How-To: Tune Throttle Valve Cutaway
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.
How does the throttle valve effect jetting?
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.
Examples of cutaway sizes
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.
What throttle position will be impacted?
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.
How to tune throttle valve cutaway?
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.
What to expect with a richer or leaner slide?
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.
Comments
Has this information helped you out? Please comment below.
Hi I have this problem with my 32 mm Mc. on my 250 Honda 4 stroke Carburetor set up 3 cut away 35 pilot set it one and a 1/2 turns out on the air idle’s good starts good rolls on good but if you grab a handful the engine just completely dies out until you back off then it goes right to idle I’ve tried lots of things and I’m stumped can you help me thank you
Don,
What year make model? Do you have any modifications like air pod, airbox delete, or exhaust?
You might want to read this and watch the video: https://howtomotorcyclerepair.com/how-to-tune-a-motorcycle-main-jet/
That’s 1 1/2 turns out on the air.
I’m working on a 1980 Yamaha YZ250 carb issues. Just slightly above idle the engine hits like a light switch. I tried going up from a 2.5 to a 3.0 slide valve. Still wants to take off just above idle. Ideas?
So you are after less power?
Explain how you are working the throttle in this situation. What throttle position?
Sounds lean on the needle. Lift the needle on clip.