Here's a real simple homemade tool you can make for pressure testing the bottom end of your two smoker, if crankcase seals or cylinder gaskets leak the crankcase can't hold pressure or vacuum and your engine won't start well or run well, basically you seal off all natural engine openings and apply a small amount of air pressure, remove the carb and replace it with your homemade adapter, pull off the exhaust pipe and seal up the exhaust port, you can use an old piece of rubber inner tube and a hose clamp, have an accurate air pressure gauge on hand and a spray bottle of soapy water, apply 9psi of air pressure to the cylinder, don't use an air compressor set at 90psi !!! if your air source is a compressor turn it's regulator down to 9psi, or use a bicycle air pump, play around with your air pressure gauge to see how much pressure you lose each time you press it to the air valve, it's usually about a 1psi, remember this is 1psi you've lost through the gauge not from a leaking engine, when you have 9psi in the engine listen for air leaks and start spraying your soapy water, first check the exhaust port to see if your seal is working, then your carb adapter, spray the spark plug, your looking for air bubbles indicating a leak, air could be leaking through the crank seals or where the centre cases join so spray the whole engine, after a few minutes check your air pressure, if the engine is in good condition pressure should not drop more than 1psi in several minutes, if you lose more than 1psi in one minute you have serious sealing problems
Your engine may need something different than this and more elaborate engines will need more areas sealed off to hold the air pressure. this tool was for an 84 RM 250. pretty simple and cheap. a piece of abs pipe 5 inches long by 1-15/16 wide OD. the pipe and the plastic cap to seal one end you can find at any plumbing or building supply store, use silicone and a hose clamp to seal the cap to the pipe, the air valve is the threaded kind with the nut, used on custom wheels, just drill a hole in the cap to install the valve and it's done
