Little needles make big difference

[Stanley]

Figured my car was running crappy because it was tuned to lean (previously too rich). Some backfires, overrunning, etc. And the idle was always bad - that is it would idle for about 5 or 10 seconds and then it would miss - kind of a hiccup.

Also I was getting some popping sometimes through the carbs, or a miss, at 4000 to 4700 rpm, And worst, it was kind of crapping out at 4700, not really making any more torque.

It had N58 needles and worn slightly oval nozzles. Put in N27's.

Just learning the SU's so it was a real surprise that putting some different needles and new nozzles would fix all that.

[Patcon]

I don't know needle numerology so what is the stock needle. What kind of step in needle size relatively did you make?

Charles

[Stanley]

Didn't measure them. The old ones said N58 on the shoulder. The new ones look thinner, got them from MSA Motorsport! S.U. Carburetor Jet Needle, 70-71 240Z (N27) - The Z Store! Nissan-Datsun 240Z-260Z-280Z-280ZX-300ZX(Z31/Z32)-350Z-370Z Parts They're not marked but they're supposed to be N27's. N27's are stock for 70-71 cars, and N58's are stock for 72 cars with AT. My car's a 73 AT so it would have had the flat-tops, but the PO put 3-screw round tops. The new nozzles were 70-71 cause that's what MSA told me I needed with the N27's.

Maybe the worn nozzles were causing that rich-lean condition that's been posted about on the site. Maybe the gas doesn't atomize well when the nozzles are worn. Anyway it's nice to have an extra 400-500 rpm before I need to shift. I was winding it out today on the back street - couldn't make it backfire. And it's finally idles ok.

ZTherapy has more selection of needles. I have a book about SU's and needles but it's over my head. I'll leave any needle mods to someone who actually knows what they're doing.

[madkaw]

Stock needle sizes are fine for most applications. Even with my built l24, the SM needles were too rich(as seen on a dyno). I went back with stock N-27's.

[Captain Obvious]

When it comes to needles and nozzles, tiny changes can have surprisingly large impacts. Just one thousandth can have a noticeable effect.

[Stanley]

OK, question for the race mechanics. Not that I race, but this is the South Bay, after all. With the Y70 head it has great mid-range torque, not much top end. I can see how a high-revving race car might need to mod the needles for top end but I found out at the drag strip that shifting later just gave me a slower ET. Would it do me any good? Know a mechanic with tons of Z racing experience, maybe I should keep my mouth shut and just let him drive and mess with it?