I fly G90s all the time - in fact this is my standard engine and has been for over 10 years. I have the G90 original carb on all mine and they run great with proper carb tuning. Most aftermarket mufflers have low back pressure - I'm not sure why they are made this way but I think it's because some prefer to run their engines differently from the way I run mine (they don't mind the midrange issue or they use tuned pipes etc).

I used to use the bisson pitts mufflers on my G90s and they fly fine with this muffler - but if you want to avoid a rich midrange you need to block off enough of the muffler outlet tubes so the combined area of the outlet tubes is the same as the outlet area of the original G90 muffler. this gives enough back pressure so you can set the carb to get good mixture control throughout the throttle range. i do this with all my G90s. when this is set properly you will not get a rich midrange - this may be what you are getting. if you don't have good back pressure you will have to open the high needle fairly high to get a good mixture at full throttle, and this gives you the rich midrange.

is this G90 broken in at least three tanks? most g90s (I have at least 6 of them now - used to have more) need to be pretty well broken in before they will hold a low idle. I consider a low idle about 2200 rpm or lower for a G90. But you need to set the high needle first before you try to set the idle. The high needle doesn't affect the idle but there is some interaction between the high needle and the idle needle in the midrange and this will keep changing until the ring is seated against the inside of the cylinder.

what prop are you using? i use an APC 14-6 on my 60 size warbirds (I fly mustangs) and this prop suits the g90 very well.

anyway- if you have the carb tuned well this engine will respond quickly to throttle changes both up and down. my G90s will idle for several minutes without loading up or dying once they are broken in and tuned right.

My G90s last for at least 6 years with everyday flying. some of them are over 10 years old - I think I have only worn out one of them, and this one would probably run as good as new if I bought new rings and bearings and rebuilt it.

Ed