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post #1 of 7 (permalink) Old 02-10-2009, 09:23 AM Thread Starter
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Valve Adjustment

Just recently repaired the intake manifold gasket and valve seal guides on my 1987 350 tpi Corvette. Before the repair I was getting 28 mpg and good power at all levels. After the repair 22 mpg with normal low end power but was a dog on top end acceleration. One area I wanted to check was the valve adjustment. I tightened the rocker nut 3/4 of a turn after zero lash. I wanted to add another 1/4 turn to make it a full 1 turn. A friend of mine who originally suggested the 3/4 turn said that the valves may be to tight and that I should back it off 1/4 turn. I'm not sure what to do. Help!
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post #2 of 7 (permalink) Old 02-10-2009, 10:07 AM
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Should be a 1/4 turn after zero lash not 3/4 turn.
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post #3 of 7 (permalink) Old 02-10-2009, 05:27 PM
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Quote:
Originally Posted by C4 for sure View Post
Should be a 1/4 turn after zero lash not 3/4 turn.
That is not correct. The FSM says "One turn down from zero lash". Everybody has a favorite setting for various reasons. However, the "adjustment" isn't a lash adjustment or any other adjustment that will affect how the engine runs. What you are setting at 1/4 turn, a half turn,, or even the recommended full turn is the amount of preload on the hydraulic lifter plunger. As long as you don't set it so loose that there is no preload, or so tight that the valves won't seat, the "adjustment" won't affect how the engine runs.

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post #4 of 7 (permalink) Old 02-11-2009, 12:12 PM
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I don't want to sound dumb but what is lash?
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post #5 of 7 (permalink) Old 02-11-2009, 04:01 PM
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Quote:
Originally Posted by mushroomcloud View Post
I don't want to sound dumb but what is lash?
In this case it refers to the free play between the push rod and the rocker arm. or the rocker arm and the valve stem.
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post #6 of 7 (permalink) Old 02-11-2009, 04:47 PM
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In this case it refers to the free play between the push rod and the rocker arm. or the rocker arm and the valve stem.
Regards : TGR
Correct. The components of the valve train, the lifter, push rod, rocker arm, and valve, expand when heated. Also the head and block expand with heat. Therefore it is necessary to have a "gap" in the components of the valve train to deal with the expansion. With no play between components, parts would expand to the point where the valve would no longer be able to close onto the valve seat and seal the combustion chamber. A spec of any solid lifter cam is the amount of valve lash (the size of the gap) to be adjusted into the valve train. Hydraulic lifters compensate for the expansion of parts and are designed to run with no lash. If with a hydraulic lifter the valve train weren't tightened up enough to remove all the play or lash, at best you would end up with a very noisy valve train.

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post #7 of 7 (permalink) Old 02-11-2009, 05:10 PM
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Have you checked for vacuum leaks? Perhaps a hose that did not get connected back up...or an out of place gasket on the intake?
As has already been pointed out, the lifter adjustment would not cause what you are experiencing...not unless it was way, way off....
...and I always though "lash" was something a kinky girlfriend did to you when you were bad....

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Last edited by CorvetteMongo; 02-11-2009 at 05:14 PM.
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