Like cutaway says about the ST needle assembly, others may do better. I flew compitition CL stunt from around 1957 to 1973. In my later years, mid '60s to '72, I did very well. I used Fox .35s, Fox .40s, one Veco .35, and the best of all, in the last several years, a George Aldrich modified Super Tigre .40. However the Fox .40 Stunt was an awesome engine after a careful break-in. I never used a K&B much as they wore out too quickly and loved to throw crankshafts. [:@]

Now, here is a secret that few would ever try, but those that did found a significant ease of running these engines. Remove the stock needle valve assembly. replace it with an OS assembly or an Enya, almost the same as OS. NOW HEAR THIS: As cutaway said, use washers to place the spraybar hole in the center. Then rotate that hole to JUST out of sight facing the rear of the engine. That old Fox 35 will do a perfect 2-4 break as well as the other engines.
In fact i may do that for my Super Cyke Spark after I try it standard, and a new Anderson Spitfire .65. My Sup. Cyke was a good runner with the stock NV back in the early '60s both spark and glow, but I wasn't doing anything fancy then.

I am now playing again with some CL stunt hopefully to get back to some Old Time Stunt flying. All engines get the OS/Enya treatment. It still works. BTW when using those old engines, 5% nitro and LOTS of oil. 25% or more like Fox Superfuel is the ticket.