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The Mikuni BM38-32 carburator was used on Ventures and Phazers from 87 to 96.

The main jet mixture is OK but the midrange is lean because of ethanol in the fuel.

I have not figured out how to change the midrange mixture.

The parts list does not show any alternate part numbers for anything but the main jet.

There is no needle jet or jet needle to adjust or replace.

How is the mid range mixture adjusted?

I'm not familiar with that type of carb. But if you call a Gary at Fox Distributing he can help you. (630) 513-9700 9am-4pm central time.
He is a Mikuni parts distributor and is very helpful.
He also stocks most all parts and will ship same day.

From http://www.totallyamaha.com/ (Technical pages)

Phaser Carb Update Bulletin

by: NY_SXR700


Service bulletin 1996 . Basically you can change the primary and main nozzles for a richer mixture. The primary nozzle part #80L-14489-00-00 has two .9mm holes as opposed to the stock nozzle's one 1.0mm hole.
There are at least 3 different main nozzles

Part #8Y2-14941-3A-00 is STD. has an OD of 6.5mm, ID of2.2mm and 12 0.7mm air bleed holes.

Part # 8Y2-14941-1A-00 has an OD of 5.5mm, ID of 2.2mm and 10 0.7mm air bleed holes.

Part # 8Y2-14941-2A-00 has an OD of 5.5mm, ID of 2.4mm and 10 0.8mm air bleed holes.

A smaller OD will richen the mixture at snap throttle and lean it at mid to WOT during steady throttle.
A larger ID will richen the mixture at mid to WOT conditions
Fewer and larger air bleed holes will richen the mixture at mid to WOT conditions.

You can use any combination of the listed nozzles for a wide range of mixtures, depending if you want to richen the mixture during snap or steady throttle conditions.

Full rich, at steady mid to WOT would be using the 80L primary and 2A main nozzle. Leanest would be STD primary and the 1A main nozzles.

For quick snap throttle conditions, richest would be 80L primary and 2A main nozzles. leanest would be STD primary and STD 3A main nozzles.

There is a chart that lists the calibration options from lean to rich in the service bulletin. Also the hole in the STD primary nozzle must face towards the air box.

The second image was a thumbnail and was unreadable.
I found the original at totallyamaha and read it there.
It was extremely helpful but not very understandable the first I read it.
I had to read it several times before it made sense.

I talked to Gary.

The BM38 is an OEM carburetor. He can only get main jets and pilot jets for it. Any other parts need to come from the dealer.