Mikuni Tm40 Tuning Manual

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The throttle valve slide is definitely overlooked when tuning a carburetor. Most people, including myself, will change out pilot jets first to alter low speed running conditions.

The throttle valve will have a “cutaway” on the air box/intake side of the carburetor, and it basically impacts the pressure signal across the pilot and needle jet.

The best way to describe how this signal varies is to imagine placing your thumb over a garden hose. When you cover most of the garden hose with your thumb, the water will shoot out faster right? And what happens in faster flow? Pressure decrease! This is known as Bernoulli’s principle. So by altering the cutaway size on the throttle valve, the pressure/signal of airflow through the carburetor can be changed. Therefore a smaller cutaway will have a stronger signal due to creating lower pressure, thus a richer condition, than a larger cutaway will.

Mikuni Tm40 Tuning Manual Diagram

Below are 3 different Mikuni VM round slide throttle valves. On the left is PN VM32/65-2.0, middle is PN VM32/65-2.5, & on the right VM32/65-3.0. The slide on the left will cause the motorcycle to run richer than the middle and rightmost pictured slide. Check the Mikuni VM Roundslide Parts Diagram for additional part numbers.

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  1. Mikuni Motorcycle Carburetor Theory 101. Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything.
  2. In this video we show you how to connect your throttle cables and then an easy way to synchronize Mikuni TM and VM carbs. Leaking needle & seat causes and removal In this video we discuss the common causes of float valve failure and also show how to remove the common types of Mikuni needle and seat assemblies.

According to the Mikuni VM Carburetor Super Tuning Manual, the throttle valve impacts jetting at 1/8 to ½ throttle positions, especially at ¼ throttle. See picture & graph below which illustrate where the slide come into play.

Mikuni Tm40 Tuning Manual Diagram

Mikuni Tm40 Tuning Manual

Since the largest impact will be at ¼ throttle, install a fresh of plugs and perform a plug chop. If the motorcycle coughs or spits through the carburetor, it is lean. If the exhaust note sounds dull or blubbery, it is running rich.

I was recently tuning a carburetor with a 3.0 slide, 35 pilot jet, and air screw 1.5 turns out. I experimented with a 2.0 & 2.5 slide to see what effects it would make. I noticed that since the signal is stronger, it will pull more fuel from the pilot circuit at idle, and I ended up leaning the pilot air screw to 2.5-3.0 turns out. You may have to drop down in pilot jet size is you experience this as well. You’ll want the pilot air screw to be 1.5 turns out.

Mikuni Tm40 Slide

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