Cylinders not working after su conversion

[shadesh]

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.

[ChrisZ]

What about float hight?

Chris

[sblake01]

Carbs are so much easier that EFI. Right?

[doradox]

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

[shadesh]

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..

[shadesh]

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.

[doradox]

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

[ChrisZ]

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]

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...

[ChrisZ]

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

[ChrisZ]

Try to check this guy out

Chris

[sblake01]

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.