engine starting

One important part of the start process is the idle speed.  If you have it set too high, say higher than 1600 to 1800 RPM, the choke will be ineffective as the butterfly is open too much for the choke circuit to work correctly.  Just a note to look at. As pointed out by a few others be sure that the throttle is at idle during choke operation.  On the Bing you can run with partial choke to get it warmed up also.  

One other point.  I dont know what fuel you are using.  if it is auto fuel be sure it is fresh.  if you have 100LL you may need some preheating of the engine if you have cold weather as 100LL has a very low RVP.  (see if you can warm up the engine block, especially the cylinder heads. )

Cheers