Years ago when I flew a lot of Super Tigre engines-2500, several 3000s and a 3250-I found the carbs after the initial small throat carb just wouldn't work very well. At first we tried an OS 7D, but they, as you mentioned, are expensive. Next, we tried an ASP 1.08 carb. Worked like a charm. Our local dealer got them for us for $26! My 3000s would 1-flip start and never missed a beat with the ASP carb.

You did need the spacer ring that the STs used to have. In a pinch I have used aluminum flashing tape and trimmed it to a snug fit, then sealed with hi-temp silicone.

Is that a Sirex Wasp in your avatar?