OK, I've got it.
I'm pulling as much of a vacuum as I'm compressing. Roughly half the
pressure of a fully swept cylinder is to be expected.
See, I told you it was cobwebs on the brain.
Thanks for all the answers.
Regards,
Lew
> -----Original Message-----
> From: owner-mg-mmm@autox.team.net [mailto:owner-mg-mmm@autox.team.net]
On
> Behalf Of Mark Palmer
> Sent: Monday, March 25, 2002 8:05 AM
> To: mg-mmm@autox.team.net
> Subject: Re: J2 Engine Mystery
>
> 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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