Leak-down testing on a two stroke is another essential tool that can get you out from troubles. In contrary to the compression tester, this test measures pressure loss but both can be used in conjunction by an experienced user to reveal possible issues on an engine.
Testing can take place after a newly assembled engine to verify that everything is leak free, on a bike you just bought and you need to check crank seals condition etc. A leak down tester can be fabricated easily out of parts found on a hardware store but there also kits dedicated for this work at reasonable prices.
Procedure is pretty simple, you need to block all air passages out of the engine, like the carb and exhaust manifolds and the spark plug hole. For the later one, a simple spark plug will do the job fine but you will have to construct some bungs for the carburetor and exhaust manifolds and also acquire a pressure gauge and an air hand pump.
Here is a pic of a plumber pipe plug on the exhaust outlet of a Honda Nsr250 engine. These plugs squeeze the exhaust walls and seal very well.
Next step is blocking the carburetor manifolds. This is the best place to position the pressure gauge and air pump. The gauge should be in the region of 0-15psi, as bigger readings will not be very sensitive to a possible pressure drop. Any style of pump will do the job well. Depending on application and cylinder configuration, you may or may not have to block all of the inlet manifolds.
When everything is in place, it's time to make the test. Pump the engine up to 5PSI and wait for 10 minutes. If there is a pressure drop of up to 0.5psi at max, then the test is successful and the engine is assembled correct.
If there is a pressure drop of more than 0.5psi, then use soapy water to find the leak. In the aforementioned test, left hand side of the engine tested fine but the right hand side had a pressure drop of about 0.5psi in only just a few minutes. Using some soapy water on a detergent spray bottle, a small air leak was revealed between the mating surfaces of the crankcase and RH cylinder.
Sadly, even though a leak down testing is an easy job, most of the time any possible air leaks will require the dismantling of the parts that leak. Using higher torque values on the bolts etc is not the way to go and actually this might increase the air leak as well. I suppose it's your call, do it right and correct the leak or use a bigger main jet on the carb and live with it. But I believe if you've gone that far you have already took a big breath and started picking the right tools.
Have fun, keep the two strokes alive.