carb pistons and idle tuning

[escapedan]

I’m trying to understand what the pistons inside the carbs “do”. The reason I ask is that after taking all the smog equipment off my 1973 L24 engine and capping all the left-over ports, my engine is idling at 2400+ RPM at 7degrees advanced. (even after warming up for 3+ minutes in already warm California)

The only thing that seems to drop the RPM is when I manually lift the pistons inside the carbs leading me to believe they should automatically lift when the engine is idling. When the pistons are fully down, the high RPM’s return. Additionally, the car “diesel’s” (continues to run, chugging and spitting raw fuel out of the rear carb) when I shut it off.

The air/fuel screws are 2 ½ turns open and the idle screw is full up (not pressing down on the throttle at all). I have removed the choke cables from the car entirely (as it will be an autocross car) but I’m not sure how to determine if the car is in “open choke” position. This could certainly be a cause for the high rpms…??

Finally, would a vacuum leak cause the high RPM’s? I have everything well sealed, but its very hard to be 100% positive all the seals are fully seated. Thanks in advance for any advice; I am eager to get the car running properly.

Dan

[240ZX]

Dan,

The piston in a SU carb is what lifts the needle assembly up, increasing fuel flow, when vaccum acts upon the top side of the piston. Basically, when you step on the throttle, a low pressure (vacuum) is created in the intake manifold and across the carb venturi. This low pressure pulls the piston assembly up, increasing the amount of fuel/air needed to increase engine RPM. In your case it sounds as though you my have a vacuum leak from removing the smog stuff....double check it! However, if it is not a vacuum leak issue you should go ahead and go through the process for a tune up. Adjust the valves, check the plugs/wires, set the timing, sync the carbs, etc, etc. Hope this helps you out a little.

[240ZX]

Dan,

I forgot one thing. The way the choke works on a SU is as follows.

There is a jet assembly, metered orifice, that the needle assembly resides within. You already know how the needle moves but, when you pull the choke on it actually moves the jet down from the needle increasing the fuel flow but not the air....hence a richer mixture for start-up.