Is your Jikov the square float chamber or the old side float chamber?
mine was the older one on a 632(I think! Jawa model numbers defeat me) and I could not get it to pull. I had a square float carb which I tried but could not get the float to control the fuel level. When running it felt powerful and smooth but I decided in the end to spend my way out of trouble. I certainly got more than 42mpg. The down side was getting the filter housing to match up with the Mikuni!