Rough idle and takeoff 250 inline

[68gmcdude]

Originally Posted by '68OrangeSunshine

I asked at Don's Hot Rod Equipment in Tucson, but you should be able to get these anywhere:
Cornet....3/8 - Inverted Flange Plugs...p/n: 106-6....$1.60 [each] x 6 =$9.60

Prices may vary nationally. Other manufacturers may offer similar products with different prices and quality.

Thank you!

[geezer#99]

If you’re setting the valves, make sure to check all the rockers are moving the same amount.

[Rust_never_sleeps]

Maybe check that the points condenser is healthy with a multimeter. I know it's new, but chaotic timing and backfires might be caused by a dying one

[68gmcdude]

Originally Posted by geezer#99

If you’re setting the valves, make sure to check all the rockers are moving the same amount.

Will do. Plan on doing that after work tonight.

[68gmcdude]

Originally Posted by Rust_never_sleeps

Maybe check that the points condenser is healthy with a multimeter. I know it's new, but chaotic timing and backfires might be caused by a dying one

I will check that. I also have another condenser I can try putting on.

[68gmcdude]

Originally Posted by geezer#99

Redo the lifter adjustment.
Only do 1/4 turn from zero lash.

I readjusted the valves and set them a 1/4 turn from zero lash. When I did a 1/2 turn it was still idling a little rough. I did this with the engine running. The idle is smooth now. Before I put the valve cover back on, I'm going to let it sit overnight and start it again tomorrow to see if it's still ok.
So are the 1/4, 1/2 and 3/4 turns on a valve adjustment simply recommendations and each motor is different? I originally did a 3/4 turn on each valve.

['68OrangeSunshine]

Cut-Out Valve Cover. That's a Pro move!

[68gmcdude]

Originally Posted by '68OrangeSunshine

Cut-Out Valve Cover. That's a Pro move!

My buddy had one laying around. Makes a nice splash guard.

['68OrangeSunshine]

Originally Posted by 68gmcdude

My buddy had one laying around. Makes a nice splash guard.

Especially as the little spoon clips they sell for adjusting V8 heads just wobble off L6 rocker arms. I have an old valve cover to use iike that, too.

[Rust_never_sleeps]

Originally Posted by 68gmcdude

I readjusted the valves and set them a 1/4 turn from zero lash. When I did a 1/2 turn it was still idling a little rough. I did this with the engine running. The idle is smooth now. Before I put the valve cover back on, I'm going to let it sit overnight and start it again tomorrow to see if it's still ok.
So are the 1/4, 1/2 and 3/4 turns on a valve adjustment simply recommendations and each motor is different? I originally did a 3/4 turn on each valve.

The book says preload is 1 turn from zero lash for everything from 250-427
For SBC the consensus seems to be 1/2 or 3/4 turn, but for a battleworn six, I'd guess you're going to know better than anyone else where it's happiest

[68gmcdude]

Originally Posted by Rust_never_sleeps

The book says preload is 1 turn from zero lash for everything from 250-427
For SBC the consensus seems to be 1/2 or 3/4 turn, but for a battleworn six, I'd guess you're going to know better than anyone else where it's happiest

When I adjusted them the first time, I tried the one full turn like the shop manual said, but the motor really didn't like, it barely kept running, so I ended up doing 3/4 turn. At the time it seemed ok. When I readjusted last night, it didn't like the 3/4 turn, so I tried a 1/2 turn and it was still idling rough. I backed it off to 1/4 turn and it idled good. Gonna go back out there this evening, start it up, let it run and see how it goes. If all seems good, the valve cover is going on and I'll take a test drive.... We'll see.

[68gmcdude]

UPDATE:

I did a compression test on the cylinders. Here are the results...
#1 - 160
#2 - 165
#3 - 165
#4 - 165
#5 - 65
#6 - 160

Another thing to note. All of the valves were backed off and tightened 1/4 turn, except the fourth valve from the rear, which I believe is the exhaust valve for cylinder number 5. That one needed to be tightened almost two full turns before the lash would go away. I'm guessing something is going on right there.

What does this mean and what are my next steps?

