Notes about SM needles and SU needles

[Firepower]

First off what I run:

I have a 240z body, L26 engine, rebuilt E88 head, with the dome top carbs, 5-speed tranny, R200 3.90 diff and 6-into-1 header. 

So - after I rebuilt the dome top carbs, I replaced the stock (N27) Needles in my carbs with SM needles, as these were recommended to me.

My experience is my L26 ran worse.  Way too rich at idle, to the point where you had to go to 10+ degrees advance and it still idled lumpy. 

So I tuned it over several tries, balanced the carbs etc, and then have to lean down the mixture at idle, but then the top end pull suffers (for me) at 3-5K RPM.

So I swapped out for N54 - which I think are original 240z needles. And these let me tune idle a lot better and still get more response from the engine in the ‘normal driving’ acceleration ranges.  Much better for the driving I do.

I don’t have a needle profile for the N54 - if someone has these numbers It would help me understand why they work in my case.

I suspect they will be close to the N27 or maybe the SU BC needles (part is AUD1063). From what I can find out, the BC were what Hitachi used when adapting over the HS6 SU over to the 240z.  BCE is considered a ‘240z needle’ but after idle will run richer even than the SM.  Maybe an L28 stroker would need that much fuel.

See: http://www.teglerizer.com/cgi-bin/needle100db.cgi?Needlecode=BC

For a list of all the BC needle profiles.

What I’ve learned is that you can go to a needle with a leaner idle profile (Stations 1 & 2) and then the rest of the needle can give a RICHER mixture than the profile suggests.  That’s because of where you have to position the jet when getting the correct idle air/fuel mixture.

Conversely, when selecting a needle with a richer profile for idle, it can give a slightly LEANER mixture than the profile suggests.

SM needles can work, provided the carbs are ‘as new’ with no jet wear, and your ignition and timing are all good. (Good vacuum advance, no vacuum leaks). I now consider the SM to be “Race Needles” and if you order some, they are SU part AUD1328. 

The ‘stations’ of a needle profile are measured in 1/8” intervals   Station 1 is immediately below the shoulder of the needle, Station 2 is 1/8” below that, etc.

For the HS6 (and thus the Dome top carb): Stations 1 and 2 are idle,  3 to 7 are cruising and acceleration (with about 35mph around Station 4, 50-55 mph around Station 6) and 8 to 13 are closer to the top end rev range of the engine.  You can ignore the Stations beyond that for a 1 3/4” carb. 

This is what I have for the SM profile:

.0990 .0950    .0915    .0885    .0855 .0825    .0795    .0765    .0735    .0710      .0690    .0670    .0650    .0630

Again, anyone that has numbers for the “stock” N27 or N54 needles can you please provide them.  I don’t think the attached which I found online can be correct for the N27.

Thanks, Richard