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Old 06-18-2010, 09:47 PM
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Bad Valve?...How do yah tell

Me and my friend were tinkering on his car. everything seems and sounds fine. I took the air cleaner off to adjust the idle. At idle everything seems and sounds fine, but i noticed if i gave the throttle a quick flick of the wrist it would shoot some air up through the carb, not a backfire but just air. He had adjusted the valves recently and i thought he might have overtightend. We readjusted the valves and the problem was not resolved. It is not noticeable with the air cleaner on. Im guessing there is a valve that isnt seating? What do you fellas think?...if so is there a way to find out which one

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Old 06-18-2010, 09:47 PM
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Bad Valve?...How do you tell?

Me and my friend were tinkering on his car. everything seems and sounds fine. I took the air cleaner off to adjust the idle. At idle everything seems and sounds fine, but i noticed if i gave the throttle a quick flick of the wrist it would shoot some air up through the carb, not a backfire but just air. He had adjusted the valves recently and i thought he might have overtightend. We readjusted the valves and the problem was not resolved. It is not noticeable with the air cleaner on. Im guessing there is a valve that isnt seating? What do you fellas think?...if so is there a way to find out which one
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Old 06-19-2010, 09:51 AM
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valve?

What engine is it?
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Old 06-19-2010, 02:25 PM
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Quote:
Originally Posted by High School Hot Rod
Me and my friend were tinkering on his car. everything seems and sounds fine. I took the air cleaner off to adjust the idle. At idle everything seems and sounds fine, but i noticed if i gave the throttle a quick flick of the wrist it would shoot some air up through the carb, not a backfire but just air. He had adjusted the valves recently and i thought he might have overtightend. We readjusted the valves and the problem was not resolved. It is not noticeable with the air cleaner on. Im guessing there is a valve that isnt seating? What do you fellas think?...if so is there a way to find out which one
A compression test would be a good start
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Old 06-19-2010, 02:29 PM
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Quote:
Originally Posted by High School Hot Rod
Me and my friend were tinkering on his car. everything seems and sounds fine. I took the air cleaner off to adjust the idle. At idle everything seems and sounds fine, but i noticed if i gave the throttle a quick flick of the wrist it would shoot some air up through the carb, not a backfire but just air. He had adjusted the valves recently and i thought he might have overtightend. We readjusted the valves and the problem was not resolved. It is not noticeable with the air cleaner on. Im guessing there is a valve that isnt seating? What do you fellas think?...if so is there a way to find out which one
Hook a vacuum gauge to manifold vacuum. There are many things you can learn about a motor with a vacuum gauge.....
http://www.earlycuda.org/tech/vacuum2.htm

I suspect what you are experiencing when you wing the throttle is simply reversion. As the piston comes down on its stroke, it pulls in air fuel mixture with the intake valve open. As the piston approaches bottom dead center, the intake valve has begun closing, but is not closed yet because the column of fuel/air mixture has inertia and will continue to pack the cylinder even after the piston bottoms and heads back up the cylinder. So we keep the intake valve open for a little while as the piston comes back up to compress the mixture. This is where the delicate balancing act comes with a camshaft, closing the intake valve at exactly the right time to capture all the mixture possible, but also closing it in time to prevent the piston from pushing an excess of the mixture back up the intake tract through the "still open" intake valve.

With a performance cam, the cam will not be efficient until higher up in the rpm range, so what you are seeing is reversion where the piston is pushing some of the mixture back past the intake valve and out the carburetor air horn. This phenomenon will disappear as the revs increase and the motor "comes up on the cam". This effect can best be observed at night with a very strong light shone across the top of the carb. With a lumpy cam, you'll see a formation of mixture outside the carb. It's being blown out of the motor, but sucked back in at the same time. Pretty cool.
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Old 06-21-2010, 08:44 AM
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try running a compression test on all the cylinders. the one that is off the most is probably the culprit.
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Old 06-21-2010, 08:46 AM
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spark plug tester fitting

blow air into the cylinder at TDC with the valves closed. you can buy or make and adaptor that screws into the sparkplug hole then hook up an air hose, listen at the carb, exhaust pipe. oil filler, radiator cap for air getting out somewhere turn the engine to next position to test all the cylinders
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Old 06-21-2010, 09:14 AM
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A vacuum gauge will tell you if valves are not sealing properly, pretty obvious if the gauge is jumping all over the place when its idling.
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Old 06-21-2010, 11:05 AM
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we did a compression check
and found that compression was horrible on two adjacent cylinders. Pulled the heads and the gasket was blown between the to bores. Put it back together but still had a little bit of the same problem, but to less of a degree. Im guessing its reversion like techinspector stated.
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Old 06-21-2010, 11:35 AM
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Quote:
Originally Posted by 4 Jaw Chuck
A vacuum gauge will tell you if valves are not sealing properly, pretty obvious if the gauge is jumping all over the place when its idling.
Hey 4 Jaw Chuck,

Is there any chance you could combine the three identical threads on this same thing? All three threads have posts.

Thanks in advance.
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Old 06-21-2010, 11:39 AM
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Did you re-check the compression after you put it together. You may have a warped head. Usually something causes head gasket failure, overheating, to high of a compression ratio for the given gasket material, failed head bolts or rot from coolant to name a few.
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Old 06-21-2010, 09:25 PM
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we checked compression all seemed swell. We are hoping that it was just a cheap gasket. We couldnt find any cracks on the block or heads, and dont think its been overheated. Guess he'll just have to keep an eye on it and see what happens
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