It would be extremely unusual indeed (or it is simply faulty i.e. seized)
for a car engine distributor *not* to have mechanical advance i.e *not*
change with revs. Mechanical advance takes account of the fact that pinking
is less likely at higher revs than lower (other factors remaining the same).
If the timing *isn't* advanced as the revs increase then more and more of
the mixture burn takes place with the piston lower and lower down its
expansion stroke which results in more being converted to waste heat and
less to forward motion. OD or not is irrelevant.
It is vacuum advance that is optional to some extent, it primarily
contributes to cruising economy. It is not usually found on competition
vehicles as cruising economy is irrelevant, it has additional components
that can fail and cause problems, but again primarily it is much easier to
tune a distributor to produce maximum power at full throttle through the rev
range without the complication that timing variations vacuum advance
introduces. Some late model American spec MGBS *may* have had vacuum-less
43D4, 43DE4 or 43DM4 distributors, but many had the usual 45D versions with
the vacuum advance system but the advance plate was pinned to prevent
movement if it was the only way to get it to meet the emissions
requirements.
>From late 76 American spec MGBs had a switch on the gearbox that meant
vacuum advance was only operative in fourth gear (Transmission Controlled
Spark Advance), i.e. you would not see the effects of changing vacuum i.e.
changing throttle opening when timing the engine (maybe it is this you are
thinking of?), and in cars fitted with this you do *not* need to disconnect
vacuum to set timing. Originally this was an extra switch and overdrive
(where provided) continued to be available in 3rd and 4th. Whether this
additional switch proved troublesome or in one of their periodical
penny-pinching exercises subsequently the OD and TCSA functions were
combined and both vacuum advance and OD (where provided) were available in
4th only. TCSA is said to have been necessary to control 'surging' in lower
gears caused by the cumulative effects of all the other emissions equipment,
but like pinned vacuum advance not all may have experienced it. If you
spend any time in traffic and lower gears you could try bypassing the TCSA
solenoid in the engine compartment i.e. connect the vacuum capsule direct to
the inlet manifold, and so benefit from improved mileage in lower gears.
However that will only be of benefit if you *don't* have a pinned vacuum
advance plate!
If you have OD and it works in 4th only then that is telling you the 'OD
advance' switch is working correctly. But there is the rest of the wiring
and the TCSA solenoid to consider, and to check that i.e. the correct
functioning of the overall system you need to turn the ignition on, put the
gearbox into 4th, remove the distributor cap, and suck hard on the end of
the tube removed from the inlet manifold. This should cause the points
plate to twist clockwise. Ignition off, or not in 4th gear, and sucking on
the tube should do nothing, and especially not pass any air.
PaulH.
----- Original Message -----
> Can someone tell me how the distributor in an 80 B with
> OD works with regard to timing ?
_______________________________________________
Support Team.Net http://www.team.net/donate.html
Mgs@autox.team.net
http://autox.team.net/mailman/listinfo/mgs
http://www.team.net/archive
|