Help with carb sync/disabling front carb engine quits

[882993md]

Hi everyone:

I spent three days using the 10 turn down method to set my float levels. It’s maddening but I was so happy when I finally did it. 

I'm trying to set idle and sync the carbs. Both jets are set at 2.5 turns. I am able to get the engine to stumble along when I disable the rear carb. However when I disable the front carburetor the engine quits immediately. I lowered the rear jet by a 1/4 to 2.75 turns and still quits. Lowered rear jet to 3 turns and still quits. 

So I took off the rear dome and turned down the jet ten turns to check the float level. The fuel level is just a hair below the opening. I called it the day at that point and now I’m hoping for some insight. Thank you.

Howard 

1974 260z

[Captain Obvious]

Off the top of my head, I would guess that the carbs airflow at idle isn't balanced and one carb is doing "more work" than the other at idle. Theory being...When you disable the one doing less work, it'll kill three weak cylinders and the engine will continue to run on the other three. But when you disable the carb that is doing MORE work, it'll kill the three strong cylinders and the three weak cylinders won't have enough power to continue to run, and the engine will die.

What happens if you hold your hand over the mouth of each carb? Do you have "about the same" suction on each? Does the engine die with a hand over each carb?

[882993md]

1 hour ago, Captain Obvious said:

Off the top of my head, I would guess that the carbs airflow at idle isn't balanced and one carb is doing "more work" than the other at idle. Theory being...When you disable the one doing less work, it'll kill three weak cylinders and the engine will continue to run on the other three. But when you disable the carb that is doing MORE work, it'll kill the three strong cylinders and the three weak cylinders won't have enough power to continue to run, and the engine will die.

What happens if you hold your hand over the mouth of each carb? Do you have "about the same" suction on each? Does the engine die with a hand over each carb?

When I hold my hand over the mouth of each carb the engine will stumble then recover. The suction was about the same for each carb. If I leave my hand too long over the mouth the engine will die. If I move my hand away fast enough it will stumble then recover. 

I tried increasing the idle on the rear carb. When I restart the engine the idle is high but as soon as I disable it shuts down immediately.

Edited March 5, 2018 by 882993md

[Mark Maras]

1 hour ago, 882993md said:

Hi everyone:

I spent three days using the 10 turn down method to set my float levels. It’s maddening but I was so happy when I finally did it. 

I'm trying to set idle and sync the carbs. Both jets are set at 2.5 turns. I am able to get the engine to stumble along when I disable the rear carb. However when I disable the front carburetor the engine quits immediately. I lowered the rear jet by a 1/4 to 2.75 turns and still quits. Lowered rear jet to 3 turns and still quits. 

So I took off the rear dome and turned down the jet ten turns to check the float level. The fuel level is just a hair below the opening. I called it the day at that point and now I’m hoping for some insight. Thank you.

Howard 

1974 260z

I've not tried the 10 turns down but I believe (someone correct me if I'm wrong) the 10 turns down, is read with the float removed. In your last sentence you said you removed the dome. If the float was still installed, the fuel level should have been 1/16" below the nozzle tops at 2 1/2 turns. 

[Captain Obvious]

12 hours ago, 882993md said:

I tried increasing the idle on the rear carb. When I restart the engine the idle is high but as soon as I disable it shuts down immediately.

Just want to make sure I understand... You mean that you increase the idle on the rear carb and the overall idle speed goes up. But even after doing that, if you disable the (FRONT) carb, it shuts down immediately?

How much did you raise the idle? Maybe you just need to go a little more? Depends on how far out of balance the two of them are. Open the rear carb some more, and then close the front one to compensate and bring the idle back down to where you want it?

What happens if you push on the linkage for each carb independently Does the engine speed come up about the same for each?

[882993md]

6 hours ago, Captain Obvious said:

Just want to make sure I understand... You mean that you increase the idle on the rear carb and the overall idle speed goes up. But even after doing that, if you disable the (FRONT) carb, it shuts down immediately?

How much did you raise the idle? Maybe you just need to go a little more? Depends on how far out of balance the two of them are. Open the rear carb some more, and then close the front one to compensate and bring the idle back down to where you want it?

