Valve trouble, diagnostic help

[Ride The Snake!]

Hello everyone,

Wondering if anyone can shed some light on my situation. In advance, please forgive my lack of knowledge and experience, as I am truly a newb home mechanic. Done little jobs on my old car and this truck for a few years but never any motor work.

1970 GMC C1500 with some sort of stock 350 manufactured between '77-79, heads from '74, and 2g carb (no choke). I bought the thing for a rack and it had no history, and the PO kinda shrugged his shoulders about where the motor came from. But it ran strong as hell so I was sold. I always loved my dad's '71 'Burb so I really wanted onna these.

I developed a miss that seemed to happen over time. I realize now that the rebuilt carb I bought had a float height set to keep the fuel bowl too full, as wetness forms outside the housing through the upper gasket (but does not flow out lower through the throttle plate overflow holes). In short, I've been running a rich condition for awhile now I guess. Oops.

Did a vacuum test and had a consistent needle bounce over about 4 inches. 20 inches at idle, dropped to 0 on acceleration, bounced to 30 on rapid deceleration. Slowly accelerating did not steady the needle bounce. According to SecondChanceGarage, this means I have a sticking valve. Just glad it wasn't looking like blown rings from the rich condition.

After pulling the plugs, which all had some degree of carbon fouling, #6 looked particularly bad. I don't have enough of an eye/nose to see if there was oil present on the plug vs gas/carbon, but it had some wet, chunky black deposits. Valve cover gaskets were leaking oil on all the plug threads too--replacing those now.

Compression test read 150 psi on all cylinders save for #1 (160 psi??) and #6 (135). I forgot to block the throttle open during the test. I didn't perform a leakdown.

Yanked off the valve covers and cranked it, turns out #6 intake valve is barely moving. It does move, but not a whole lot. Wasn't sure what to make of it, so went through and adjusted the lash on all the valves as Chiltons describes for '72 and later motors. No change. Pulled the pushrod and it had some minor wear but wasn't bent; rolled on a flat surface well. This was my stopping point today.

Obviously, I need to go deeper. Should I be investigating the cam lobe and lifter for wear? My logic tells me it sounds weird for a single cam lobe to wear out by itself, but maybe lifters do? Or is this more likely a burnt valve? I would think if the valve was burnt or bent or whatever, there'd be some noticeably bad noise, and I can't hear any. And as stated, there is [I]some[I] degree of movement in the valve.

Just from my naked eye, it looks as if the rocker arm isn't being actuated fully by the pushrod. I'm hoping it really is just a worn lifter. But why would a single lifter wear out?

[70SBC10BOB]

For being a new home mechanic, you nailed it! If #6 valve is not moving up and down much = flat cam and/or bad lifter.

Replace the cam and lifters and rock on.

Nice job of diag.

Bob

[Ride The Snake!]

Cheers, and thank you for the compliment! Ugh, so I have to replace the entire cam? Or is it possible only the lifter is worn?

[Ride The Snake!]

Guess it'll be a good time to change my timing chain too, which has about 10-11 degrees of slack as I measure on the crank and dizzy rotor.

edit: Just read the Wiki entry on hydraulic tappets. Seems they're vulnerable to oil quality, and my motor definitely seems to have a fair amount of sludge, despite my frequent enough oil changes. Guess I'll try cleaning the pan and do a short-duration interval subbing one quart with ATF.

[john]

Make sure the rocker stud is not puling out. they will do the same thing.

[Jurassic-1]

I agree with the possible flat cam. I hear with the lack of additives we should be adding Zinc to our oil to help prevent cam wear. Some people say Rotella still has enough additives. My engine rebuilder disagrees. I purchased the additive on line in 4 OZ bottles I just received it and plan to change the oil in my old vehicles and add it. Any thoughts to this?

[87Skier]

I don't remember where I heard this but supposedly zinc is having less of an affect because ethanol fuel breaks it down.

[Ride The Snake!]

Originally Posted by john

Make sure the rocker stud is not puling out. they will do the same thing.

What do you mean by pulling out? Isn't the stud threaded on both ends? You mean it just works its way loose? I'm not sure I understand.

['68OrangeSunshine]

If you have not done this before, get a cardboard box and some empty egg cartons, and a Sharpie. Mark the carton so you know what lifter came out of what hole. You will want to reassemble them back in the same sequence. Same with the pushrods.
Get some black moly lube to coat the lifters when they go back in. That's if you reuse the old parts.
Sorry if You've heard all this before, but you did admit to being a "Newb."
New cam means new lifters. Not sure if old pushrods will be OK. Roll them on a flat surface and see. Not sure what the experts will say. You could go whole hog and get all new, or you could save a buck and not replace something that ain't broke.

[trac209]

Probably a issue mentioned above but I highly doubt zinc content has anything to do with cams going bad. That whole zinc debate is a crock as far as I'm concerned and a sales gimmic used to sell oil. Zinc was removed due to cat converter fouling issues. Oil that's made today is just a good a lubricant as years ago and zinc isn't a magic additive to save your old flat tappet cam engine. I've logged tons of miles on old sbc' with flat tappet cams with off shelf oils without issue.

[hamjet]

I feel its a flat lobe. if it were the lifter, whether colapsed or worn down it would be ticking when running.( I didn't see any mention of that). A single lobe can wear if the lifter stops rotating, or real dirty oil can speed wear.

