I really don't think the carb is too big, by guess is you have other issues with the engine, timing etc.
I think you just need to double check everything on your engine, something is not correct.
I do have a 625 street demon on a stock 318, its been on there a few years and it a real nice carb, all I had to adjust OTB was the idle. Would not get rid of it for any other carb.
