Quote from: WestTexasKing on November 10, 2012, 05:23:24 am

Quote from: Toydoc on November 10, 2012, 05:12:00 am

I'm with you. But how do we know if either one is "right on" with jetting, and what was the target jetting for (drag, trail, dune)?

I'd have to say that it would only take one or two sizes to be "right on" with jetting if it's at all out of tune.
It surely doesn't account for the difference between a 500 and 620 main jet.
Try putting a 620 main jet in a carb that was tuned for a 500 main jet and it'll be ridiculously rich, regardless of what type of riding you're intending to do.

Not always true. Every needle jet has a flow limit. I've had some with no flow change in MJ after 480. So could you add more needle jet to the 38 and bring it down to the 500? They are all combos that work, but is it the only one that works, and is it the best or right on? I really don't know so I'm asking.

I agree that there is a needle jet and needle combination that works best for a given engine.  I have encountered the same problem many times when increasing the main jet size or when removing the main jet will not richen the mixture.  When this condition exists, the flow area between the needle jet and the tip of the needle is controlling the fuel flow and not the main jet.  The solution to this problem is usually corrected when I install a needle that has the same shape up to about ¾ throttle and has a steeper taper from ¾ to full throttle. 

If the flow area between the tip of the needle and the needle jet is restricting the fuel flow at full throttle, installing a larger needle jet would help the main jet problem but will probably make the mixture too rich at small throttle openings.   If you have a needle that has a very sharp point and increasing the main jet size has no effect, then it is time to increase the needle jet diameter and the needle's major diameter.

We make our own needles for our alcohol carbs because of the above problems.  Most gasoline carbs have needle jets and needles that have their largest diameter around .100”.  Our alcohol needle jets are usually in the .150” to .160” diameter.  Our alcohol needles have a major diameter in the range of .140” to .150” diameter.  As the needle jet and the major needle diameters increase, it provides more flow area at ¾ to full throttle while allowing the correct flow area at the lower throttle openings when the needle's major diameter is optimally sized.