Adjusting SU Carbs

Octagon Newsletter September 1988

By Bob Wilihnganz

I keep running into people with cars that have SU carbs that they have trouble adjusting. I ran across the accompanying pictures in the 'T 'Series Handbook. It seems to make more sense to most people then the verbal descriptions one usually finds. For what it's worth, here is how I adjust my carbs.

(1) I check the points setting and the timing before anything else. This is critical.

(2) Then I take my TD for a spin and get it up to operating temperature.

(3) If you have SUs with a lifting pin like that shown in Figure 'A' , push up on it until you feel resistance and then go a bit more a bit more is about 1/32 of an inch. If your carbs don't have the lifting pin remove the air cleaners and use a knife as in Figure 'B' to lift the piston 1/32 inch. If the engine speeds up momentarily and then settles back down to the initial idle speed (just like the middle curve in Figure 'A') your carbs are just fine and don't change anything. If the engine speeds up and stays at fast idle as shown in the top curve then you are set too rich. What you need to do is to turn the large nut (as shown in Figure 'C') to the right. That will cause the nut to move up. Move it only 1/6 of a turn and try again. As you get closer to the right setting the idle speed will increase somewhat as well. If the engine slows or even stalls when you lift the piston then you are too lean. This is shown by the bottom curve in Figure 'A'. You should turn the nut to the left which will cause it to go down until you get the correct response.

If this doesn't help you engine run smoother, either you need to do other things to your carbs or there is some non-carb related problem. If you want to learn how to do a complete set up and adjustment of your carbs I recommend the material in the 'T' Series Handbook.

Figure A

Figure B

Figure C

Return > Link - Return< Return