Fuel problem with 8hp

[labougie]

My 8hp 1992 Enduro stutters and misses at certain throttle openings. Tickover is fine (although a little smoky, even at 75:1) and half throttle and above is perfect. The problem is between tickover and half throttle where it runs unevenly. I can't think that it's an electrical problem because it only happens at the throttle opening I've mentioned. I'm very poor on carburettors - is there a jet con*****ing this specific throttle range? Is it adjustable? Thanks in advance.