S50 Quattrovalvole; Slightly Higher Performance
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Addicted
2010 S50 Modified "Punkin"; 2003 ET4 150 "ISO"
Joined: 18 Feb 2017
Posts: 536
Location: Deepinnaharta, Texas
Sat Oct 10, 2020 2:51 pm quote
Greetings:

Desperately seeking a size 36 N424-21 Pilot jet for ET4 ISO, I stumbled upon the answer to a mystery described Lo these many moons ago. What is the significance of the "S" behind the Size 35 Pilot Jet that came stock on the S50?

Power Barn in Wisconsin has a website that shows a side-by-side comparison of the Keihin jets. (For Honda XR650's "Secondary" carb, in this case... Yes the 600-650 singles had Two (2) carburetors. Not fun to tune.)

So the "S"-marked jets have much smaller emulsion holes in the stem of the nozzle. Evidently this was to improve fuel atomization when low-intensity of vacuum pulses were present. Mystery Solved! Not sure why Punkin's 4-valve engine had such low-intensity pulses, but that's the answer.
Addicted
2010 S50 Modified "Punkin"; 2003 ET4 150 "ISO"
Joined: 18 Feb 2017
Posts: 536
Location: Deepinnaharta, Texas
Wed Oct 14, 2020 8:33 am quote
S-Jet Mystery Solved
Greetings:

S50 came with this #35 pilot jet - I think there's a pic back a few months. Here's PowerBarn's comparison shot.

KeihinSandNonS.jpg
Emulsion holes in the stem are pretty different. Size of the Vertical/parallel hole defines the jet size.

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