Wanted:   O.S. 4BK carburetor.   I am told that these came on various O.S. 40 size engines. 

Do not have pic or dimensions.  The O.S. carbs I have do have a marking on them, but I guess there can be exceptions. 

I'll look, I may have a couple of grungy ones, If I do I'll PM you a picture of them.  They will be take offs from engine's I planned on stuffing a CL venturi on lol  You'd need to clean em and would need a new O ring  seal for it.

Edit:  No luck, have a handful of 4A's off of early 40H's and one 40D off an LA40.

The 4BK came on the 40-45 FSR. They were/are very popular carbs in profile carrier. The 4D came on the 40 and 46SF. It had a .5mm bigger neck (.530"- 13.5 mm), and also a slightly bigger barrel bore. This is what we used when I was heavy into carrier. Unless you absolutely must have a 4BK, the 4D will give better performance and be easier to come by. I have a used 4D I can part with. Email me if interested.

Bill Bischoff

I believe that the 4A came off the baffle-piston 40, and it is an air bleed carb. The 4D is a two-needle carb, which offers much better adjustment capability for smoother, more linear throttle response. I wouldn't use an air bleed carb unless the rules required it (like NW Sport 40 carrier). You mentioned that you had a 40D carb. That is a totally different carb than the 4D.

OK, looks like Bill will be taking care of my need.  Thanks for the responses.