[ROVERNET - UK] TC CARB TUNE

[Glen Wilson]

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

Larry's question mixes up about three or four different issues. This is the 
kind of stuff that drives me crazy about dual-SU setups because most of the 
procedures and advice go out the window right at the beginning if the carbs 
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 steps 
in a particular order because several of the adjustments are interactive and 
affect each other. Sometimes when you are dealing with two adjustments, one 
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 screw 
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 to 
refresh my understanding.

1. Basics...Get in the ballpark by setting the plug gap, static timing, point 
gap/dwell angle, and set the jets to the same number of flats on both carbs 
(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. The 
weights change your timing curve and ignition timing depending on the speed 
of your engine. Usually, when you use a timing light, the vacuum advance is 
disabled and the engine rpms are kept low enough that the centrifugal weights 
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 go 
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 lean. 
I would think that the mixture (air/fuel ratio) would have to be correct and 
pretty much the same in each carb before you fine tune the balance, timing, 
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 evenly 
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 same 
be it rich, lean or just right. You can use a carb balancing tool, listen to 
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. To 
my way of thinking, balancing the carbs is really a rather fine adjustment to 
compensate for minor manufacturing differences between two carbs that are 
both basically within spec and with the same needles and jets. You adjust the 
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 all 
that much difference between 92 octane unleaded on the current octane rating 
scheme and 100 octane on the old P6-era scheme? I would think the best thing 
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. No 
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 power 
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 
description?

Glen
-- 
~ Glen Wilson ~
http://stores.ebay.com/EasyAuctionServicesPA
Adding new stock daily...