Re: [T3] First time timing (second time)

["Dave Hall" <dave@hallvw.clara.co.uk>]

Ross and list,
Take an aspirin first!   Here goes:-

The vacuum thingy on the distributor is basically a vacuum retard
device, which when disabled by opening the throttle gives another 7
degrees of advance to add to the centrifugal advance, making a total
of about 40 deg (yes, 40) of advance at high RPM.

Yours has a twin chamber vacuum retard (T 07... engine).  The two
pipes originally fitted have different bores, and take their vacuum
from either side of the throttle butterfly, so the retard is a
different amount when the throttle is just opened, or just closed
compared to that in its steady state at idle or open throttle.
The one nearer you (rear of car) is the larger bore outlet and goes
to the manifold (header) below the carb.  The further one (towards
front of car) is the thinner outlet and should connect to a pipe on
the left-hand side of the left carb.  Presumably yours has been
sealed to prevent air being drawn in.

The vacuum unit gives around 7 deg of retard at idle, so timing at 0
deg (hoses ON, stock twin carb) means when you put your foot on the
throttle, it gets about 7 deg of advance (once the vac both sides of
the membrane is the same).

This is explained rather well (better than me) in 'your'
(Australian) Scientific Publications manual 47 (VW Type 3
1500/1600).  I've not found the details anywhere else.

"At the initial point of starting the chamber connected to the
vacuum source below the closed throttle moves the distributor plate
7¡ in the direction of distributor rotation, thus retarding the
ignition.  When the throttle is opened connecting the inner vacuum
chamber to the vacuum source above the throttle butterfly, i.e. both
chambers subjected to a common vacuum source, the vacuum to the
diaphragm unit is progressively equalised returning the distributor
plate to the original and static timing point.
Further progression of the throttle opening and subsequent increase
in engine rpm causes the centrifugal advance mechanism to come into
operation.
The centrifugal advance assembly operates directly on the
distributor shaft cam and independently of the vacuum unit which
operates on the distributor plate only.

NOTE: The vacuum unit does not advance the ignition timing.  It
retards the ignition 7¡ at idle speed and returns the plate to the
static position after throttle opening".

If you can hold vacuum in each side of it, why not connect it up
again?  They are generally considered the best and most economical
distributor for a stock engine.
If the hose on yours is the one to the nearer (rear of car)
connection, and onto the manifold below the throttle plate, it is
still getting vacuum (the upper connection on the carb body is close
to atmospheric pressure at idle).

Incidentally, those with twin vac and FI should time with hoses OFF,
on the 5 deg BTDC mark (no. 2 from right).  The FI pulleys are
marked differently from the (stock) twin carb ones.

Time for another aspirin!

Spark timing is critical to get the flame to hit the piston at the
best moment in its stroke.

Advance is needed when the engine is running faster, as the piston
covers the distance quicker.  You have to give the spark a head
start on the piston.  This advance is supplied by the centrifugal
mechanism.

It is also needed when accelerating as the mixture is richer and the
cylinder fills up better, so the flame travels slower in the more
dense gas than before, and needs to start earlier to get there on
time.

At tick over, the less dense mixture has a higher flame speed so can
be set off later, so ignition is retarded.

Does that make sense?  I hope so!

If you want a fun demonstration of this flame front speed change,
fill a wide (1" dia.) glass or perspex tube with gas and light one
end.  The flame travels down the tube faster and faster as the
remaining gas gets progressively diluted with air, and pops out of
the end.  (Don't try to compress the gas, as in an engine!)

Dave.
UK VW Type 3&4 Club
www.hallvw.clara.net/
-----Original Message-----
From: Ross O'Shea <Ross.Oshea@med.monash.edu.au>
To: type3@vwtype3.org <type3@vwtype3.org>
Date: 24 September 1998 01:50
Subject: Re: [T3] First time timing (second time)


Hi again.  Saga continues (timing on '70 dual carb auto notch)

> T 0 690 001 is Aug '69; T 0 805 013 is July '70.  Numbers are
> sequential, so T 07 ... is approximately between Sept '69 and the
> end of June '70.

That's right; it's the original engine (just gone over 60,000
miles).

Last night I set the timing to TDC at 850 rpm (was previously at
around 8 BTDC).  Still advances beyond 30 BTDC at 3000 rpm, but
obviously not as far as before.



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