[Steeveedee]

Check the height of the rocker stud, compared to the others. It could be pulling out of the head. Try a wet compression test. If that cylinder's pressure comes up it's rings leaking by. If not, it's an exhaust valve. I'm curious if you used a vacuum gauge on it. Is, was there a lot of fluttering? IIRC, the one you indicate is the intake valve. Did you check the compression after making the lash adjustment? It would be rough at idle and at low speeds, but the rotational inertia of the engine at higher rpm can cover up the weak cylinder to some extent.

[geezer#99]

Could be the rocker stud is pulling out of the head.
Or the cam is going flat.
Did you check all the rockers are moving the same amount?

['68OrangeSunshine]

Cam lobes can flatten if you're not cafeful to use oil with the right amount of ZDDP. Because the Zinc ruins the platinum in catalytic converters, Lube Manufactorers have steadily weaned it out of modern motor oils. Engines of Our vintage have no Cats and relied on the Zinc to minimize tappet/cam lobe wear.
I use Valvoline VR1 SAE 30 Racing Oil in my 292. It has the right amount of ZDDP. There are other ZDDP additives, including STP, but one also has to be careful not to ''overdose'' the zinc.

[68gmcdude]

[QUOTE=Steeveedee;9346511]Check the height of the rocker stud, compared to the others. It could be pulling out of the head. Try a wet compression test. If that cylinder's pressure comes up it's rings leaking by. If not, it's an exhaust valve. I'm curious if you used a vacuum gauge on it. Is, was there a lot of fluttering? IIRC, the one you indicate is the intake valve. Did you check the compression after making the lash adjustment? It would be rough at idle and at low speeds, but the rotational inertia of the engine at higher rpm can cover up the weak cylinder to some extent.[/QU

I'm assuming all the studs should be the same height? So I check by laying a straight edge across them all to see if that one is higher than the rest?

What is meant by a "wet" compression test? How is that done?

I apologize for what may seen like dumb questions, but I am new to a lot is this. Thanks!

[68gmcdude]

Originally Posted by '68OrangeSunshine

Cam lobes can flatten if you're not cafeful to use oil with the right amount of ZDDP. Because the Zinc ruins the platinum in catalytic converters, Lube Manufactorers have steadily weaned it out of modern motor oils. Engines of Our vintage have no Cats and relied on the Zinc to minimize tappet/cam lobe wear.
I use Valvoline VR1 SAE 30 Racing Oil in my 292. It has the right amount of ZDDP. There are other ZDDP additives, including STP, but one also has to be careful not to ''overdose'' the zinc.

This is the oil I've been using and I also use a lead additive when I fill the tank.

['68OrangeSunshine]

Originally Posted by 68gmcdude

This is the oil I've been using and I also use a lead additive when I fill the tank.

High Zinc is good. When or if you have your head in for a Valve Job you can request Stellite valve seats and burn unleaded without worrying.
Real Lead was a additive with actual TetraEthylLead that really made old engines zoom again. I think it's illegal now. It disappeared from shelves 24 years ago.

[RichardJ]

>>one full turn like the shop manual said, but the motor really didn't like, it barely kept running, so I ended up doing 3/4 turn. At the time it seemed ok. When I readjusted last night, it didn't like the 3/4 turn, so I tried a 1/2 turn and it was still idling rough. I backed it off to 1/4 turn and it idled good.<<

>>1/4 turn, except the fourth valve from the rear, which I believe is the exhaust valve for cylinder number 5. That one needed to be tightened almost two full turns<<

You had to go 2 turns below zero lash when adjusting 1/4 turn, but you did not when adjusting at 1/2 or 3/4 or 1 turn ? That inconsistency makes no sense. I would check for a broken valve spring.

At 2 turns below zero lash, a hydraulic lifter turns into a solid lifter. That alone could cause the "#5 - 65 psi". The adjustment could be holding the valve open or have even bent the valve. You are correct on the 4th rocker from the rear being the #5 EX.


