems to be sealed good but will definitely check it closer

[Sheepdip]

Reading all of your post I personally think you have a cam going flat leaning towards the #1 exhaust lobe.....but that's just my opinion.

[1970cstblazer]

A bad cam is all too common especially in the last 15 years. People blame low zinc content in the oil. Reality is the metal hardness or lack thereof of the cams. The cam companies are cutting corners. There has also been suspect machining.

[Sheepdip]

A worn exhaust valve lobe will not allow a complete normal cylinder evacuation and eventually a less than pure/designed mixture of fuel and fresh air cannot get into the combustion chamber and it stumbles.

Same reason a performance cam lopes at idol, valve overlap pushes a small amount of exhaust gas into the intake passage giving it a less than pure mixture.

[clyde72]

my first thought was the cam, and it could very well be, Im thrown off by the good compression and visually the rockers look to be moving the same, i have a video of them, Ill try to post it.

[Sheepdip]

Originally Posted by clyde72

my first thought was the cam, and it could very well be, Im thrown off by the good compression and visually the rockers look to be moving the same, i have a video of them, Ill try to post it.

An exhaust lobe won't really affect the compression as it's not a sealing problem it's an opening/exhausting problem. You would have to set up a dial indicator to properly check or compare the difference in the lobes.

[clyde72]

O.k., so a problem has , as they say "manifested itself", wound it out pretty good leaving my driveway, 4500, resulted in a buncha backfires followed by noise, I turned around and slowly drove it back to the garage, noticing that it really wasnt running any different, same skip. noise was coming from #1 side so I pulled the valve cover finding an obvious problem, the pushrod on the very first valve was pushed right thru the rocker.
giving the history of consistant issues with the #1 cylinder what do you think is the cause, Ive had the cover off and have a video of that side running and there alway seemed to be oil shooting out of it

[Steeveedee]

Do you have some way of forcing the valve to move (engine off)? It may be seized? Binding could cause the wear you're seeing. See in the valve stem is the same height s the rest.

[57taskforce]

Been there done that. Over time the push rod cup on the rocker wears out sometimes. Mine was on an exhaust valve so it was backfiring thru the carb pretty good. Glad I don’t use Holley junk so no blown power valves. I just replaced the rocker arm and have driven it for close to 2 years since this happened.