Re: Beach cruiser!!!!
First off. Invest in some caliper or V brakes because if you slam on that coaster and are relying on it as your sole means of stopping on an 80cc bike you can A) lock the coaster brake up, meaning suddenly your pedals will start pedalling without you doing it, and no stopping. B) The chain on the pedal side can break or pop off meaning again you have no brakes. I use beach cruisers and always have an extra caliper brake on board as a back up.
As far as stalling it out goes, that's not a big issue. The kill switches on these kits are crap anyhow, and when they fail they more or less short the electrical and the bike won't go anymore anyways. If this is your introduction to the motorized bicycle world, you should get past the illusion of it being a motorcycle. it's not. These are pedal assist engines, meaning you shouldn't be firing it off from a dead stop anyways w/o pedalling.. ((unless you really like changing our and adjusting clutches)). Also remember that beach cruisers aren't built to go 50 mph and if you torque on the engines with wild abandon you'll be buying a new kit or throwing it back up on craiglsist as scrap in no time flat. Don't worry about getting to places fast on these things. These engines need attention and a healthy amount of respect and understanding in order for you to get the best out of your money. I've got over a dozen builds under my belt now, and with the exception of 3 I totally disregarded that kill switch. You should be concerned with brakes, lights, mirrors, a horn and a helmet. Loctite the engine mounts and blue loctite the other nuts and bolts as vibration WILL SHAKE THINGS LOOSE.
As far as your mix goes, there's an easy way to get aroubnd that heady math. It's Opti-2, and it's sold at most ACE hardware locations. 1 packet (about $2.00) to one gallon of gas for all 2-stroke motors and it goes. No adjustments necessary. The trick behind that is to clean the engine out of all the gunk from the previous oil the last owner used because putting in a full synthetic without a flush will cause all that crap to come loose and muck up the works.. I will tell you that all of the builds I've run with nothing but Opti run as smooth today as they did 4+ years ago when I started building with minimal engine wear and little to no grime associated with two strokes. Hope this helps you a lil bit. There are a TON of folks on here that have run the gamut of things that can go wrong, and we're all here to help along the way.