SU/Hitachi Carburetor Theory/Operation

[akorna]

This may be repetitive - but I've spent a lot of time researching and playing with the SU/Hitachi carbs – in my case three screw round tops. Here are some statements mostly about mixture that I’d like some feedback on - I think are all true, but please correct me if I’m wrong.

1. The rough setting for fuel mixture is the float level.

2. The fine setting for fuel mixture is the nozzle height adjustment screw.

3. The fuel level in the float bowl needs to be high enough to ensure adequate fuel supply at full load, but low enough to ensure that flooding will not occur. (approx. 23mm from the top of the float bowl)

4. If the fuel level is at the lower end of the acceptable range, then more counter-clockwise turns (lowering of the nozzle) of the mixture screw will be required to reach optimum mixture strength. A higher fuel level will require fewer counter-clockwise turns (lowering of the nozzle).

5. The number of counter-clockwise turns of the nut to reach optimum mixture is dependent on temperature and elevation. Lower elevation increases turn requirement, lower temperature increases turn requirements. Typical range is 1 to 4 turns.

6. One turn of the mixture adjusting screw moves the nozzle approximately 1 mm.

7. If the front and rear carburetor float levels are different, then the number of turns for optimum mixture will be different for each carb.

8. The front carb primarily feeds cyls 1, 2, 3, and the rear 4, 5, and 6. However, the balance tube reduces the direct relationship and the fuel mixture to the cylinders is more of an average of the two carburetor settings.

[tlorber]

Its also very important to have the car level when tuning the carbs. Incline and decline can effect idle rate.

[akorna]

tlorber,

So I would add to 5 that the front to rear inclination would effect the adjustment? - I would think it would have to be on a pretty severe slope.

[tlorber]

My driveway is about a 3% slope as is my street. All I know is that for both the Z and the roadster (which has the same carbs,) the idle changes when parked on the slope vs. when the cars are parked in the garage or on a flat surface.

If you're asking whether a 1" or so difference in spring height over a roughly 90" wheelbase would make a difference (thats slightly over 1% grade) I can't say for sure. However, it's all relative. As long as the car is adjusted on level ground, which is probably where it will spend most of its running time, the height difference will be "cancelled out" so to speak.