Great, glad to hear you found a good way to adjust the carb.
As too the slipping, I will post a couple of pics tomorrow showing what I did to my Foot Peg/Roller, one thing that will help is if you will put a different tire on the rear of the bike that doesn't have the nobby type tread or at least has the nobs much closer together, it will give the roller more contact area than it has with those nobby tires with the big gap in between them, I'm going to my local bike shop tomorrow to pick up a couple inexpensive street tread tire for the Diomand Back BMX bike I'm gonna put my weed wacker on.
below are pics of the type I think would work great on the friction drive.
keep me posted on what you do to the bike and how it's working out, the weedwacker set up is something I'm playing with right now myself, it will be a bit rough built right now, but if it will push me down the road I'll be happy, the engine I'm using hasn't run in a couple of years, but I got it all cleaned up and running like a top today so I know I have a darn good little engine to start out with it seems.
best wishes!
Peace, map