[ROVERNET - UK] TC CARB TUNE

[Slatskars]

Glen,

You are correct, but sure use a lot of words to get there. The carbs are a 
separate issue from the rest of the tune-up. The mixtures can be set no 
matter what condition the plugs, etc. are in. Depending upon which of the 
four types of SU's are used the system can be different. The most common H 
or HS types have a button that lifts the carb piston. Mixture is set so that 
the RPM momentarily increases and then dies off. If it dies without 
increasing it is too lean. If it increases and continues to increase too 
rich. After the mixture is set then balance wit unisyn or hose. That simple! 
The carbs are very mechanical devises and once set properly they will remain 
that way for an extended period of time. Air leaks can affect the ability to 
adjust mixture properly. This can sometimes be compensated for by changing 
to a slightly different (richer)needle.

Slats
----- Original Message ----- 
From: "Glen Wilson" <glenwilson at cavtel.net>
To: <rovernet at lyris.ccdata.com>
Sent: Sunday, July 02, 2006 5:43 PM
Subject: Re: [ROVERNET - UK] TC CARB TUNE


> (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...
>
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