What happens if you push on the linkage for each carb independently Does the engine speed come up about the same for each?

Just want to make sure I understand... You mean that you increase the idle on the rear carb and the overall idle speed goes up. But even after doing that, if you disable the (FRONT) carb, it shuts down immediately?

Yes that is correct

How much did you raise the idle? Maybe you just need to go a little more? Depends on how far out of balance the two of them are. Open the rear carb some more, and then close the front one to compensate and bring the idle back down to where you want it?

I went up two full revolutions on the #10 screw.

What happens if you push on the linkage for each carb independently Does the engine speed come up about the same for each?

When I do that the rpm's do increase but I noticed that it's not quick and instant like in the past. It seems sluggish for each but the rpm's do increase.

I'm going out outside now to try again. I will see if the car will start with the front carburetor disabled and try to get the rear to idle. 

Howard

Edited March 5, 2018 by 882993md

[240260280]

Also be sure to check the 2nd and 5th plug colour to see the difference between the front (plug2) and back carb (plug5).

[David F]

Are you checking mixture at each carb by slightly lifting each piston to see how the idle changes?  I am guessing that the engine immediately slows and stumbles on the rear carb (lean).  If so, set the fuel level higher.

Edited March 5, 2018 by David F

[882993md]

ok here are the results.

I attempted to get the rear carburetor running (stumbling along) with the front disabled and I couldn't. Even with full choke it will run really rough for a few seconds and quit. I again checked the fuel level at 10 turns down and it now it was way low by 3 to 4 mm . I bent the float tab down (it is now much lower than the front tab) was able to get the fuel right at the opening after two tries. I tried to start it again on just the rear and same result-stumble for a few seconds then quit. I had the mixture at 3 turns for the rear and 2.5 for the front.

I enabled the front carburetor and the car started with no problem. I noticed the following with both carbs running:

-Rear carburetor is sucking A LOT of air versus the front (sounds like a jet almost)

-Rear carburetor was back firing. When I blipped the throttle at the rear it back fired even more.

Howard

Edited March 5, 2018 by 882993md

[882993md]

1 hour ago, 240260280 said:

Also be sure to check the 2nd and 5th plug colour to see the difference between the front (plug2) and back carb (plug5).

OK i will do that

[240260280]

If you have an air fuel meter after your are able to lift each carb piston to disable*** one-at-a-time, then you can adjust the jet turn out until you get ~29:1 air fuel ratio for running on each. This will accurately balance your jet heights and get an idle a/f of ~14.5:1.

Once you get 29:1 on each, re-measure air flow and adjust idle stop screw so that both are balanced then re-do the 29:1 adjustment. Repeat until balanced in air flow & fuel AND idle rpm is ~ 800rpm.

 

Next dial up the rpms to 3000rpm using the fast idle screw at the top of the carb assembly then balance the air of the back carb to match the front using the balance screw where the throttle rods couple. You can also do the front back check of 29AF to see how it checks out at high speed.  You may need to adjust jet height to balance a/f at 3000 therefore you will disturb you idle setting as early SU's have only one fuel adjustment for idle and off idle..

 

*** Lift the piston all the way to the roof of the carb to disable it.  Some British tests recommend using a lift pin to lift it  just 1/64" then listen to engine. Do not confuse the two.  The method I suggest is the American Dragster method (from Norm the SU Dude) where he lifted each piston so that carb was disabled and the other carb ran the engine on its 3 cylinders.  This step is further enhanced by the Canadian Scientific Method where you use an Air Fuel meter to set the jet height so that each carb gives the same amount of fuel when running only on each's 3 cylinders.

Edited March 6, 2018 by 240260280

[882993md]

1 hour ago, David F said:

Are you checking mixture at each carb by slightly lifting each piston to see how the idle changes?  I am guessing that the engine immediately slows and stumbles on the rear carb (lean).  If so, set the fuel level higher.

David when I lift the pin for the front it stumbles then recover. When I lift the pin for the rear the engine dies.