Seems like if a WT 456 carb would work, so would any other Walbro..The shindaiwa 4 stroke I just converted uses a WYL carb, no adjustable high speed needle.....There were no extra springs in the WYL on the Shindaiwa...It worked OK, but the Shindaiwa is like a YS, with oil in the fuel and another reed in the crankcase..There are no crankcase vents, so there is a good pulse for the carb....I got almost 7000 rpm with a Mejzlik 18-6 and about 6300 with an 18-8...A 20-5 PK turned about 5400..
I made an adapter from 3/8 G10 for a WA167 to get an adjustable high speed needle, it worked out OK...
A WYK90 is 10.5mm, has a high speed needle, costs about $43 from Walbro...Didn't get one to try, the owner will be doing some more tests next week when he gets it back...
the Shindaiwa is PRICEY..[8D]Not at all like the $79.95 Harbor Freight Ryobi..