Geoff Kirkpatrick britcarnut at yahoo.com
Mon Jul 3 16:55:18 BST 2006

I can vouch for the Gunson's Colortune.  I bought one on Ebay several
years ago and have used it to set up one of my 3500S' carbs.  It was
running very rich and popping and backfiring before I started.  Being able
to watch the spark through the clear replacement plug that's part of the
Colortune kit is a very effective way to get the carb mixtures balanced,
and the car ran massively better when I was done.  One of these is well
worth seeing out.



Message: 6
Date: Sun, 2 Jul 2006 20:43:44 -0400
From: Glen Wilson <glenwilson at cavtel.net>
To: rovernet at lyris.ccdata.com
Message-ID: <200607022043.44921.glenwilson at cavtel.net>
Content-Type: text/plain;  charset="iso-8859-1"

(Eric, 1000 rpm sounds awfully high for a Rover...)

Larry's question mixes up about three or four different issues. This is
kind of stuff that drives me crazy about dual-SU setups because most of
procedures and advice go out the window right at the beginning if the
are not in tip top shape. Any leaks or worn parts can mess it all up.

A calibration procedure must be broken down and performed in a number of
in a particular order because several of the adjustments are interactive
affect each other. Sometimes when you are dealing with two adjustments,
will change the other but not vice versa. Therefore you have to procede so

that when you have one setting adjusted properly, you don't immediately
it up when you make the next adjustment.

Correct me if I'm wrong in the following because I am not the guru. This
is a 
general conceptual description written as sort of an intellectual exercise
refresh my understanding.

1. Basics...Get in the ballpark by setting the plug gap, static timing,
gap/dwell angle, and set the jets to the same number of flats on both
(how many, I don't know). When you do all of this, you should be aware of 
whether or not you have the distributor vacuum timing advance disabled or 
functioning, and you should make certain that the little weights in the 
distributor for centrifugal advance are lubed and able to swing freely.
weights change your timing curve and ignition timing depending on the
of your engine. Usually, when you use a timing light, the vacuum advance
disabled and the engine rpms are kept low enough that the centrifugal
are not activated and changing the timing. The engine speed at which you 
adjust the timing using a timing light may or may NOT be the same as the 
basic idle speed of your engine.

2. Mixture...Regardless of the amount of air flowing through the carb, the

ratio of air to fuel must be within a certain range. I would think that a 
tool like a Gunson Colortune sparkplug thingamajig is the rational way to
to get the mixture pretty close. Once you're going down the road, you can 
check the color of the sparkplugs to see if any cylinders are rich or
I would think that the mixture (air/fuel ratio) would have to be correct
pretty much the same in each carb before you fine tune the balance,
or idle speed.

3. Balancing...This has to do with balancing out the volume of air flowing

through each of the two carbs in a given time at a given engine speed. I 
guess the real point is to have all four cylinders firing more or less
because they are getting about the same amount of fuel-air mixture. Loosen

the link between the carbs before attempting to balance. If the carbs are 
good, putting the jets to the same setting should result in a pretty well 
balanced situation. Mixture-wise, this should make them pretty much the
be it rich, lean or just right. You can use a carb balancing tool, listen
a tube for equivalent hissing sound, or whatever. Regardless of what these

techniques tell you, the bottom line is that the engine must run smoothly.
my way of thinking, balancing the carbs is really a rather fine adjustment
compensate for minor manufacturing differences between two carbs that are 
both basically within spec and with the same needles and jets. You adjust
jets to get the correct mixture, then adjust the basic reference throttle 
opening of each carb separately to achieve balance between the carbs, lock

the linkage between the carbs to keep them the same in relation to each 
other, and then set the idle speed using the adjustment that adjusts both 
carbs as one unit.

3. Timing...Haven't we established before on this list that there isn't
that much difference between 92 octane unleaded on the current octane
scheme and 100 octane on the old P6-era scheme? I would think the best
to do without a rolling road would be to ballpark it and then adjust the 
timing at the distributor until you just eliminate the pinging under load.
matter what numbers you adjust to, you still have to get rid of the 
pre-detonation under load, and adjusting much beyond that just reduces
without gaining anything. You may then have to go back and fine tune your 
idle speed, but your mixture and carb balance don't need to be readjusted.

So, what did I leave out and what mistakes have I made in this general 

~ Glen Wilson ~
Adding new stock daily...

"This is the final test of a gentleman: his respect for those who can be of no possible service to him."
- William Lyon Phelps

Geoff Kirkpatrick, 382 Riverside Avenue, Ben Lomond, CA 95005, USA

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