Lew,
With both valves closed, when the piston starts downwards, it will create a
vacuum in the cylinder and draw air up from the crankcase past the rings.
Remember, to seal properly, rings have to have some compression (positive
pressure) to force the ring out against the cylinder wall. So, in your
case, the cylinder will draw SOME air past the rings on the "intake" stroke,
even tho the intake valve is closed.
Once the piston starts up on the compression stroke, positive pressure
(compression) seals the rings and therefore compression takes place, and
that's what you're measuring. Not as much compression as you get when the
intake valve opens, because the "leaking" rings don't pass as much air into
the cylinder as an open intake valve would ... but the rings are passing
enough air to allow some compression to take place.
Since you are getting 45 psi on all cylinders taht's telling you that all
the valve seats & rings are OK (or at least, in similar condition). I'll
bet that when you set the valve clearances correctly, you'll be back to 95
psi in all cylinders.
Regards,
Mark (unrelated) Palmer
>From: "Lew Palmer" <lpalmer@roundaboutmanor.com>
>Reply-To: "Lew Palmer" <lpalmer@roundaboutmanor.com>
>To: "MMM List" <mg-mmm@autox.team.net>
>Subject: J2 Engine Mystery
>Date: Sun, 24 Mar 2002 22:12:47 -0600
>
>OK, folks. I have another mystery.
>
>
>
>I've been finishing the restoration on a J2 for a fellow club member.
>Unfortunately, I did not assemble the engine, and the person that did
>apparently had no clue on how valves are adjusted on triple-M engines.
>Therefore, when the head was assembled, there was insufficient (read NO)
>clearance on the intake valve on #4 cylinder and precious little on the
>exhaust valve. A compression test showed around 95 pounds on each of
>cylinders 1, 2, and 3, and zero on #4.
>
>
>
>Hoping to discover it was only the tight valves causing the problem, I
>then removed the cam shaft so that all valves should have been closed.
>To my surprise, #4 cylinder now showed about 45 pounds. To my further
>surprise, #1, 2, and 3 now read between 45 to 50 pounds.
>
>
>
>What would cause a normally operating cylinder (with valves opening and
>closing properly) to show 95 pounds pressure and that same cylinder to
>show 45 pounds with both valves closed?
>
>
>
>Please help clear up the fog from my brain and give me a hint of what
>I'm missing.
>
>
>
>Regards,
>
>Lew Palmer
>
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