Rough idle and takeoff 250 inline

[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.

[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.

[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.

[Steeveedee]

#5 intake appears to have a worn guide and or valve, as the darkening is generally from oil leaking by. Ain't real bad, you might get away with new valve seals, though it is only a matter of time.

[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.

[67 twins]

Originally Posted by 68gmcdude

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

Well now is the time to drop a 292 in there instead. You will love the extra torque.

['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.

[RichardJ]

A couple of quick questions?

You put the head back on before pulling the block? I'd rethink those lift points. If the head is torqued to the block during the rebuild, you don't want to remove those bolts before placing the long block back into the chassis.
You left the radiator in place while pulling the block? There are four bolts right there on top and the radiator simply lifts out.
You stated earlier that the Int/Ex manifold to head gasket was not leaking. That doesn't appear to be the case.
I've never seen anyone remove the lower mount brackets with the engine. Not wrong, just never seen it.

[68gmcdude]

Originally Posted by RichardJ

A couple of quick questions?

You put the head back on before pulling the block? I'd rethink those lift points. If the head is torqued to the block during the rebuild, you don't want to remove those bolts before placing the long block back into the chassis.
You left the radiator in place while pulling the block? There are four bolts right there on top and the radiator simply lifts out.
You stated earlier that the Int/Ex manifold to head gasket was not leaking. That doesn't appear to be the case.
I've never seen anyone remove the lower mount brackets with the engine. Not wrong, just never seen it.

I won't put it back in using those lift points. But which bolt holes should I use?
I didn't have any issues getting it out with the radiator in there.
The intake/exhaust manifold gasket to the block looked ok when I took it off. The small square gasket between the manifold was cracked.
Seemed easier to me to just disconnect the lower mounting brackets.
I'm learning as I go...

[RichardJ]

At this point, I assume you are trying to decide on rebuilding or buying a remanufactured engine.
Two comments, no matter which way you go. There are two threaded studs and also two locator pins on the side of the head. They are threaded into the head. Remove them now and save. Head rebuilders remove them and never put them back in.
Engine rebuilders will sometimes install a fiber cam gear to save a few penny's. Specify an aluminum cam gear.

[68gmcdude]

Originally Posted by RichardJ

At this point, I assume you are trying to decide on rebuilding or buying a remanufactured engine.
Two comments, no matter which way you go. There are two threaded studs and also two locator pins on the side of the head. They are threaded into the head. Remove them now and save. Head rebuilders remove them and never put them back in.
Engine rebuilders will sometimes install a fiber cam gear to save a few penny's. Specify an aluminum cam gear.

I've decided to have my original motor rebuilt. A shop about an hour away from me that rebuilt a motor for me in the past and several motors for a friend of mine that has a body shop.
I'm aware of the two pins on the side of the head and I'll be sure the remove them. I didn't realize they screwed in. I don't recall seeing the two studs. I'll look for them. Thanks.

['68OrangeSunshine]

Those Pin/Studs are threaded... Can't recollect thread and fastener size but they are a common size, and if you should ever want to go with long tube headers, you can use bolts in those holes on the ends.

[67 twins]

Originally Posted by '68OrangeSunshine

Those Pin/Studs are threaded... Can't recollect thread and fastener size but they are a common size, and if you should ever want to go with long tube headers, you can use bolts in those holes on the ends.

3/8-16 I used studs in all the holes when mounting my cast dual manifolds. Makes it easier to get everything lined up while installing. If you don't mind me asking, what kind of mufflers are you running?

['68OrangeSunshine]

Originally Posted by 67 twins

3/8-16 I used studs in all the holes when mounting my cast dual manifolds. Makes it easier to get everything lined up while installing. If you don't mind me asking, what kind of mufflers are you running?

I was told they were turbos. That was about 40 years ago. I have dual exhausts with a diagonal crossover. An ''N'' Pipe if you will...

[RichardJ]

Originally Posted by '68OrangeSunshine

I was told they were turbos. That was about 40 years ago. I have dual exhausys with a diagonal crossover. An ''N'' Pipe if you will...

I assume you did that because of the staggered collectors on your long tube headers. I had about 6" stagger and wanted the crossover to exit the EX pipes at 90*, and equal distance from the collectors. I also wanted to be able to separate the two Ex pipes for easier removal. I carried the 6" stagger all the way through the mufflers.

Getting way off topic here.

[68gmcdude]

Originally Posted by RichardJ

There are two threaded studs and also two locator pins on the side of the head. They are threaded into the head. Remove them now and save. Head rebuilders remove them and never put them back in.

I talked to my engine builder. He will be putting the studs and the pins back on because he will be breaking in the motor and attaching the manifolds when running it