[Ride The Snake!]

Thanks for all the input, guys. Tomorrow I'm going to be pulling the intake and taking a gander. I'm guessing I'll have to pull the cam itself out in order to judge its wear. Hopefully I can manage to do that as well. And I'm going to replace the timing chain while I'm in there.

I guess now I have to decide what sort of cam to get. Any recommendations for moderate gains on a stock motor without sacrificing all the bottom end torque?

[bigrigpardo]

the stud is pressed into the head and on rare occasion work itself loose. put a straight edge over the tops of the studs and see if that one is higher than the rest.

[geezer#99]

If you have a flat lobe and it's likely you do, you gotta be aware that all those fine metal shavings never got caught by the oil filter. They're sitting everywhere in your motor, especially in your crank and rod bearings.
You can take a chance and just replace the cam and lifters and timing chain, but it's likely they'll be wasted parts when you turn a bearing. Then it'll be motor rebuild time.
It's motor refresh time now. Torn down, hot tanked, flushed out, polished crank and new bearings along with the new cam etc. Pistons should be good if you didn't wash the cylinders to much from your leaking carb. But you might have scuffed a skirt or two.
So, toss a coin!!
You can spend a little now, or a bunch later!!

[Ride The Snake!]

Originally Posted by geezer#99

If you have a flat lobe and it's likely you do, you gotta be aware that all those fine metal shavings never got caught by the oil filter. They're sitting everywhere in your motor, especially in your crank and rod bearings.
You can take a chance and just replace the cam and lifters and timing chain, but it's likely they'll be wasted parts when you turn a bearing. Then it'll be motor rebuild time.
It's motor refresh time now. Torn down, hot tanked, flushed out, polished crank and new bearings along with the new cam etc. Pistons should be good if you didn't wash the cylinders to much from your leaking carb. But you might have scuffed a skirt or two.
So, toss a coin!!
You can spend a little now, or a bunch later!!

This sounds like it's becoming quite an endeavor. Yikes. I can't afford a shop to pull the motor out and apart, so I'd be looking at renting a cherry picker and taking it to a shop to tank it. IF I was to do all the major labor myself, what kind of cost am I looking at here? This is starting to sound over my head and/or budget, but I'm not running away yet.

You are saying I would reuse the crank, but it would be "polished"?

[Ride The Snake!]

How long do you all estimate it would take me and my dad to tear the motor apart? My old man has some experience rebuilding an engine--he rebuilt his Alfa GTV motor about 20 years ago, and has always wrenched on his cars as need be, but he's not a mechanic by trade--rather, a cabinet maker.

[geezer#99]

LIke I said, you take your chances!!
I did that 40 years ago!!
Went a month before the rod bearing went.
You might get lucky, you might not!
If the bearings haven't started to score the journals, you can just polish the crank. Polishing is done with super fine emery cloth. JUst takes off a hair of material compared to cutting the journals enough for new bearings.
But don't jump to conclusions. Do some exploratory first. Pull the valve covers off and check the rocker studs are all the same height.
Then if that checks out, pull the intake and look at the cam, lifters. A bad lobe is usually easy to see.
Take pics and ask more questions as you go. Lots of sharp people here.
If you've never done this tear down before, ask before you tackle something. We can save you a lot of grief with tips now. Could save you head scratching later.

[Ride The Snake!]

Good points. This will be my starting point tmrw. Measure the studs, pull the intake and look at the lobes and lifters. I'll take some pics of what we find.

I feel some degree of confidence knowing you guys are such a wealth of knowledge here should I get stuck. Already, I can't thank you enough for your sage advice.

[geezer#99]

Do you need any tips about pulling the intake?
Have you done it before?

[Ride The Snake!]

No, I haven't. Anything I should be aware of?

[geezer#99]

Several things!!
All to do with the distributor.
But lets wait till you check the rocker studs.
One thing at a time.
Once you get the covers off, take some pics. We might see something out of character.
Remember, slow and easy!

[Ride The Snake!]

I have already pulled the covers and observed the valves during cranking. The #6 intake valve's rocker arm moves only a small degree. It looked like the pushrod wasn't actuating it fully. I went through and adjusted the lash on all of them, but didn't yet measure the studs, which I'll do tmrw. Valve covers are still off, since the thing is sitting inside my dad's wood shop.

I have pulled the dizzy before, back when I rebuilt it. Anything to know besides noting its orientation?

[geezer#99]

Well then you've got the dizzy covered!
Burn us off a pic of what you have right now.
If all the studs look good, then pop the rocker off #6 and check the pushrod movement.
Hold the pushrod down as you crank it so it won't pop out of the lifter.

[tcrist]

Unless you really know what you are doing it would be best to either have yours rebuilt or purchase a long block. Unless you have someone around that has done it before and you really want to learn how to rebuild an engine. But how often are you going to build an engine.

There are quite a few engine rebuilders in the Bay Area. Some might have long blocks in stock. Could save you time and $$. At least it is worth checking into.

[Ride The Snake!]

Originally Posted by tcrist

Unless you really know what you are doing it would be best to either have yours rebuilt or purchase a long block.

There's no way I have the money for a new motor or a rebuild.

The only thing I think could be viable for me is doing much of the tear down myself and having a shop clean it and refit the rod bearings.

Or, throwing a cam in there and seeing how long I go before I turn a bearing.