From: F Crocker <jafcjr@presys.com>
Subject: Vacuum advance and Crane ignition question
>While poking around under the hood of our '79 B with another MG
>afficionando, we discovered that the vacuum advance is not hooked up.
>
>Query: Does the VA need to be operational with a crane ignition?
I hesitate to chime in as I don't know what the vacuum mechanism is doing on
a '79 MGB. It could either be Vacuum Advance or RETARD, oddly enough. Also,
you have to be careful where the vacuum comes from on intake. Some may be
off the manifold, some can be either in front of, underneath, or slightly
behind the throttle plate -- thus will modulate based on throttle position.
You should really check with Bentley to see how the vacuum is used to
control ignition for your car and then see what DPO bodges it has been
subjected to over time.
Normally, vacuum advance ported to a 'normal' vacuum source will see vacuum
under steady state cruising and advance the timing for better driveability
and economy. With the engine under load, there is usually less vacuum
available which brings the timing down to correct advance for power
production.
Vacuum retard does some weird stuff. I think the vacuum port is uncovered
only when you're off the throttle thus it limits advance on overrun which is
supposed to cut down on NOx emissions. But I'm at the end of my experience
here.
A long time ago there was a long-winded and noisy thread on the proper use
and port location for vacuum advance (which I'm proud to say that I started)
and, when I went back to the manuals, I found three or four distinct vacuum
advance port strategies over the lifetime of the 'B'.
Having said all of that, the worst thing that can happen when the vacuum
hose is disconnected is the undesirable air leak on intake. Even when the
crab is adjusted for a lean condition, you still are preventing air from
coming across the jet bridge which is needed to pull fuel into the air
stream correctly.
My LBCnonMG had a broken vacuum advance that caused a pretty significant air
leak at part throttle. It was a bear to diagnose the off-idle stumble until
I figured that one out. Which begs the proverb: "When you have carburation
problems, fix the carbs last."
As for the Crane ignition, they do absolutely nothing for or against the
performance of the car -- they are merely a switching mechanism that
function exactly the same as standard points. There main difference is that
they break differently.
Bob Allen, Kansas City, SU-less
"The more opinionated the 'A' driver, the shorter the shifter."
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