Excel Spreadsheet Functions
It breaks the needle codes to tell you the needle taper angle, L1 length, shaft diameter, and straight length to taper start.
This helps if you want a different taper angle or straight shaft length. For the 3 letter needles for the 33-38mm PWK/PJ/PE/PWM carbs it also shows if what you want is available from SUDCO (whose inventory is basically the same as Jets-R-Us but I have never had any quality issues when buying from SUDCO so I prefer to buy from them. Email Chad at chadt@sudco.com)
As with all my spreadsheets, the data entry cells are the light blue ones. Just put the pointer over the cell, click, then type in the data, then press the Enter key.
Straight Shaft Length
First you need to try different needle clip positions to find which position gives the best off idle transition from closed throttle to 1/4 throttle. You should mark your throttle body and handgrip with White-Out to see the 1/4 throttle position while riding. Gradually open the throttle while noting the engine response. It's normal to have a lean zone just off closed throttle but beyond that you should have normal power. If the best transition power requires the needle clip to be at the first or last position for that to heppen then you need a needle with a different straight shaft length. If you are at clip position #1 then get a needle with a 2mm longer straight length. If it is at #5 position then get a needle with a 2mm shorter straight length. It is best to not go by the L1 length (I'll explain later).
Needle Taper Angle
A too-lean needle will cause engine bog when cracking the throttle open from a mid throttle position. A too-rich needle will give an inconsistent exhaust note at mid throttle and then accelerate good when cracking the throttle open. A richer needle will have more taper angle, and a leaner needle will have less taper angle. People erroneously try to make up for incorrect taper angle by changing the clip position but then that screws up the transition power off idle. The correct way is selecting a needle with a better taper angle.
Needle Diameter
About half of the gasoline going to the engine at idle comes from the needle/needle-jet space. The way to tell if that clearance is correct is by analyzing the slide open distance at idle. If the slide is less than 11% open at idle then probably the needle clearance is too big and you need a fatter needle or skinnier needle jet. If the slide is more than 11% open at idle then probably the needle clearance is too small and you need a skinnier needle. The 11% only applies to reed valved engines. Click here to read more.
Engines 200cc or larger eventually wear out the needle jet to have a larger hole due to the needle vibrating with engine vibrations (unless it has an extra balance shaft). If you don't want to replace that needle jet (often they are just a press fit) then you need a fatter needle to make up for it.