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

[RustyPile]

Originally Posted by 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

Carburetor issues very seldom cause a particular cylinder to misfire. Seldom enough that carburetor replacement should be far down on the suspect list.. Doesn't miss until the engine is up to operating temperature is a good clue.. It's probably a lean miss due to a vacuum leak on one of the runners. During initial start up and running, the choke is partially closed creating a rich condition thus "masking" the miss. Check for any cracked, broken, or disconnected vacuum hoses.. Don't forget the vacuum source for the transmission modulator valve if so equipped. Intake gaskets are another source for a cylinder related vacuum leak.

{EDIT} Go back and properly adjust those valves before you damage a lifter, valve stem, or rocker arm...

[clyde72]

thanks, you folks got me to stop chasing my tail and narrow things down, stopped thinking carb, Ive done what I consider a pretty thorough vacumm leak check, the intake all seems sealed good ,of course I cant see or test the back. I took the valve covers off today and all rockers are definitely moving the same, I adjusted them up tighter last nite.
This morning I threw in a pint of stp along with a pint of Lucas oil stuff...started it up and went on some errands...it ran on 8 for over 20 miles!!!!
I could feel a miss coming back slowly as I was getting home, I read the headers with the temp gun and #4 was even with the rest and #1 was hotter than Ive seen it.
So heres the latest theory, (theres been many) as HO455 suggests, its a lifter problem. Im really not interested in rebuilding the 350, been there with previous trucks and my kids are hot on the LS swap, but Id like to get the summer out of it without being too annoyed. I have no issue time and money wise to pull the intake and drop in new lifters...
Bad Idea or worth a shot?

[Steeveedee]

I wouldn't bother with new lifters. I don't think that it is actually valve train related. They may last until summer, but the difference in the grind angle on the old cam and new lifters after this much wear is pretty sure to wear them down, possibly before then. Which Lucas oil product did you use? If it's thin and some kind of cleaner, it'll defeat the purpose of the STP. Summer ain't far off. Maybe just nurse it along?

[clyde72]

I used the thick gooey Lucas, I could drive it the way it is but the wife and I had a couple rallies planned with the 74 camper, they make a good looking combo, the stock orange stripe on the Shasta matches the truck , it's really annoying with a load , but Im not going to bother if it wont help.

[Dashman]

You know there's a procedure to adjusting valves, right? ...setting certain pistons to TDC and adjusting certain intake and exhaust valves, right? Just checking.

You've checked the condition of the #1 and #4 plugs after it's been missing? So you're sure it's not coolant diluting the charge, right?

What if you replaced the camshaft, lifters, timing chain, and gaskets? Seems like it's a decent engine, if you've been towing with it. You could be done with it in a weekend perhaps. Then when you do your LS swap, you could sell it as a running engine. There's risk involved if you go through the effort and expense, and it's not issue though.

[clyde72]

Ive had them adjusted by a real mechanic a while back, but ive been messing with them so much it wouldnt hurt to do it again, Im not seeing any sign of a head gasket leak, the #4 plug always seems to be relatively normal, light brown, # 1 is usually dry black.....,
I have thought about the cam, lifter job, it does run quite nice when its got all 8....just trying to justify the time..
Question, whats a running 350 with a new cam, ignition worth these days?

[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.

[clyde72]

my stubborn self just wasted more time trying to find the lifter with the pushrod to put on a rocker i had here, I cant seem to land it, who knows if the lifter is still in the hole,...Im thinking its time to throw in the towel on this one, its been annoying me because ive been thinking it might be something simple where I wouldnt be tearing into the motor which would end up as a total rebuild, and i have/had a thought I might keep the truck more stock with the 350, Ive had a few smallblock quadrajet vehicles in the past and have good memories along with bad. The amount of time Ive put into chasing this down I couldve had it yanked and been well along with detailing the engine bay for the 5.3., which is going to make it a better truck, why am I fighting to preserve my 9 mpg motor...
I think its time to switch over to the "LS swap" threads. Thanks for all your help.

[clyde72]

O.K.,after picking up the 5.3 and reviewing the work thats going to be needed to do that right,(detail engine compartment, dod delete, fuel tank, etc) I,m putting that off till winter, maybe not even this winter. I couldn't resist taking it apart...definitely some worn cam lobes and a broken valve spring, decided to do the cam and lifters, dropped the heads at the machine shop, rest looks good , no ridge in the walls etc., Will be a good motor for our 57 4wd long bed when I take it out. The 400 thats in there from when we got it is pretty cobbed up.

[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.

[clyde72]

more drama, put in a mellen rv cam, new lifters rockers, push rods ,recommended by machine shop, heads redone, did the whole breakin oil, no idle routine, had the valves , timing set, fired right up, ran the breakin process....50 miles later, starts ticking...oh ****..pull the valve cover, #8 intake rocker loose.. maybe I adjusted wrong, tighten it up, 20 miles later ticking again, same rocker. I pulled the lifter, large amount gouged out....fk me...
Machine shop says been happening..lotsa bad cam/lifters, I,done plenty years ago and never had a problem, no "break in ", start ,em up and drive away..
I dont want to just change cam lifters for two reasons.How do I know it wont happen again,and what about the ground up metal in the motor. what a shame, 52 year old motor f*2ked..
machine shop guy is "thinking about" what,s next...not really too upset with him, he didnt build the stuff, and I installed it so he really doesn,t have much skin in the game other than hes done all our work for years.