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TechTalk: SU Carb tuning by Marc Sayer 
Including excerpts from a discussion on the Z Car List about tuning SU's


Re: SU mixture adjustments

> The problem I had was that although the mixture screws were turned equal
> number of turns, I could raise the front carb's piston all the way up without
> any real change to the idle. But when I lifted the rear carb's piston
> juuuuust slightly the engine died almost immediately.
> I did try setting the mixture nuts differently (did not spend too much time
> on this) and it did not seem to have any effect.

OK,… the fact that the adjusters are turned equal numbers of turns is not the important thing here. That adjustment will not set the jet's position on the needle. By setting them both the same you are essentially setting the jets the same distance from the jet bridge. Unless both the needles are positioned at exactly the same height in the body, this setting has little to do with setting the jets to the same positions on the needles which is what determines mixture strength.

Needle positions can vary one carb to another for several reasons

Throttle body/shaft/plate wear (or any other vacuum leaks) can give false readings with the synchronizers which can lead to the two throttles being set differently from one another and this will alter piston position. Also needle and/or jet wear can require the needles to be set differently from one another to achieve the same A/F ratio. Different fuel levels in the float bowls of the carbs can also require a differential in jet location to achieve the same A/F ratio.

You must balance the airflow through the carbs and ensure that the pistons are both at the same height (there is a tool set for British cars that lets you do this but it is not configured properly for the Z car - ZCCD is looking into developing one specifically for the Z Car). Then you can adjust the mixture "screws" to achieve best idle.

The "Change the Idle Speed With the Mixture Screws" Test
I don't trust the "lift the piston" test alone. After doing that I run my "change the idle speed with the mixture 'screws' test". Here's how that works, I turn one of the mixture 'screws' in slowly until the idle drops a fixed amount (say 200 RPM) and I keep track of how much I turned the mixture screw to get this RPM drop (say 1/2 turn). I then set that mixture 'screw' back to its original position and adjust the mixture 'screw' on the other carb the same number of turns in as I did the first one. If the idle speed drops the same amount and my lift the piston test was good too, I'm done. If not, then I adjust that mixture 'screw' until I get the same idle speed reading as I did with the other carb. I then note the difference between the number of turns on the two carbs. Lets say that to get the 200 RPM drop on both carbs I had to lean out the first carb 1/2 a turn and the second carb 1 turn. This tells me that there is 1/2 a turn difference in settings between the two carbs. Since the combined mixture is about right (best idle setting) each carb must be off by half the amount, but in opposite directions. Since carb 1 was running leaner (needed less leaning out to get the required drop in idle speed) it will be the one to be set  richer. So carb number 1 needs to be set 1/4 turn richer and carb number 2 has to be set 1/4 turn leaner. Now I go back and try the lift the piston test and then I try my "change the idle speed with the mixture 'screws' test" again.

The "Richer/Leaner"Test
If this doesn't do it I then try the "richer/leaner test". It goes like this, carbs are balanced, piston heights are equal, and the mixture is set for best idle. Turn the mixture "screw" on carb #1 IN until idle speed drops (say 200 RPM) and count the turns. Turn it back out the same amount (we're back where we started). Turn the mixture "screw" OUT until the same (200 RPM) drop in idle speed is achieved. Compare the IN and OUT adjustments. If they are equal cool go on to next carb. If not (say 1/2 turn IN & 1 turn OUT) then I split the difference (1 turn + 1/2 turn = 1.5 turns, half of that is 3/4 turn) and run the mixture screw back IN (I haven't touched it since I ran it OUT) 3/4 of a turn. Repeat for other carb. This sets the mixture screw in the middle of the range of an adjustment of plus or minus 200 RPM.

>Overall the car runs fine, and the spark plugs do not look any different
>between the front and rear bank of cylinders.

The adjustments you've made are probably close to right. Often the biggest difference will be at idle and light throttle, and this is not reflected well by plug color. In fact you can only use plug color to test full throttle mixture, and then only under the right conditions. Additionally, some gas additives now in use alter plug color to a uniform tan and mask mixture. With SU's, If you can't get the carbs set right, you should look for worn throttle shafts or other vacuum leaks. If these are present, you just can not get a proper adjustment, and you will have to live with the best you can get. In that case worry about the normal running phases more than the idle phase. A lean idle will be an annoyance but a lean full throttle will ruin an engine. You have to keep in mind when working on SU's that their biggest benefit, the fact that the mixture for all phases of operation is controlled by one thing - the needle, is also their biggest drawback. Any change made to the mixture at any phase of operation affects all phases of operation. In a Weber or Mikuni sidedraft, a change to the idle circuit will not have any affect on the mixture at full throttle, but on an SU it will!


Created: Thursday, June 20, 1996


Last Updated: Tuesday, June 15, 1999