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Carburetor tuning - setting mixture

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M

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A few days after I purchased my TR3 in 2009, my mechanic gave me some helpful pointers on how to set the mixture. One thing he said, which went in one ear and out the other because of my inexperience, was "Always remember that the carb you are lifting is not the carb you are checking. If you lift the front, you adjust the rear."

In the ensuing couple of years, I have read and been told by many people that when you adjust the mixture, you lift the pin on a carb and you turn the adjusting nut ON THAT CARB.

But recently, I was reading a manual for a 1954 XK 120, with H6 SU carbs, and found following:

"Next check the mixture strength by starting the engine and raising the lifting pin of the FRONT CURBURETTER. ... If...

... (a) The engine speed increases and the engine continues to run at increased speed, this indicates that the mixture strength of the REAR CARBURETTER is too rich....

... (b) The engine speed increases and the engine then stops, this indicates that the mixture strength of the REAR CARBURETTER is too weak.

... (c) The engine speed increases momentarily and then decreases but the engine continues to run somewhat bumpily, then the mixture strength of the REAR CARBURETTER is correct.

Repeat the operation, lifting the REAR CARBURETTER piston and testing the mixture setting of the FRONT CARBURETTER."

In other words, When you lift the pin or piston on one carb, it is THE OTHER CARB THAT NEEDS ADJUSTING.

Any thoughts on this procedure?
 
Ed, I find that very interesting, as over the years, using the other system has made me adjust the other carb to get things to even out. The only true test is the five mile plug test,, then you get the internal picture of each cylinder.

Wayne
 
Wayne,

You are right about the really true test. My mechanic, after telling me about the procedure, added: "The proof of proper running mixture is a 'cut test.' This is done by running the car at full throttle in third gear, and then letting off and pushing the clutch down. Then shut off and coast to a stop. Remove spark plugs and look at their color." I have done this to check on some problem I thought I was having with #1 plug.
 
It may vary with the intake manifold configuration; but on a TR3 I find that the result from lifting the pin indicates primarily the carb you are lifting the pin on. There is some impact from the other carb's mixture though; so you do have to work back and forth until both are 'right'.
 
Randall,

Yes, indeed... back and forth seems to be the way.

I also start with a colortune check to set a kind of "baseline," and then turn to the lift-to-pin/lift-the-piston routine.
 
The Brit manufacturers all set their main jets on SUs 35-38 thou below the jet bridge and provided the float heights are correct and the needles sealing, the car will run perfectly.

My Bentley, my Healey and My TR are set this way and they run exactly as they should.

Make sure the butterflies are also carefully synchronised.

Ash
 
I use an O2 meter but then I have to I have the cursed Webers infinately adjustable!@@##%$%#$%

you know who

Hondo
 
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