Jerry, how much difference is a #6 slide vs a #7... If someone were running a G diameter needle with a #7 slide with proper AF mixture, and then went to a #6 slide how much would the needle diameter need to increase to compensate for the change? H or J diameter? I noticed slide selection is rarely discussed in people's mention of jetting. My understanding is you typically use the leaner slides for large bore applications like a cr500, when should someone consider a slide change and what do you recommend for 250r's? Thanks!
If you have the wrong slide you cannot compensate by changing the diameter of the straight portion of the needle just like you cannot change the pilot to compensate for having the wrong needle diameter or slide. All the fuel circuits overlap each other but, each fuel circuit has a primary throttle position that it has the largest influence on fuel control.
The fuel flow area between the needle and needle jet does not change from closed throttle to the point where the taper starts to change the fuel flow area. The taper on the needle starts to change the fuel flow area at around 1/2 throttle opening. The cutaway on the slide changes the angle that the air flows over the needle jet shroud. At small throttle openings a slide with a large number directs the air more downward as the air passes over the needle jet shroud, causing less suction and thus a leaner mixture than a slide with a smaller cutaway with the same needle diameter. The effect of the slide cutaway is greatest at about 1/8 to 1/4 throttle and has diminished to zero at around 1/2 throttle.
Understanding what fuel circuits are working and the throttle position where each fuel circuit has the heaviest influence is the key to being able to tune like a veteran tuner. Without this understanding one is wasting their time trying to tune a carb.
The pilot jet and air screw have the heaviest influence upon the idle mixture after all of the surfaces in the crankcase have become saturated with fuel and oil and have stabilized so that air fuel ratio from the carburetor passes through the crankcase to the combustion chamber unchanged. It is not uncommon for this to take 10 seconds or more when the engine is returning to an idle after a few seconds of partial or full throttle. Not understanding this phenomena is one of the main reasons guys should not waste to much time trying to make a two stroke idle like a four stroke. A four stroke only has the intake port surface area that has to saturate and stabilize before the air fuel ratio from the carburetor passes through the intake port unchanged.
