Lew,
Another thought -- one might initially suspect that air might get sucked in
past the valves, even tho the cam is not opening them -- i.e. valves acting
as simple spring-loaded poppet valves & being sucked slightly open by the
vacuum created when the piston goes down the bore.
I think you can pretty quickly disprove this thought, however. I don't have
the exact figures on J2 valve diameter, stem diameter, and valve spring
pressure, but using rough estimates: if the valve head is 1.5" and the stem
is .312", the net AREA of the valve head is about 1.7 square inches. If the
descending piston could generate a perfect vacuum (which obviously it
can't), the resulting force trying to open the valve would be 14.7 psi times
1.7 sq in, which is about 25 lb.
I am pretty sure that the valve spring provides a much larger force than
this, probably something like 80 - 100 lbs "seat pressure" (it's really a
force, not a pressure, but that's what everybody calls it). So the force
created by the vacuum in the cylinder would NOT be enough to overcome the
valve spring and crack the valve open, in my opinion.
If you REALLY wanted to be sure, you could place a piece of paper over the
carb intake & crank the engine -- if the paper tends to suck into the carb,
I'm wrong.
Regards,
Mark Palmer
>From: "Mark Palmer" <mgvrmark@hotmail.com>
>Reply-To: "Mark Palmer" <mgvrmark@hotmail.com>
>To: lpalmer@roundaboutmanor.com
>CC: mg-mmm@autox.team.net
>Subject: Re: J2 Engine Mystery
>Date: Mon, 25 Mar 2002 04:29:08 +0000
>
>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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