N424-74C is the incorrect Stella 4T idle jet

[Neurotic-Hapi-Snak]

After having some trouble with switching out the idle jet after installing a new exhaust, I did some research online. Turns out, while the Keihin N424-74C is the standard idle jet for PB carburetors, the Stella 4T uses a special idle jet designed to properly atomize fuel at low venturi vacuum. Mainly the atomizer holes on this special jet are smaller than the N424-74C, and the former has only 6 holes vs 8 for the latter. The jet orifice is also lower on the stock jet. The N424-74C will not fully atomize the fuel, resulting in erratic idle, poor performance off idle, incomplete combustion, and carbon fouling.

I believe the Honda Ruckus and some other Honda models use a similar or identical Keihin idle jet as the Stella 4T. Aftermarket idle jets for the Ruckus come in sizes 38, 40, 42, and 44, orifice sizes being universal across all Keihin jets. I may order a Ruckus 42 idle to check if it works.
https://jetsrus.com/a_jet_kit_scooter/h ... ruckus.htm

Otherwise you can try filling in the N424-74C atomizer holes with solder, and drilling out smaller holes, or drilling out the stock idle jet.

[misterbrackets]

Check this out:

http://www.ebay.com/itm/HONDA-ZOOMER-RU ... 52&vxp=mtr

Looks like it has the smaller holes and 6 vs 8.

[mfavata]

I ordered a 42 idle and a 95 main jet from jets r us and installed them yesterday and have not had a single idle issue, and I've put about 20 miles on the bike since the change. I notice that the idle jet I took out did only have the six holes, while the new one did have eight. I adjusted nothing on the carb other than installing the two jets and it runs GREAT. I can't say I've achieved any higher top speed, but my acceleration and overall power seems much better. I can climb hills a lot better now.

[misterbrackets]

Out of curiosity (and the the fact that jets aren't all that expensive), I tried swapping out my #42 idle with one with the 6 smaller holes. The difference wasn't huge, but I do notice a very slight improvement starting in the morning and the idle seems a bit smoother. I also had to adjust the mixture by about a half a turn - clockwise.

[ericalm]

Hm. I've been running super rich since rejetting and haven't gone back in to correct it. I may go back to the stock idle jet and the 95 main. (Think I put the 98 in…?)

[misterbrackets]

I've had the 98 main jet in for about a year. It was about right (a tad rich) with the 150 cylinder and Pinsaco exhaust, but seems perfect now with the 165 installed.

Though you might want to leave the 98 and see what happens with the original idle jet in first.

Did you change your air filter as well as the exhaust?

[Cangurito]

Hello to you all,

I'm very interested in this topic. I have a 125 cc 4 Stroke Star (Stella) and i would like to upgrade it with the GPR exhaust and rejetting the stock carb. Reading the post it seems that the Idle Jet N424-74C in our stock carb is different to the spare you can buy for example by JR MotorSports, isn't it?

Hello Neurotic-Hapi-Snak, I read somewhere you finally moved to the Polini Mikuni option, but during your attempts of tunning the stock carb with the spare idle jets (42 or 45), did you find some 42 with 6 holes instead of the spare 8 holes?

For a 125 cc 4 strokes, upgrading only the exhaust, the Mikuni 22 choice is too much. I'm sure that it won't work well. I think my only chance for rejetting properly, without losing patience and money would be use the idle jet (42 with i suppose 6 holes) used in the Keihin PB20 that mounts the Stella 200 4 Stroke.

Any help would be appreciated,

Saludos.

My lovely babies!

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[Neurotic-Hapi-Snak]

I never ordered any new jets to try after that, I just switched to the Polini (Mikuni) 22mm carb. The Honda Ruckus idle jets, from what I read online, seem to be similar to the stock idle jets. Otherwise you could try the 42 idle jet from the 200, but I don't know if the 200 has the special idle jet.

The Mikuni 22mm oval bore (the carb in the Polini kit) is often used on 100-140cc Honda clone pitbikes, so I don't think it would be too big for a 125.

[lastusername]

Sorry for necro-posting, but I just thought I'd chime in with some feedback. I just installed a RamAir sock with this scoop from Tasso: http://www.tassoparts.co.uk/cgi-bin/sto ... &uid=14097. I've also been running a 4T GPR exhaust, but definitely been running lean for awhile. I jetted up tonight to the standard 94 main jet, along with the 42 from here: https://jetsrus.com/a_jet_kit_scooter/h ... ruckus.htm

Fired up fine and idles great. I'll take it out tomorrow and really give it a check at speed, but as far as idle, that Ruckus jet worked out of the box with no problems.