On Tue, 08 Apr 1997 10:05:36 +0000 Robert Allen <boballen@sky.net>
writes:
>By golly, you're sure right. MGs use manifold vacuum for their
distributor and the carbs are CD -- constant depression (or continuosly
drips) so there will always be vacuum on each side of the throttle plate.
Curious. That would imply that vacuum advance contributes to total
advance even under periods of moederate acceleration. From a traditonal
carb perspective, that is pretty strange.
> ..... [snip] .....
>Also, there is the perverse "vacuum retard" on emission MGs (that I have
zero experience with) so I'm curious where that vacuum source is. I
understood that the "retard" feature was supposed to be active only at or
near idle so I wonder where the vacuum source sees idle. Or I could be
wrong again.
MGA and early MGB have ported vacuum from the rear carb and vacuum
advance distributors. Later MGB with smog stuff use manifold vacuum and
vacuum retard distributors.
Ported vacuum is low at idle, comes on strong on acceleration at low
speeds, then increases slightly with engine speed. Manifold vacuum is
high at idle, drops dramatically under hard acceleration, climbs again at
higher RPMs, and hits the highest when you close the throttle. Ported
vacuum with the vac-advance dizzy and manifold vacuum with the vac-retard
dizzy have roughly the same effect. Spark advances when you hit the gas.
As I recall, the primary reason for using the manifold vacuum retard
setup is to retard the timing dramatically on overrun to prevent
backfire.
Vacuum spark timing modulation is usually limited to less than 10 degrees
advance and maxes out before 2500 RPM. Mechanical spark timing advance
is nearly linear, starts just above idle, and maxes out around 3600 rpm.
Total spark advance should be limited to no more than 32 degrees on a
stock MG 4-cylinder engine.
A hot cam may tolerate up to 36 degrees advance for a racing setup, but
often a radical cam cannot take much spark advance at low speeds. So, if
you install a wild cam in your street machine without changing the dizzy,
you may have to back off the idle timing to zero (TDC) or even a few
degrees ATDC.
Now I know someone is going to tell me I'm in way over my head here, but
hey it's free.
Barney Gaylord
1958 MGA
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