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.
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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.
- Please keep in mind, this is a generalized guide to tuning and jetting a Mikuni round-slide carburetor. This does not provide all the answers to all carburetor issues. But, this is really valuable information for anybody who works on motorcycles - from weekend warriors to pros.
- The Mikuni TM40 with the remote choke and the stock Kehin on the XR650 use a similar plunger. If I remember correctly, there wasn't as much travel in the cable with the Mikuni carb as there was with the Kehin, but I was able to fully activate the choke anyway.
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.
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.
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.
Mikuni Tm40 Tuning Manual Pdf
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 Tuning Manual Diagram
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