I just completed my SU converson on my 1975 280Z. I have the 3 screw round tops and could use some advise

Upon starting the car, I find that the front three cylinders (Front Carb) are not working in that when I pull the plug wire out, the engine continues to idle without stumbling. When I pull the plug wire out for the other 3 cylinders, the engine stumbles.

When I spray carb cleaner in the front carb, the engine begins to idle normal.

When I push the piston up in the front carb, the engine does not die. When I push the piston up in the rear carb, the engine stumbles or dies.

When I push the choke for the front carb, the engine idle picks up.

The mixture nut on the nozzle is out 2 1/2 turns on both carbs.

Whats my problem.?

Can i switch the nozzles between carbs to see if the problem persists?

Lean mixture in front carb?

not sure here... Any assistance will be appreciated. Thanks.

What about float hight?

Chris

Carbs are so much easier that EFI. Right?

Will it run on all 6 other than at idle? That's one lean carb, partially plugged up maybe. Won't run on all 6 at all. No fuel to the carb or something in the carb completely plugged up.

Steve

2 fast 2 Z.. I will recheck but it should be 12mm for rear and 16mm for front for a 3 screw carb..

sblake01.. Respectfully, I know you are a proponent of EFI systems and there will always be discussions/arguments over which is better but your comment above does not help me..

Will it run on all 6 other than at idle? That's one lean carb, partially plugged up maybe. Won't run on all 6 at all. No fuel to the carb or something in the carb completely plugged up.

Steve

How do I test to see if all six runs when not idling? Do I rev up the engine and have someone pull the plug wires to see effect? sorry, just trying to clarify..

I had taken apart the carb and cleaned it, blew compressed air in all passages.

How do I test to see if all six runs when not idling? Do I rev up the engine and have someone pull the plug wires to see effect? sorry, just trying to clarify..

I had taken apart the carb and cleaned it, blew compressed air in all passages.

I think you can tell the difference between the power of a 6 cylinder engine vs. a 3 cylinder engine. Take it out and drive it.

Steve

Shadesh, i don't know the measurement for 3 screw carbs, but where dit you get those measurements?, common sense to me would be that there have to be the same front and rear, i just looked at my FSM and the 4 screws have to be 14-15 mm. front / rear.

Chris

Shadesh, i don't know the measurement for 3 screw carbs, but where dit you get those measurements?, common sense to me would be that there have to be the same front and rear, i just looked at my FSM and the 4 screws have to be 14-15 mm. front / rear.

Chris

Chris,

See the attachement.. I dont really understand why the measurements would be different for a 3 screw vs 4 screw but here it is...

Maybe the float bowls sits in different hights, still i think it is a adjustment problem, SU's are simple carbs and it seems that your front one is starving, so if there is no gasket leaks, i still vote for incorrect float hight..

Do you understand the SU function?

EFI the bastard:eek:

Chris

Try to check this guy out

Chris

sblake01.. Respectfully, I know you are a proponent of EFI systems and there will always be discussions/arguments over which is better but your comment above does not help me..

I apologize for throwing in that rather moot comment. It was meant to be tonge-in-cheek not derisive. Actually, I'm only a proponent of EFI systems on vehicles that were originally equipped with it. Do you have access to a tool like a Uni-syn or Color Tune? I've used both of those in the past to assist in balancing carbs.