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
yes
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."
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.
Indeed they did.
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!
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.
they were for tuning . I just gave a set of them in the case away for amass, actually.
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