[TR] Is My Distributor Worn Out?

Thu Jul 9 19:40:10 MDT 2015

Tim,

This was a god-send. Today after work I broke out the timing light and the
electronic tachometer and performed your tests.

The vacuum advance works fine, but boy does the timing mark bounce back and
forth erratically. About an 1/8" to 3/16" at 1000 RPM. Or about 6-8 degrees
I'm guessing. I've been fighting a rough idle for so long and now I know
what it is. OK, I'm off to the Advanced Distributor website.

Thanks again Tim!

Johnnie
'67 TR4A with a 25D/40795A distributor

> -----Original Message-----
> From: Triumphs [mailto:triumphs-bounces at autox.team.net] On Behalf Of John
> Macartney
> Sent: Thursday, July 09, 2015 6:52 AM
> To: 'Ann and Tim Buja'; triumphs at autox.team.net
> Subject: Re: [TR] Is My Distributor Worn Out?
> 
> Wow! Did I learn a lot from that!! That post is definitely one for the
archives. Well
> done Mr. B. and I wish I could be at the VTR bash - but 'tis a wee way to
come.
> 
> Jonmac
> 
> -----Original Message-----
> From: Triumphs [mailto:triumphs-bounces at autox.team.net] On Behalf Of Ann
and
> Tim Buja
> Sent: 09 July 2015 02:49
> To: triumphs at autox.team.net
> Subject: Re: [TR] Is My Distributor Worn Out?
> 
> </lurk mode>
> 
> Johnnie asked:
> > How do you know if your distributor needs to be replaced or rebuilt? I
> > have a Lucas 25D installed.
> 
> Randall replied
> > Hook up a timing light, start the engine, shine the light on the
> > timing mark.  If it jumps around, the distributor is worn...
> 
> Disconnect the vacuum hose from the vacuum capsule and hook up the timing
> light sensor on the ignition coil lead so that every spark produced by the
coil will
> trigger the timing light.  Shine the light on the timing mark - if you see
a single
> mark that moves per the advance curve as you increase engine rpm, your
> distributor is in good shape.
> 
> If you see two marks that jump back and forth and change the gap between
them
> erratically, then your upper shaft bushing is worn out.  If the marks move
around
> relative to one another, you're seeing that the timing for #1 relative to
its TDC is not
> the same as the timing for #4 at its TDC because the shaft wobble is
changing the
> point gap which changes the dwell angle and the coil firing point at every
> revolution.  It's hard to get a car to run well when the timing for every
cylinder
> changes each time the points open.
> Timing issues from a wobbly shaft can be cured with the installation of
electronic
> ignition (Crane recommended) but this is only postponing the inevitable
rebuild
> 
> If the mark(s) stay in a fixed position between 800 and 4000 rpm, your
mechanical
> advance is not working.  This could be due to frozen weights or corrosion
between
> the concentric shafts in the mechanical advance mechanism.
> This will cause lousy performance above 2000 rpm.  You can check for this
by
> gently twisting the rotor shaft (you may want to remove the rotor itself
so you don't
> break the plastic key that holds in it a specific position on the shaft).
You should
> be able to turn it about 10 degrees or so in one direction against spring
tension
> that returns it to its original position.
> If it is firmly locked in place for both clockwise and anti-clockwise
rotation, the
> concentric rotor shafts are corroded together and will need attention.
> 
> TR4s should see advance starting at 700-900 engine rpm.  The tables below
give
> you the min/max values for mechanical advance in distributor degrees at
> distributor rpm when the distributor is on a distributor machine.
Multiply by two for
> crankshaft degrees and crankshaft rpm when it's on the engine.
> The 40xxx number is the Lucas model number that's stamped on the side of
the
> distributor body.
> 
> TR3/4		40698A
> Dist	Degrees Advance
> RPM	Min	Max
> 225	0	0
> 350	0.5	2.5
> 750	8.5	10.5
> 2700	13	15
> 
> TR4	61-62	40734A
> Dist	Degrees Advance
> RPM	Min	Max
> 250	0	0
> 450	3.5	5.5
> 1550	11	13
> 2800	14	16
> 
> TR4	61-onward	40795A
> Dist	Degrees Advance
> RPM	Min	Max
> 225	0	0
> 350	0	2
> 600	5	7
> 1200	9	11
> 
> Use this Excel spreadsheet to print your own advance curve for a variety
of
> Triumph cars.  Each tab in the spreadsheet is a different model
distributor
> http://www.snic-braaapp.org/media/DistAdvance.xls
> 
> If you plot the curve on the chart and you see a straight angled line with
no knee
> between two separate slopes, then you have incorrect springs inside the
> distributor.  At rest, you should see two springs under the breaker point
baseplate.
> One should be made of thin wire and should be tight on its mounting pins.
The
> second spring should be made of heavier wire and should be loose on the
pins.
> As RPM increases, the light spring controls the advance at a steep rate
until the
> pins move apart enough to contact both ends of the heavy spring.  At the
point,
> both spring control the advance at a shallower slope.  On the tables
above, this
> happens at 750, 1550, and 600 distributor rpm, respectively.
> 
> The maximum advance is controlled by a cam that will hit one of the spring
pegs at
> full advance.  It should have a number stamped on it to indicate the
maximum
> number of distributor degrees advance it will allow.  You should be able
to see this
> through the holes in the baseplate.  The distributor models listed in the
tables
> above should have 14, 15, and 10 degree cams, respectively
> 
> With the engine running, put a vacuum gauge on the vacuum hose that you
> removed from the distributor vacuum capsule.  Vacuum advance capsules
should
> have a vacuum source that provides zero vacuum with the throttle closed
and
> increasing vacuum as you open the throttle.  Vacuum retard capsules should
> have a vacuum source that provides full manifold vacuum with the throttle
closed
> and quickly decreasing vacuum as you open the throttle.  Stromberg carbs
have
> the retard source near the throttle plate on the bottom of the carb and
the advance
> source on the top.  SU carbs have the advance source on the bottom since
throttle
> shaft rotates in the opposite direction from Stromberg carbs.  If you get
no vacuum
> at any throttle position, your vacuum source is plugged or the hose has
extra
> (more than two) holes in it.
> 
> Once you've verified you have a good vacuum source, have an assistant hook
it
> up to the distributor while you check the timing mark with the timing
light.
> Advance units should be tested at part throttle, retard units at closed
throttle.  A
> Mity-vac usually works better for this test than the vacuum source on the
carb.
> 
> Shameless plug:
> Come to VTR 2015 in Fontana, Wisconsin from August 11 - 14 where we will
have
> a vintage Sun distributor machine in action at our Distributor tech
clinic.
> http://www.vtr2015.com
> 
> Tim Buja - Rockford, IL - 80 TR8, 73 Stag, 72 TR6 VTR 2015 Registrar and
Tech
> Session presenter <lurk mode>
> 
> 
> ** triumphs at autox.team.net **
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