You shouldn't have any difference between 1/4 and 3/4 turns, especially at idle. You should be able to make the adjustment anywhere in that range and never have to re-adjust within the next 200,000 miles.
You can deaden the sound of a ticking rocker by simply grabbing hold of the rocker with your fingers. I won't bite you. Make sure your adjusting socket isn't sliding down and making contact with the rocker and deadening the sound, throwing off what you think is Zero lash.

[Dashman]

Check the compression on your #5 cylinder again. It won't matter what your valve adjustment is as long as you know the valves are closing to get the compression. This is a separate problem than your valve adjustment. You could have low compression due to cylinder wear and piston rings, you could have valve train issues, or both. Valve adjustment is never going to get that compression any higher.

If you like your truck original, just find another 6 cylinder block to rebuild, continue to drive your truck, then swap the long block into your truck, and swap over all the accessories. A machine shop could rebuild someone else's head, or your own. ...or start planning a V8 swap.

[68gmcdude]

At this point, I'm thinking of just pulling the motor and having it rebuilt. Truck has 56k miles on it. Thoughts!

['68OrangeSunshine]

56K is only about a quarter of its projected potential. Equivalent to about 5 years as a DD with a short commute.
I ran a 292 to 250,000. Of course I hand built it using all the HD components I could find at the Chevy dealers. And I ran it on Valvoline SAE 30 Racing oil with 2000 mile changes. That build occured in 1977 when L25 parts were readily available from GM, if not in stock locally. By the 24th and 25th year, I had upscaled to SAE 50, and it was blowing blue smoke rings on every take-off from the light. A Sun scope in a shop showed only 4 cylinders were running, so I put the Stepside on jackstands and started rebuilding another 292.
All the same, a well maintained L6 is a long distance runner.
You may only need a Valve Job -- which is a euphamism for a Rebuilt Head.

Have a mechanic, qualified at the Old School, listen to it first.
Unless some clueless Grandma ran sand and gravel thru the engine at a 1000 miles a year, every year for the last 50, it shouldn't need a rebuild that young.

[68gmcdude]

Originally Posted by '68OrangeSunshine

56K is only about a quarter of its projected potential. Equivalent to about 5 years as a DD with a short commute.
I ran a 292 to 250,000. Of course I hand built it using all the HD components I could find at the Chevy dealers. And I ran it on Valvoline SAE 30 Racing oil with 2000 mile changes. That build occured in 1977 when L25 parts were readily available from GM, if not in stock locally. By the 24th and 25th year, I had upscaled to SAE 50, and it was blowing blue smoke rings on every take-off from the light. A Sun scope in a shop showed only 4 cylinders were running, so I put the Stepside on jackstands and started rebuilding another 292.
All the same, a well maintained L6 is a long distance runner.
You may only need a Valve Job -- which is a euphamism for a Rebuilt Head.

Have a mechanic, qualified at the Old School, listen to it first.
Unless some clueless Grandma ran sand and gravel thru the engine at a 1000 miles a year, every year for the last 50, it shouldn't need a rebuild that young.

Thanks for the input. Digging this deep into a motor is all new to me so my first thought is the entire thing needs rebuilt. I guess I'm jumping to conclusions. I will do a wet compression test to see what that suggests. If it suggests the rings are ok I will pull the head off and have it looked at.

[HO455]

One of the best troubleshooting tools for this would be a quality vacuum guage. Old school mechanics used them regularly to direct themselves towards the problem. Every Chilton and Hayes repair manual has a section on how to use one.

After reading your initial post my first thought was that you had a burnt valve. At this point if you don't want to use a vacuum guage and you want to check your compression do a leak down test instead. Properly done a leak down test will tell you if you have a valve sealing problem, a ring sealing problem, a bad head gasket or cracked block.

https://www.mobil.com/en/lubricants/...-leakdown-test

https://youtu.be/h993q16Oul4?si=t8FTGGfHq9_H6nNS

https://youtu.be/bfZvOxGt52o?si=tl6ty8ic1NNphIzc

[geezer#99]

Nice thing about a six banger is you can pull the side cover off and inspect your #5’s lifter.
A wet test on compression won’t bring that #5 bore up much if any.
If the lifter looks good then suspect a bad valve.