One thing you can try is taking the vent tube off. Looks like you may not need it. I'd also look at the carb crap trap inside the carb. If it has a restriction, you will be running the low needle out much more than its supposed to be and that could be affecting your mid range. Clean the carb good and see if that helps. Your low needle should not be out more than 1 and 1/4 turns open and are usually around 1 turn open. If you are more, than I'd clean the carb and see what happens.