Back in the dark ages, when Blechfire Mammoths roamed freely on the open
roads, ignition timing control was a fairly simple thing. The theories
said that as the engine speed increased, the spark needed more lead
time. Simple to cover this with a centrifugal advance. The theories went
on to state that under light loads, even more advance was needed. The lower
cylinder pressures of a throttled engine caused a slower flame front
propogation. This lead to the two normal control systems Tim Mullen
described. They either used some centrifugal and added more advance based
on manifold vacuum (high vacuum at low loads), or used lots of
centrifugal and then used venturi vacuum to retard timing under high loads.
There was one variation on the theme that a few used. I call it the Ford
system, as the first time I ran into it was while sorting out a modified
351C. It is manifold vacuum, but the passage into the manifold is actually
*just* upstream of the butterfly. When the butterfly is closed, the passage
is covered; no vacuum. But as soon as the throttle is opened, the port is
uncovered, and acts just like a manifold tap. This is what Greg has on
his shiney new XJ12-HE.
Alas, some designs resulting from the US emissions standards totally
ignored these theories. MGB's came through okay, with only the last models
getting a 4th gear control switch. Spitfires, on the other hand, ended
up with two different systems, both of them "backwards". Early 1500s
had a retard can hooked to manifold vacuum. Later cars had an advance
hooked to venturi vacuum. Both systems operated with abnormally low
timing advance during normal (cruise) operation, and only allowed the
advance to come up during high load.
Not knowing which of the two systems (or, since it's a Canada car, a possible
third system) the Spitfire carb conversion has, the only suggestion I
can make is to keep the vacuum type the same as stock. The centrifugal and
vacuum advance curves of any distributor are designed to work together.
Changing the vacuum source may render the total advance curve unusable,
possibly harmful to your pistons.
>
> Hum... I've got a 75' B and i put twin sw HIF-4's ( not bad and better than
> what i had) you made a point in your last post about "ported vacuum" and i
> wonder if that's the major problem with me setting my timing accuratly on
> my 'B'.
> Rich.P
>
This one isn't as complex. The system didn't change much over the life of the
18V engine, and theres a bit of "been there, done that". Vacuum advance
should be hooked to the manifold still. It was for the entire range, HIF
and CD175. For starters, try setting the timing at 15 before @ idle, vacuum
advance disconnected and hose plugged.
Randy
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