Guess the problem
Thread Starter
Race Director
Joined: Feb 2004
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From: CT
Guess the problem
Engine has been running rough in the '62. The engine is a 70 LT-1 short block, Dart Heads, headers, weiand manifold, Hei dist. All added components are are only a few years old.
Engine started to run rough during the summer on a trip. Most noticed at low to mid RPM. It is not the carb, not the ignition. Today I had a chance to check the compression. All cylinders are right around 150lbs, except #3 which is 60 or less. There is no fouling of the spark plug, no valve train noise like a broken rocker or spring. There no smoke from exhaust so no burned piston or broken ring.
I haven't taken the valve cover off as yet, but was wondering what others might think the problem might be.
Engine started to run rough during the summer on a trip. Most noticed at low to mid RPM. It is not the carb, not the ignition. Today I had a chance to check the compression. All cylinders are right around 150lbs, except #3 which is 60 or less. There is no fouling of the spark plug, no valve train noise like a broken rocker or spring. There no smoke from exhaust so no burned piston or broken ring.
I haven't taken the valve cover off as yet, but was wondering what others might think the problem might be.
Instructor
Joined: Jan 2012
Posts: 103
Likes: 0
From: Sonoma Ca.
60 lbs is alot if its a guess I guess compression ring or if it has a hydro lifters a sticky lifter,valve, weak valve spring, leak at intake gasket at #3,while running spray some carb cleaner around #3 see if it picks up, pull the valve covers run the starter and see what the valve train does
Le Mans Master
Joined: Feb 2008
Posts: 6,482
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From: Folsom CA
If it is quiet, lifters are working but might be adjusted poorly.
If it is low on compression an intake leak would not affect compression in any way.
Weak valve spring, should show up at high RPM with a floating valve.
Be interesting to find the answer on this one.
If it is low on compression an intake leak would not affect compression in any way.
Weak valve spring, should show up at high RPM with a floating valve.
Be interesting to find the answer on this one.
Skip Shift
Joined: Jan 2007
Posts: 2,031
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From: Citrus Hills, Florida
Might try a leak down test to see if cylinder can hold compression with valves closed. If no big leak and rocker/rocker stud/pushrod look OK then maybe a cam lobe but I would be skeptical. Listen to dipstick to hear if air is going into crankcase or carb to see if is going topside. Compression is leaking out somewhere at only 60lbs. Let us know what you find.
Fred
Fred
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From: Canada's capital
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I suspect a burnt valve/seat, not enough to smoke yet, or a scuffed wall! Did it get overheated ever? 
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Melting Slicks
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From: ct
George. This may sound way to simple but it can't hurt to try. Pour some water down the carb and keep the motor running. I'm sure you know this trick. You may just have some carbon build up on an exhaust valve keeping it open just alittle. I know when your car was here it did run alittle on the fat side.
If you lost a lobe I would think you would here it in the valve train.
Let me know what you find. If I can help in any way let me know.
Brian G.
If you lost a lobe I would think you would here it in the valve train.
Let me know what you find. If I can help in any way let me know.
Brian G.
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
Engine has been running rough in the '62. The engine is a 70 LT-1 short block, Dart Heads, headers, weiand manifold, Hei dist. All added components are are only a few years old.
Engine started to run rough during the summer on a trip. Most noticed at low to mid RPM. It is not the carb, not the ignition. Today I had a chance to check the compression. All cylinders are right around 150lbs, except #3 which is 60 or less. There is no fouling of the spark plug, no valve train noise like a broken rocker or spring. There no smoke from exhaust so no burned piston or broken ring.
I haven't taken the valve cover off as yet, but was wondering what others might think the problem might be.
Engine started to run rough during the summer on a trip. Most noticed at low to mid RPM. It is not the carb, not the ignition. Today I had a chance to check the compression. All cylinders are right around 150lbs, except #3 which is 60 or less. There is no fouling of the spark plug, no valve train noise like a broken rocker or spring. There no smoke from exhaust so no burned piston or broken ring.
