David Hall wrote:
>
> Robert..
>
> Thanks for the detailed information! I would guess that the huffa-huffa is
> coming partly from the cam, and partly from worn valve seats. Also, thinking
> about it, the rocker shaft is probably worn pretty bad as well. I am
> guessing this because there is a noisy vibration around 3K which smoothes out
>at
> about 3.5 - 4K. Also, around 2K, the variation in the intake pressure seems
> to not be as bad. (Hence the cam.)
Well, you could do a leakdown test to see what you've got. Do you own
one of those Mighty-Vac brake bleeders? You can buy an adapter for them
to hook to the pump exhaust port and then test leakdown at TDC. As your
compression test was pretty even I wouldn't think a single valve seat is
bad -- pressures were too even. Either the motor is uniformly worn or, I
dunno, test a couple pots on your newest driver. Maybe the gauge is a
cynic.
You saw less pressure on 4. That is the hottest running cylinder. That
could be a bad exhaust seat as it is sitting way out there on the end.
Conversely, if number 1 is low, it might be rings as that cyclinder gets
hit directly off the water pump and rings tend to coke on the front
side.
The main advantage of roller rockers is to lessen the parasitic losses
from the valve train. Our LBCs valve lift is pretty modest, not much
more than a third of an inch, but that still requires our little rockers
to tilt something like 30+ degrees. That is a lot of rotation and force.
The cups at the end of the pushrods keep everything in contact but there
is some stress there and the rocker shafts can wear. I don't know how
you would replace the bushings in those rockers. I guess it only takes
money.
I don't know what to think of the 3K rpm vibration. I don't see how that
could be the drivetrain. Maybe the motor is just coming "on the cam" if
it runs stronger from that point onwards. More detail.
Well, I gotta git...
Bob Allen, Kansas City
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