How do I identify roundtop needles?

[lombrozo]

Ebay roundtops aparently from 72, with water chambers at the bottom of the carbs.

Motor is stock 73 with Schneider 270 degree cam, pertronix ignition. Smog all stripped, water supplied to carbs and manifold. Stock exhaust. Mechanical fuel pump.

All seems to be going well, but I am having trouble getting a stable idle when hot.

Have the Ztherapy video, but no idea how to tell which needles are installed.

How low should I be able to idle with a 270 degree cam?

I made a larger heat shield thinking that I was cooking the float bowls after a hard run. It will idle all day in the driveway, but first stoplight after the mountains and the idle slowly drops until it dies if I don't pedal it.

Experimenting with leaning the idle now on the alternate theory that it richens up with all that heat raising the vapor pressure in the float bowls.

Looking forward to your theories. Tired of three-footing at the stoplights.

[kenz240z]

What year is your Z? The '73's have an electric fuel pump mounted back by the fuel tank. There is a filter inside the electric fuel pump that can cause problems with fuel delivery if it is dirty and clogged.

[Bruce Palmer]

Although I don't think it has anything to do with your problem, you can pull the dome off and lift the piston out, release the set screw and carefully pull the needle out. The letter/number designation is on the head of the needle, more than likely smashed unreadable by the set-screw. Just another of those Murphy's Laws deals in life........

First thing I would do is the "spray around" with carb cleaner to see what you have going on for vacuum leaks.

Palmer's first law of SUs: The vast majority of 30 something year old SUs are worn out and all the fiddling in the world is not going to make them well...... ;^)

[hls30.com]

Palmer's first law of SUs: The vast majority of 30 something year old SUs are worn out and all the fiddling in the world is not going to make them well...... ;^)

This is from a man who knows SUs-take it to heart, and the carbs to ZTherapy!

Will

[Arne]

I totally agree. I've had three sets of 240Z SUs, and one set on an old MGB. The only set out of the four that weren't totally worn out are the set on the red 240Z - it only has 60,000 miles on it. All the other sets were shot and had huge vacuum leaks at the throttle shafts. Since the mixture throughout the range on an SU is dependent on the idle mixture adjustment, if the shafts suck air at idle, you're going to be way rich everywhere else. And if you lean it so it runs OK at higher RPM, the idle mixture will be dangerously lean.

[wfl_colorado]

Along these same lines I had my 73 w/ SU's running pretty good, but replaced the entire exhaust and it will idle fine while adjusting it, but once I drive it a bit and make my first stop it behaves like lombozo's where it wants to die. I have been told the back-pressure has been affected, but I don't know how to account for this. Could the new exhaust(which is off an old Z) be plugged or restricted causing this?

Tx,

Bill

[lombrozo]

The car is a 73, bone stock, dad bought it new; it does not have the electric fuel pump, and the filter is good, tank was boiled out.

I did the ZT video instructions, pulled the needles, could see no trace of marking s on the heads (no doubt about that Murphy fella).

I have sprayed extensively and find no vac leaks. I don't have much vac at idle, I don't know how much to attribute to the cam (I misspoke, it is Racer Brown 270)

I took it a quarter turn leaner when it was hot last night, and it raised the idle, I was able to back off the idle screws and get a bit better idle, but still wants to fade away at long stops. I do have a UniSyn.

Does anybody sell stock 72 needles so that I can at least get back to a baseline?

[DRBall]

Yeah ZTherapy.

[beandip]

Stock needles are 54s . You should be able to find them an the Nissan dealer. I found that these needles work the best in my 280 engine and also the same needles in my 240 . This surprised me in that when I changed engines and the ZX engine being larger and with a moderate cam and headers . I thought I would need richer needles. I tried several different ones and came back to the 54s for overall drive ability and performance. this is My experience any way. I tried the SM , the 42 from Nissan and they were both WAY to rich . I was getting unburned fuel in the crankcase. I am at 300 ft elevation , If you are in Co or NM you may look to the 58 needle as it is a leaner one . The only way I have found to determine the needle number after it is used . Is to mike a known needle and then mike the one in question at the precise same length for comparison. factory needles are printed with the numbers . SMs are stamped with the letters. Gary

[nwcubsman]

Okay, you may be tired of hearing this from me, but this was my personal experience.

Almost the same setup here, except I still have the electric fuel pump and newly rebuilt Z Therapy carbs (what a beautiful thing to see). Cold idle was good, but after a 30 or more minute romp down the freeway, the farther I drove on the surface streets the more idling problems I had, to the point of dying at a traffic light or in my driveway. I had the luxury of having personal help at Z Therapy due to my frustration. After futzing (is that a word?) around for months and consulting with any number of people, someone drove it and decided my valves were too tight. Wow!, what a difference.

Best of Luck!

[wfl_colorado]

I have been wondering about that myself. Thanks Bryan, I will have to take a look.

Bill