Ruttly wrote on 01/18/17 at 05:40:30:Dave , On that chart , what does "straight dia" indicate ?
From what I can tell.....that is the diameter of the jet needle. The top 3 shaded areas on the chart are all related to the jet needle.
The I.M.S is the Idle Mixture Screw, and you can see that it has almost no effect on anything but the mixture at idle speed. This is partly because on the stock carb, the fuel/air mix from the pilot jet goes into 2 separate channels after it leaves the pilot jet - 1 channel goes directly to a port in the side of the carb on the engine side of the throttle plate, the other channel passes through the idle mixture screw before it goes to a separate port on the engine side of the throttle plate. This is why the engine can still run when your pilot jet is too big and you turn the fuel mixture screw all the way in.....the other port is flowing enough fuel to permit the engine to run even though no fuel is passing through the mixture screw port. In the attached image you can see the 2 paths for the idle fuel flow.
Here is a link to where that chart came from - it if for Keihin carbs. Although it is useful, it may not be entirely accurate for our carb. The factory has already done the work to get the slide cutaway and other factors very close - and we can do the fine tuning to correct the lean (EPA induced) condition by changing the pilot jet, clip position and main jet size.
http://www.keihin-na.com/assets/1/7/slide_valve.pdfAnd if you are really "anal" about this stuff...you can see in the above CV carb image why it doesn't make any difference if you use a pilot jet with holes in the sides of the jet.....or no holes. The pilot jet screws into a hole that has no openings on the side, so nothing can flow through those holes.
In the Mikuni Round Slide carb, the mixture screw is an "air screw", and it changes the idle mixture by changing how much air is allowed to pass through those holes in the side of the pilot jet. You can change the mixture characteristics by the size and number of holes in the side of the pilot jet. (The diagram is a bit hard to understand clearly - but there is an air passage that goes past the air screw and then into the side of the pilot jet.