If you have manifold vacuum (instead of carb) then pulling off the pipe
changes both the mixture and the timing. Block off the pipe to the manifold
and that restores the mixture to what it was, so you can just see the effect
of the timing. In theory, if everything else is correct, pulling off the
vacuum pipe *should* cause the engine to stall or nearly so, as not only
will the mixture be weakened but the timing will be significantly retarded,
and either should cause a reduction in revs.
Pinking can be caused by low grade fuel, glowing carbon deposits in the
cylinders (which also causes running-on) as well as overadvanced timing
under load. Overadvanced timing can be caused by faulty weights and springs
(centrifugal advance) in the distributor. With the cap off you should be
able to turn the rotor arm anti-clockwise against spring pressure. When you
let it go gently it should return all the way. Ideally there should be no
slop or free play at all, but a little is inevitable on an old unit.
Retarded timing can worsen running-on as the engine runs hotter, giving more
chance for glowing carbon etc. Get the timing right for running first of
all, then tackle the running-on if it is still an issue.
----- Original Message -----
From: "Marc Siegel" <smarc@abs.net>
To: "MG List" <mgs@autox.team.net>
Sent: Thursday, June 26, 2003 3:55 PM
Subject: Update (Was: RE: How to diagnose poor running?)
> Pulled into the garage, and it was idling pretty rough. I was going to
> recheck the timing, and pulled off the timing advance vacuum line from the
> manifold (engine was still running). I noted that as soon as I did that,
the
> idle smoothed out immensely. I figured that it must be quite rich and the
> added air being pulled in through the now open vacuum port had leaned it
out
> enough to make the engine happy.
>
> The other thing I noted is that I can now hear pinking under heavy
> acceleration. I did a compression check about three weeks ago, with
results
> of 150-145-150-150. I'd been having a run-on problem, & it was suggested
to
> me that perhaps the head had been shaved, and to run higher octane gas. It
> was also suggested that I advance the timing a bit, which I also did, by
> about 5 degrees. The end result being that the run-on problem seems to be
> about 90% better.
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