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Plastic Mikuni main jets?

Started by IR8D8R, January 22, 2026, 01:41:01 PM

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IR8D8R

January 22, 2026, 01:41:01 PM
Did Yamaha/Mikuni ever produce plastic main jets?

An eBay seller I've bought NOS stuff from has it listed for various models including RD350. Assume nylon.

https://www.ebay.com/itm/135375987488.

IR8D8R

m in sc

#1
January 22, 2026, 01:46:05 PM
yes

SoCal250

#2
January 22, 2026, 02:34:37 PM
Pretty sure that's not a genuine Mikuni jet but an aftermarket version made by Pro Jet. To my knowledge Mikuni only made jets in brass.
The listing even mentions "Part is an aftermarket nylon Pro Jet carburetor main jet."
75 Yamaha RD125B   75 Yamaha RD125B (project)
75 Yamaha RD250B   75 Yamaha RD200B (project)
73 Yamaha RD350     77 Yamaha RD400D   79 Yamaha RD400F  
91 Yamaha TZR250R  89 Yamaha FZR400   05 Yamaha FZ6   
05 Yamaha XT225TC  82 Honda MB5  02 Aprilia RS250 Cup (sold)

1976RD400C

#3
January 22, 2026, 03:10:02 PM
I have some of those that are 40 years old. The holes seem to be what they say they are compared to brass Mikuni. There's a bit of nervousness when tightening them.
'76 RD400 green  '76 RD400 red   '84 RZ350

quocle603

#4
January 22, 2026, 03:46:53 PM
Indeed they did.
Do not underestimate the power of a two-stroke.

1975 Yamaha RD350 (modified), 1973 Yamaha RD350 (stock), 1971 Suzuki T500, 1981 Yamaha XS650 HS2, 1982 Honda MB5, 1980 Puch Maxi, 1979 Puch Magnum, 1993 Tomos Bullet, 2003 Malaguti Firefox F15 LC

85RZwade

#5
January 23, 2026, 09:51:41 AM
I have a whole set of those that I've never used. They were old when I got them, and I've always worried that they'd break off during installation. Worse yet if they broke off during a ride!
I post waayyy too much

RDnuTZ

#6
January 23, 2026, 10:30:00 AM
UNI filters also sold nylon jetting kits in the 1970s. Single cylinder applications in multiple sizes. I have 2 of those kits: 1 lived in the toolbox that went to races with me and another stays at home for initial jetting on new builds.
1987 TZR250R Restricted Japan Domestic market bike (Project)
1977 RD400 (Project)
1974 RD350 (2) (Projects)
1973 RD250 (Project)
2022 Beta 300 X-Trainer, Yamaha Vintage MX, YZ (18)

m in sc

#7
January 23, 2026, 10:50:21 AM
they were for tuning . I just gave a set of them in the case away for amass, actually.

IR8D8R

#8
January 23, 2026, 01:52:58 PM
Quote from: m in sc on January 23, 2026, 10:50:21 AMthey were for tuning .

Thanks! That makes way more sense. I didn't notice the line about it being aftermarket. I've never heard the idea of plastic factory carb jets in a road-going vehicle from that era. Old nylon threaded fittings break off at the threads every time. Automotive Induction systems are mainly plastic now...

PEEK might be an acceptable plastic for jets. It's very hard, heat resistant, and you can machine it.

IR8D8R

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