Ok, I looked at my 10cc engine's carb and I discovered that the idle needle is too short to cover the spraybar orifice. What happens is as you try to screw in the idle needle, it bottoms out in the end or tip of the spraybar. In the second pic you can see where if you screw in the needle, you cannot cover or block off the orifice in the spraybar any farther than what I show in the second pic. In the second pic the idle needle is the outer tube surrounding the right end of the spraybar. You can see the slotted orifice in the spraybar in the two pics.
it needs about 1mm to 2mm more depth to cover the orifice. So to correct the problem, one either makes a new idle needle that is longer, or you carefully drill the hole a little deeper on the inside of the idle needle, or you shorten the spraybar a little bit on the end.
But they don't make it easy to get the idle needle out. Apparently they pinged the threads on the outside to prevent the needle from unscrewing and falling out while the engine is running. So as you try to unscrew it, it stops on the outer edge of the threads. I tried to force it out, but one of the edges of the brass needle snapped off, So now I only have 1/2 of slot to screw the needle in and out with.
I think by simply removing the idle needle, and finding a drill bit of the correct size to fit inside, you can drill the inside hole about 1mm to 2mm deeper, and it should solve the problem. It looks like one needs to carefully drill the needle while it is inside the throttle barrel without removing it.
Oh yeah, the OS 7B carb is the same design. It has the same type of idle needle, but it extends a little deeper allowing one to cover or block off the spray's orifice better. The idle needle tube is a more thin brass tube as well, so it flows a tiny bit better than the OEM carb would allow.
