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.

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

Hey Thanks.. I just saw the videos (2 parts). It seems that the float height may be the problem. I will re measure and adjust if necessary.

Does this mean that the float level is lower than it should be? or higher than it should be.. I know it needs to be level to something in the bell housing or the nozzle??

As i understand it your float level is to low.

Good luck

Chris

^^ I agree, sounds like the level is too low in front float. To test that, you could try to adjust the front nozzle down farther. You said they were both set at 2 1/2 turns down. Try turning the front only an additional 2 1/2 and see if that's any better. If it is, that would confirm that the level is too low. Don't leave it that way, correct the level so that both adjustments are similar (they don't need to be identical).

Do you understand the SU function?

EFI the bastard:eek:

Chris

What's not to understand? Perhaps the simplest carb ever produced next to motorcycle carbs. Do you understand the EFI function?

I don't understand this!

What's not to understand? Perhaps the simplest carb ever produced next to motorcycle carbs. Do you understand the EFI function?

What i meant was that if he undestand the simple function of SU's he would also know what is the problem, at least 80%, witch i think he do by now.

And if it won't work he can always EFI the car.

Chris

Don't forget to check that both carbs are opening together. We've helped too many fight problems only the find out they failed to secure both carbs to the center linkage.

Doesn't matter though cause you need new carbs anyway...... My standard friday afternoon answer.....

I don't understand this!

What i meant was that if he undestand the simple function of SU's he would also know what is the problem, at least 80%, witch i think he do by now.

And if it won't work he can always EFI the car.

Chris

My apologies Chris. I thought those comments were directed at me.

No sweat Stephen

I have to say i have the same problem. Only it is reversed the back one causes the miss. I swapped the carbs and the problem seems to be gone.

But i cant run like that no linkage. Ne ideas.

I have to bring this thread back as I am having issues with tuning my carbs. I originally had the float of my front carb set a 16mm and rear at 12mm as it is required for 1972s. Also, set the adjusting screw at 2 1/2 turns.However, the front 3 cylinders were not working.

I then adjusted the front float level to 12mm. Set the adjusting screw 2 1/2 turns and still the same problem. BUT, when I set the screw at about 4 1/2 turns, the front 3 cylinders started working.

I set the air flow using the synchronizing tool.

What is the problem? Is the needle out of sync.? Please help.

I then adjusted the front float level to 12mm. Set the adjusting screw 2 1/2 turns and still the same problem. BUT, when I set the screw at about 4 1/2 turns, the front 3 cylinders started working.

The front carb's float level is still too low. Correctly setting the level can be tricky for the inexperienced. You need to bring it up another 3-4 mm, as a WAG.

I had the same issue and it was the float bowl fuel line was kinking.

Just a thought.