Late model North American MGBs do indeed have a transmission controlled
spark advance system (TCSA) but there is considerable confusion as to just
how it is wired. From the manuals it appears that there is a gearbox
microswitch that closes in 4th gear only, to operate a solenoid, to allow
manifold vacuum to advance the timing. Since it is manifold vacuum maxuimum
advance is applied to the distributor at idle then there will be a
noticeable difference in idle speed depending on whether the vacuum is 'on'
or 'off''. It seems reasonable that this could be detected by putting the
car into the appropriate gear while stationary, but you say it happens in
3rd which goes against the manuals. Does it do it in 4th too? The strict
answer to your question is that applying the vacuum advance to the
distributor advances the timing. It should *increase* the idle speed, but
if the timing is overadvanced anyway it may cause a misfire and hence a
*reduction* in idle speed.
There is also another switch on the gearbox - for the overdrive which is
also supposed to operate in 4th gear only. So why two switches both doing
the same thing? Some have speculated that the microswitch closes in 2nd and
4th and the o/d switch in 3rd and 4th, hence wiring the switches in series
gives the required '4th gear only' operation. However factory V8s also have
o/d in 4th gear only, but from a single switch (they have no TCSA switch),
and the manuals show the North American switches are wired one in each of
two branches and not in series.
PaulH.
----- Original Message -----
From: Keith Gernert <kgernert@triad.rr.com>
To: <mgs@autox.team.net>
Sent: Wednesday, September 13, 2000 12:29 PM
Subject: OD transmission and timing switch
> I noticed by accident a while back that my 79B with overdrive transmission
idles
> better when I have the shifter in 3rd gear. Place the shifter in 1st, and
the thing
> definitely idles worse. I'm guessing there's a switch that changes the
timing when
> the shifter is in the upper two gears.
>
> My question is this: Does this device advance or retard the timing?
>
> I know my timing at idle is not factory spec. Last time I tried setting
it to spec, the
> car ran noticeably worse. After a couple days of that, I turned the
distributor a little
> more counter-clockwise (I forget now which way that effects the timing) to
return it
> to a better-running state. I'm curious about the gear-indicator timing
switch,
> thinking that may be a clue in getting the car to idle better. (Checking
the valves
> and some major carb checking are on the list of things to do, the latter
when I can
> relegate the car to non-daily-driver status in case something goes
terribly wrong.)
>
>
> --
> Keith Gernert
> 79 MGB
> --
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