I was contemplating my latest r/c challenge, tuning my Webra, and started thinking about how each of the engine speed ranges are tuned. It became clear that the taper and shape of the low speed needle maps the entire engine operating range *sort of*.

If the low speed needle has a straight taper, and is engaged into the spray bar at all throttle settings, it will define the low, mid, and high speed jetting. The high speed needle, in this case, will only be able to make the high speed setting more lean but not richer.

If the low speed needle has a double taper or single taper with a curved end, it can define the low and mid speed jetting but will disengage the spray bar at high speed and allow the high speed needle to define the jetting. This is what we want.

One last factor is the chamfer at the end of the spray bar. This can have an effect on high speed jetting depending on its position relative to the end of the low speed needle.

Of course, all this assumes that the throttle barrel has a lead so that the low speed needle is pulled out of the spray bar as the throttle is opened.

So, on my Webra, it seems as though the low speed needle has a single taper with a blunt squared off end. When the low speed jetting is set for optimum idle, the low speed needle is still in the spray bar at full throttle and the high speed jetting is made too lean. The high speed needle has no effect. If I curved the end of the low speed needle, I believe I could have a good low speed jetting and have a responsive high speed needle.

Any thoughts. Am I totally off track. :stupid: