OK here is my take. You get decent throttle resoponse from idle to 1500 rpm. Then out of go. If you richen the mixture it gets better, but you get carbon in the exhaust. How are you richening the mixture, mixture screws or jets. It sounds to me as if you need bigger jets in the carb. From idle to 1500 you are dumping raw fuel into the carb via the accelerator pumps. The fact that you have good throttle response here means your engine wants fuel. About 1500 you are probably out of accelerator pump shot and running out of fuel. Then it runs better with richer mixture. If you do this by adjusting screws you are probably getting more gas in but it is not mixing well with the incomming air. Bigger jets will let more fuel in and mix it properly. Possibly it could be a low float that is running out of fuel in the carb bowl? Don't know about this one, but sounds like your motor wants more gas to me.
Hope it is a dirty carb though. Had a similar problem and a good cleaning fixed it. But if that dosen't do it I would go after fuel delivery.
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