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Tuning Question: Which replaceable carburetor part has the biggest influence on jetting thru the whole throttle range?
1. idle jet, 2. main jet, 3 needle jet, 4. slide cutaway, 5. needle
ANSWER
The needle jet edges out the main jet by just a little as the one replaceable part that has the greatest overall effect on jetting thru the whole throttle range. Surprisingly, if a carb is jetted right, then the needle jet clearance to the needle contributes more than 60% of the gasoline at slightly above idle. But of course this depends on carburetor sizing. The bigger the carb, the less vacuum there is at the needle jet when the slide is very low.
So overall I would say the needle jet is the most important "adjustable" part in the carburetor. But ask your riding buddies if they even know what that is. In Keihin carbs the needle jet size is fixed and the "adjustment" comes by selecting different needles of different widths. The clearance between the needle jet and the needle determines the effective needle jet area. That is strictly controlled by that clearance until over 1/4 slide open. After that the needle taper allows more clearance the more the slide is opened. In Mikuni and Dellorto carbs you can select needle jets with different hole diameters.
Keep in mind also that you can assume the needle jet hole is wallowed out to be too big in a carb over 15 years old, the carbs in big engines being the worse due to the extra vibration if the engine doesn't have a balance shaft.
When that happens you have to put in an extra small idle jet for a good idle mix. When you need a smaller idle jet then often the slide opening for a smooth idle is over 10% of the carb bore.
If you want to inquire about getting a new needle jet you should email chadt@sudco.com and he'll help you out.
The graph below shows the % control that the 3 jets have thru the whole throttle range. This is from my VM18 and varies from carb to carb, especially due to sizing.
1. idle jet, 2. main jet, 3 needle jet, 4. slide cutaway, 5. needle
ANSWER
The needle jet edges out the main jet by just a little as the one replaceable part that has the greatest overall effect on jetting thru the whole throttle range. Surprisingly, if a carb is jetted right, then the needle jet clearance to the needle contributes more than 60% of the gasoline at slightly above idle. But of course this depends on carburetor sizing. The bigger the carb, the less vacuum there is at the needle jet when the slide is very low.
So overall I would say the needle jet is the most important "adjustable" part in the carburetor. But ask your riding buddies if they even know what that is. In Keihin carbs the needle jet size is fixed and the "adjustment" comes by selecting different needles of different widths. The clearance between the needle jet and the needle determines the effective needle jet area. That is strictly controlled by that clearance until over 1/4 slide open. After that the needle taper allows more clearance the more the slide is opened. In Mikuni and Dellorto carbs you can select needle jets with different hole diameters.
Keep in mind also that you can assume the needle jet hole is wallowed out to be too big in a carb over 15 years old, the carbs in big engines being the worse due to the extra vibration if the engine doesn't have a balance shaft.
When that happens you have to put in an extra small idle jet for a good idle mix. When you need a smaller idle jet then often the slide opening for a smooth idle is over 10% of the carb bore.
If you want to inquire about getting a new needle jet you should email chadt@sudco.com and he'll help you out.
The graph below shows the % control that the 3 jets have thru the whole throttle range. This is from my VM18 and varies from carb to carb, especially due to sizing.