Rough idle and takeoff 250 inline

[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]

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

[68gmcdude]

[QUOTE=HO455;9346799]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.

I just ordered a leak down tester. Thanks for the links. Great info.

[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.

[RichardJ]

>>I just ordered a leak down tester.<<

Did you back that lifter away from the 2 extra turns and recheck the compression?

Please do that before buying a tester.
If you want to do the same thing a leak down tester will do without buying the tester. Put the piston on the cylinder to be tested, at TDC. Wrap a small rag around an air nozzle to seal it and blow air into the spark plug hole. Do remember there are two TDC for every piston.
If you have a broken ring, You will hear air coming out the oil fill hole on the valve cover.
If you have a bad Exhaust valve, you will hear air coming out the tail pipe.
If you have a bad Intake valve, you will hear air coming out through the carb.
If you have a bad head gasket, you may see air bubbles in the radiator.

A leak down tester will do the same thing except give you some numbers, which won't mean a damn thing anyway.

You do have a compressor for the leak down test? Sorry, just asking as I don't know.
Some guys don't, but I always use some means to lock the crank from turning for a leak down test. You put 100 psi on a piston and if it wants to go down. it will. The first time it happens, you may need to change your shorts.

[68gmcdude]

Originally Posted by RichardJ

>>I just ordered a leak down tester.<<

Did you back that lifter away from the 2 extra turns and recheck the compression?

When I back off the two full turns the rocker arm is very loose.

[68gmcdude]

Update..

Got the cylinder head off today.

The head gasket looked decent for what I'm guessing is original. It came off in one piece. The #6 cylinder had some antifreeze in it but I'm guessing that's from pulling the head? I did drain the radiator first.

Pulled out the push rods and put them in order on a piece of cardboard. They all looked straight except the rod for the exhaust valve on cylinder #6. It was bent. The rods for cylinder #5 that has the low compression were ok.

The surfaces of the intake valves look nice and smooth. The surfaces for the exhaust valves are the opposite. They are not smooth. The surfaces are very rough.

The cylinder walls look good. No scratches or gouges that I can see. When I turn the motor by hand it all moves nice and smooth with no scraping sounds. The very tops of the cylinder walls seem to have a little buildup on them. Can that be cleaned up with some very fine crocus cloth?

The tops of the pistons look ok, I think? Just dirty. Maybe need some cleaning up.

The gasket between the cylinder head and the intake/exhaust manifolds looked good. No cracks or missing areas. However, the gasket that goes between the two manifolds was split in one spot. That was a gasket I replaced about 4 years ago.

Thoughts or suggestions?

[geezer#99]

Post a pic of your head so we can see the valves.

[68gmcdude]

Pics of valves...

[geezer#99]

Exhaust valves do look burnt.
Likely from excessive heat caused by retarded timing.

[68gmcdude]

Originally Posted by geezer#99

Exhaust valves do look burnt.
Likely from excessive heat caused by retarded timing.

Complete valve job recommended? Is this causing low compression in cylinder #5?

['68OrangeSunshine]

Originally Posted by 68gmcdude

Complete valve job recommended? Is this causing low compression in cylinder #5?

Yup. Valve Job. I recommend you replace the originals with Stellite Valves and seats. They are resistant to unleaded gasoline.

If you wanted to go Hot Rod, you could up the ante to I.84 Intake and 1.6 Exhaust valves, but maybe you better stay in the conservative lane. You would have to get a bigger cam, 4-Bbl carb, aftermarket intake and L6 Headers, to balance the higher performance.
It's a direction, you might think about. Unfortunately it's a lot more expensive now, than when I went for the Hi Perf package: 4 Barrel Holley, Offenhauser Intake, Clifford headers, .030-over cylinder bore, Forged Pistons.

[67 twins]

Originally Posted by '68OrangeSunshine

Yup. Valve Job. I recommend you replace the originals with Stellite Valves and seats. They are resistant to unleaded gasoline.

If you wanted to go Hot Rod, you could up the ante to I.84 Intake and 1.6 Exhaust valves, but maybe you better stay in the conservative lane. You would have to get a bigger cam, 4-Bbl carb, aftermarket intake and L6 Headers, to balance the higher performance.
It's a direction, you might think about. Unfortunately it's a lot more expensive now, than when I went for the Hi Perf package: 4 Barrel Holley, Offenhauser Intake, Clifford headers, .030-over cylinder bore, Forged Pistons.

