You're getting good advice from DaveW. My 2-stroke motorcycle days were mostly in the 70's which seem a little hazy now. I'm guessing I'll never get back to that collection of RD350s in the basement, too bad....
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Belt on primary to secondary to have some load from turning the jackshaft and wheels even up in the air.
Any special break in routine I should follow?
I think you should just run it till it gets warm once or twice, don't expect to be able to figure out any tuning without a load on the motor. The carb will not be running with the needle and throttle positions it would in real life for whatever RPM and power you are making. The risk is changing the setup after getting misleading results.
Make a metal lists of conditions that will win your races or kill your motor. Go down the list. Number one would be wide open throttle in your power band. Get that right and then go down your list. There will always be things ahead of testing the engine on jack stands. So don't learn what you don't need to know.
While it's open make a careful diagram on a sheet of graph paper of the actual port locations, sizes and shapes from TDC. Then hopefully you can check to see if it' stock or modified.
My motorcycles had a stock redline at 8500, after I had the cylinders ported and put on some pretty aggressive expansion chambers the motor pulled really hard to probably 12,000 or well after 10,000 which is where the tach stopped. It had a Dyke ring and used stock pistons. I started with some aftermarket units but they weighed more than the stock ones so I junked them, I think. No valve springs or floating valves to worry about.
The power band was such a big change for the motor that once it hit, if you suddenly decided to abort (from shear panic for instance) the power would continue to climb even while you were shutting the throttle as fast as possible. Just a moment mind you but it could be a very scary moment the first time.
Is 40:1 the recommended mix? When you change the oil mixture remember you are also changing the fuel mix so 40:1 is leaner than 50:1.
I used to use Permatex Aviation Form-A-Gasket to seal the cases after working on the motor. Maybe you are not going to split the cases.