SU's won't balance

[alien-e]

A little history: I bought my Z with a 4bbl Holley on it and decided I would do better with SU's, so I bought a pair on e-bay. They were gummed up pretty badly, but I put a lot of effort into them and got them cleaned up. Unfortunately, the needles were stuck in the jet tube (both). Halfway through the build, I got another pile of carburetor parts, that were the same models as the one that I currently have. So, I retrieved the needles from those SU's and finished the re-build. So, I installed the carburetors and they ran ok, but I think that the needles were bent, and the cylinders didn't "clunk", like they were supposed to. The choke on the front carburetor was sticking as well. So, I bought a pair of needles from Ztherapy which allowed the needles to seat properly and the chokes snap back in position as they should. I have the dashpots filled with "air tool oil" (which is 20wt oil) to the proper level. I have adjusted the valve lash, replaced the spark plugs and wires, and set the timing to 10 degrees. I also run a pertronix igniter, so the points aren't an issue. I did a lot of research on the subject and can't find any help in this category.

Problem: The front carburetor runs strong, and responds to adjustment of both the idle screw and the mixture nut. In fact, the engine ran pretty smooth at 500 rpm with both idle screws all the way up and the mixture nut set to 1/2 turn from the top. However, when measured with the unisys tool, the two carburetors don't balance. I adjust the RPMs to 750 and try to balance the rear. When the rear is balanced, the RPMs are up to 1000. When I lift the front carburetor to disengage it, the engine dies. When I lift the rear carburetor, it runs strong (on the front 3 cylinders). No amount of adjusting of the mixture nut makes a difference. So with both balanced at 1000 RPMs, and then adjusted up to 1500 to adjust the high RPM balance, the rear carburetor now pulls considerably more air than the front.

What should I look for to make this rear carburetor balance?

[240ZMan]

alien-e,

I had the same issue as you. In my case the problem was that the needles were not set the same amount into the pistons, even though to my eye they looked identical. Someone here suggested the following which worked for me:

- remove the pistons from both carbs

- set the needle into the piston on one of them so that a metal straight edge just touchs the shoulder of the needle. Note that the shoulder is NOT flush with the machined groove. Tighten that set screw.

- leaving the set screw on the other piston loose, gently point the two needles at each other and bring the pistons towards each other so that the needles are next to each other and touch the base of the other piston. (Sorry I'm not explaining this very well, but if you hold one piston in each hand I think you'll get the idea.)

- when the needles touch the base of the other piston, you will know that they both stick out exactly the same amount.

- tighten the other set screw and reassemble.

Hope this helps.

[zztom]

Are your throttle bushings sucking air? Spray some Carb Clean at the throttle links going into the carbs. If the car wants to die then they are leaking. I had the same problem with front and back carbs doing the same thing as yours. I found the rear throttle bushing was bad.

[alien-e]

I evened out the length of the needles and it seemed to do the trick... mostly. It was much closer to balanced at high and low idle than ever before, but it's not quite right. The rear carburetor is still not as strong as the front. I'll try the carb cleaner tomorrow.

[Zs-ondabrain]

And don't forget your 5-weight oil, chech it and fill accordingly.

Helps everything flow nicely.

[beandip]

when you had the carbs apart did you check the butterfly clearence ? If the butterfly is not adjusted properly there will be a air leak around it and you wont be able to ballance the carbs. . If this is the case what you must do is to remove the carb and look through it with the butterfly closed. Hold it up to a light and see if there is light around the disk. Be sure to have the idle adjustment backed off. If there is light seen around the disk , loosen the screws that hold the disk in place just enough that the disk will move with some resistance. Now close the throttle firmly and check for alignment . Do this untill it is seated all around and no light is seen , tighten one of the screws and check again if all looks good remove the loose screw and use lock-tite on it and tighten it down and do the same with the remaining screw . This should do it if you dont have another vacume leak. When you lifted the front carb. piston and the engine died , it's because the rear carb is too lean . Now this could be caused by the above needed adjustment , or it might be just out or adjustment. If you want a step by step on adjustment . Go to the tec page and scrowl down to the information on SU adjustment..:classic:

[Chino 240Z]

Gosh, you're close but like me want next to perfect. Kinda a process of elimination by rechecking or readjusting each 1 or 2 more times to dial them in.

1.) May have some air leaking around the shafts?

2.) Beandips info on the butterfly adjustment is a good place to check.

3.) I would get that 20wt. oil replaced with ATF or Marvelous Mystry Oil, closer to 5 wt.

4.) Float may need 2-hairs of adjustment more then fine tune with mixing nut.

5.) I set each needle to its own nozzle and carb, not one needle to the other needles set point. No 2 carbs are exactly the same. Try this: Mixing nut / nozzle all the way up then with the needle pulled out or left out a tad long and needle screw loose enough for needle to slide, gently set the piston in it's carb and softly push down till needle touches nozzle and "soft-clunk" the needle pushes up and into the perfect position. Then gently lift out piston(try not to move needle)and tighten the needle screw at that postion. This way you know each needle is closed and at the bottom of its own nozzle and when shut, it's shut. Each needle will be equal to each other and with in its own nozzle & carb. Now open up mixing nut to desired starting point and tune.

6.) If you didn't use new gaskets between carb & intake you may have leak there?

Good luck.