Yes, but he doesn't have to do that all at once. If he does indeed need a valve job, now would be the best time to go to larger valves & some port work if he ever wants to go that route. He can do intake, carb, cam & headers as he can afford them.
That's what I'm doing on mine. Mine needed a valve job so I had 1.84 & 1.6 valves installed with a little port work. I do have the cast dual exhaust manifolds but the rest has to wait. Probably won't do a cam until I build a short block with 12bolt.com new hypereutectic pistons.

[68gmcdude]

I've made the decision to do a full rebuild of the motor. Pulled it out today.

['68OrangeSunshine]

Originally Posted by 67 twins

Yes, but he doesn't have to do that all at once. If he does indeed need a valve job, now would be the best time to go to larger valves & some port work if he ever wants to go that route. He can do intake, carb, cam & headers as he can afford them.
That's what I'm doing on mine. Mine needed a valve job so I had 1.84 & 1.6 valves installed with a little port work. I do have the cast dual exhaust manifolds but the rest has to wait. Probably won't do a cam until I build a short block with 12bolt.com new hypereutectic pistons.

I wasn't suggesting he do it now. Just to think about it.

On my first build [1977] I went all stock, except for any HD parts the Chevy dealer had for the L25. I also bored out the cylinders to Thirty Over and got TRW Forged Pistons. I was in the military and had the base Special Services Hobby Shop Garage, plus 10% off parts and 20% off machine shop work. My Marine Buddies helped a lot. I built up a junkyard 292 [1967] block. When it was ready, we swapped accessories over and put up the OEM '68 block in crates. In 1978, now discharged, I found I needed higher performance just to survive local driving conditions in Tucson. Since I'd just built that engine, I wasn't talked into swapping in a 350. I qualified for a student grant, and popped it on a 390 Holley, Offy intake and Clifford headers. [School officials got wind that I'd ''squandered it on Chrome'' and I was disqualified for further grants, but Screw Them. No chrome was involved.] Anyway that '67 block lasted 25 years and pulled the Orange Stepside thru 250.000 miles.
In 2002, I read Leo Santucci's book, ''Chevrolet Inline Six Power Manual'' and proceeded to rebuild the OEM engine that was dissassembled but perfectly preserved in avionics crates I'd stowed it in. This time due ro work interfering, it tooks 3 years to complete the project. Santucci had the Lump Port Head trick outlined. Intriguing, but totally removing the head bolt bosses for big flush screws and airfoil ''Lumps'' was too radical a move for me. I did upsize to 1.84'' -- the stock valve size for 307 V8s. I got a Crane 260H cam. Rhoads lifters, too. [But my machinist said I didn't have a big enough cam lift to need them.] Also a Melling High volume oil pump. [Again ''Experts'' tell me the Stock piece is good enough. Hasn't failed yet, and my Oil Pressure is good 19 years later.] Both the Clifford headers and Offy intake cleaned up nicely and went into the 2005 motor. The Holley R8007/4160 4BBl, w/390 CFM gave up the ghost after 250K miles. I couldn't get a Carter AFB, so I got an Edelbrock 1404 w/ 500 CFM. Badger cast pistons, 030-over -- no forged were available.
I haven't seen the 2nd Edition of Santucci's book, but the ''New Trick'' I hear, is to feather the head bolt bosses into an airfoil themselves, eliminating the risks of an unsupported upper head.
The Stepside doesn't need an engine rebuild yet, but I have bodies of a '67 K/10 Suburban, and a '72 K/5 that will need fresh powerplants in the future. Sold as I am on the 292, I'm inclined to put L25s in them.
I have two cores. One already ran 250K @ .030 over, but another over-bore to 060-over would yeild a 301 CI block. The other one came out of a farm rig, and may not have been rebuilt. It's a '63, and still in green paint.

[geezer#99]

Before you rush off to rebuild that head, did you inspect the bottom of the lifters?

[Carl Spangler]

Head doesn't look bad but #5 looks 'wet'. Could be from excess oil via busted/stuck oil control ring. You're this far in you might as well pull #5 piston and inspect the rings/bore.
JMO.