I haven't taken the valve cover off as yet, but was wondering what others might think the problem might be.
pull the valve cover and check the action on both intake and exhaust valves...check lash for too tight clearance on the valves for that cylinder
apply air to that cylinder with both valves closed, this will show the condition of the valves, seats and rings
could also be a head gasket but im leaning more towards a lack of air in the intake charge for adequate compression or a leaky exhaust valve...jmo....
Thread Starter
Race Director
Joined: Feb 2004
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From: CT
I'm leaning towards the burnt valve. Don't think it is a lobe as it's the original cam and always had good oil and good pressure. Inside of engine very clean. If it was a broken spring there would be more noise and would be more noticable at higher rpm. I do remember when I check that plug on the trip it was bright white as running too lean. Other plugs were normal
Instructor
Joined: Jan 2012
Posts: 103
Likes: 0
From: Sonoma Ca.
60 lbs is alot if its a guess I guess compression ring or if it has a hydro lifters a sticky lifter,valve, weak valve spring, leak at intake gasket at #3,while running spray some carb cleaner around #3 see if it picks up, pull the valve covers run the starter and see what the valve train does
Last edited by anips; Feb 10, 2012 at 12:40 PM.
Le Mans Master
Joined: Feb 2008
Posts: 6,482
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From: Folsom CA
Good tips above.
White spark plug, good indicator. Very Lean burn, no oil in burn. Rings are fine.
Head gasket, Lean burn makes detonation and that could cause a head gasket failure.
This much compression leaking will presurize the Radiator, I did not see any mention of rapid loss of fluid in radiator. I drove an engine with one cylinder leaking in the head gasket for a couple months while building the replacement engine. I drilled a small hole in the Radiator cap to allow the pressure to release and it worked fine till the new engine was installed.
If a cam lobe is worn bad enough to kill your compression there would be a lot of valve train noise.
The Water trick may work if this is carbon issue but do it slowly so you dont bend a rod or break a piston.
I would start with a simple valve adjustment on that cylinder and see what you find.
Good advice above to pressurize that cylinder with the cyl on TDC will identify where the air is going. Your symtems indicate exhaust leak as an intake leak would be heard in your carburator. Hope the valve is not burned and with your low mileage it may not be.
White spark plug, good indicator. Very Lean burn, no oil in burn. Rings are fine.
Head gasket, Lean burn makes detonation and that could cause a head gasket failure.
This much compression leaking will presurize the Radiator, I did not see any mention of rapid loss of fluid in radiator. I drove an engine with one cylinder leaking in the head gasket for a couple months while building the replacement engine. I drilled a small hole in the Radiator cap to allow the pressure to release and it worked fine till the new engine was installed.
If a cam lobe is worn bad enough to kill your compression there would be a lot of valve train noise.
The Water trick may work if this is carbon issue but do it slowly so you dont bend a rod or break a piston.
I would start with a simple valve adjustment on that cylinder and see what you find.
Good advice above to pressurize that cylinder with the cyl on TDC will identify where the air is going. Your symtems indicate exhaust leak as an intake leak would be heard in your carburator. Hope the valve is not burned and with your low mileage it may not be.
Thread Starter
Race Director
Joined: Feb 2004
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From: CT
I do intend to dig further. Takes me awhile though as car is in an unheated barn and others must be moved to get to it. I will start with taking the valve cover off, then if needed intake and then head. I'm guessing now that it is a bent or burned exhaust value. Past couple of times briefly running the car there was a sheen of something (fuel?) coming from the exhaust on the #3 side. It did not leave an oily trace. If need be I'll buy a new head or maybe a new set. I let you know the progress and what I find.
Thread Starter
Race Director
Joined: Feb 2004
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From: CT
Pulled the valve cover off today. Appears to have wiped the lobe off of intake #3. No movement of pushrod when cranking. With the low compression I thought it would have been a hung open valve. There is no gas in the oil so it might have just happened. It had been acting up and getting worse, but I didn't drive the car once it got real bad (total wipe of the lobe). Next will be to pull the intake to pull the lifter and check the lobe. If it is just the cam any suggestions as to a replacement - solid, hydraulic or ? It is a stock LT-1 cam that came with the short block. Hard to see why it would happen now as I always use proper oil and change oil & filter every 1,000 miles. Block has about 40k miles on it now (installed in '72).