Never heard of that, and I've seen a bunch of 25 year old machines with hoppers that look like new on the outside, and only have paint worn off on the inside. Give Don a call at Keystoker and ask him about a solution. They are very good folks to deal with.
What kind of coal are you burning? Same brand all three years? Simple solution, but makes you scratch your head.
You don't run the exhaust of the 'Ole Water Buffalo' through it do you to dry the coal in the hopper?
If you can, you should post a couple pics of the GT750, and as our black flies attack us, I wish I had one of those 2 strokes to smoke em to death. Perhaps someone will chime in with a 500 Titan, or a old Kawasaki 500 or the dreaded Vibrator 750!
Smitty could easily modify the oil injection to "Black Fly" mode!! Wonder if the 500cc Wankel smoked much? That was a "different" kind of bike.................I preferred the Honda 1000/6 cylinder. That was 4K back in the day, but what a smooth rig. Hot air scorched your legs, but...................