I've updated the carbs to the F3 carb specs. If you haven't done this yet to your TZR, I highly recommend that you do. It runs much better, and its easier to tune when you make changes to the motor.

PART carburator body main jet needle jet jet needle pilot jet air correction jet air jet vent power jet

STOCK Mikuni TM28SS 230 hex Q-2, series 505 5L19, clip position 2 20, bleed type 1.5 press in jet open with hose 60

MODIFIED Mikuni TM28SS 270 hex O-8, series 166 5EJ13, clip position 3 27.5, bleed type drilled out to 2.5mm plugged disabled

Here are some pics of the F3 carb modifications that I have done.

First picture is of the air correction jet breather tube that I removed and pluggged instead of just tying off. Use some pliers and just pop that plastic barb right out. Turns out the hole is a drill tap size for a 4x0.7mm thread. I used a 4mm set screw with a bit of red Loctite to hold it in place. Looks nice and clean now.

I've done the same to the powerjet circuit. This isn't in the F3 manual but I don't see how the 'power jet' system could work on this carb the way the oulet is situated in the intake bell of the carb. Better to block it off and not worry about it. When running the bike in 2007, I didn't notice any problems at the end of the straights, so it seems I'm correct.

This is a look at the inlet of the carb. You'll see the air correction jet plug (located at 6 o'clock) removed. The 2XT carbs have a threaded in plug instead of a pressed in plug for the air correction jet.

NOTE: Remove the front plate when you drill out the plug and the air correction jet on all TZR250 carbs. If you don't, you'll drill right through the carb body and make a mess of it. The hole in the body is already the right size, so just work on the front plate.

Finally, here is the float bowl with the plug in the power jet fitting. On the 2XT float bowl, the fitting just unscrews, on the 1KT/2MA you'll have to drill it out. Once again I plugged it with a 4x0.7mm set screw and locked it place with red Loctite. Under the plug is the #60 jet for the power jet system. I just left it there, untouched.

Considering that my 1979 TZ250 has 34mm carbs, I'm sure that the TZR's breathing is severly restricted by the stock 28mm carbs no matter how you modify them.
Do NOT bore out the stock 28mm carbs. The carb body for some reason is a poor casting and has porosity. So, I've decided to install a set of brand new Mikuni TM34mm flat slide carbs.

34mm Mikuni TMX carbs are the way to go. Flat slides, large float bowl and easy to get tuning parts. However they just don't 'bolt right on'.

This will require that I CNC machine a new set of maniflods to hold on to these bad boys.
Here we go.....

Starting with a square block of 6061-T6 aluminum, I wrote a CNC program to cut the basic shape with all of its contours that I could not do on my manual mill. I cut out a hole down the center to make the future porting and blending into the reed cage a bit easier. The contour is made up of overlapping shapes of the square reed cage and the oval of the rubber carb flange. It looks a bit funny, but its necessary to make sure there is enough material for wall cross-section thickness.

Then in my manual mill I angled the top to 23º to tilt the carb down. The reed cage sits at 45º, and bolting the carb straight on would cause the float bowl to overflow into the motor even when running. A quick call to Mikuni USA confirmed that the maximum carb angle is 35º. My manifold places the carb at 22 º. After angle cutting, I drilled and tapped the two 8x1.25mm bolt holes.

If you have a close look at the stock maniflod you can see the stuffer that Yamaha designed into the lower edge to help fill in the dead zone. My new manifolds don't have this stuffer since my design has allowed me to blend the carb opening into the square opening of the reedcage perfectly.

My CAD drawings show that this is definitely the way to go. I designed the manifold to move the new carb out (back) about 25mm to give it some clearance between the float bowl and the motor.

There's not alot of room in there now with the 34's installed. As you can see, the 34 is quite a bit bigger than the old 28. The larger float bowl also means that there is less of a chance of a lean out when I wack open the throttle coming out of a high speed corner.

This is what you can see from the side of the bike, peeking past the trailing edge of the lower body work. Mmmmmmm, sexy billet aluminum.
It's a wickedly close fit. The float bowl just clears the tab for the sprocket cover on the left side and the rear upper motor mount on the right side. I'm going to slide a piece of 1.5mm rubber sheet under the carbs just to protect the carbs if there is any vibration and the carbs touch the engine case.

CONTACT ME AT TIMOTHY@NAUGHTYBOYMOTORRACING.COM