Mystery Jet on FCR39MX

I finally have my 2003 YZ450F rebuilt after buying it with a broken case this winter. It runs pretty poorly and I quickly realized I had reassembled the carb incorrectly. This was the first FCR carb I have ever worked on. I had put a type of mystery jet in the Main Air Jet hole since I could not find any other spot it could possibly go. Naturally this kept the main from working at all. My big question is what the heck is this jet/part and where does it go. It is about 1/2" long with no threads, but a screwdriver slot in the top and a very small hole running through the middle, but exiting out the side, not the end.

Sorry for the horrible pic, but my camera cannot focus super-close. I will try again later if you need a better picture to help.

Jet.jpg

By guessing from the pic it looks like the accelerator pump nozzle.... Does it have a small hole only in one side?

Yep, a small hole pointing to the side. Here is a better pic:

Jet2.jpg

Indeed it was the accelerator pump nozzle. Thanks so much for the help. This is the first carb I have ever worked on that has a secret, "DO NOT DISASSEMBLE", portion. The bike is running much better now so I can get onto regular tuning.

Thanks again.

Indeed it was the accelerator pump nozzle. Thanks so much for the help. This is the first carb I have ever worked on that has a secret, "DO NOT DISASSEMBLE", portion. The bike is running much better now so I can get onto regular tuning.

Thanks again.

I imagine it would run rather poorly with that shoved in the main air jet intake. :thumbsup:

Your carb also has a non-serviceable float needle seat, as I recall (the needle IS serviced). They started that in '03 or '04 at the latest. Don't try to remove it.

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