Crankcase compression testing.

This is pretty simple, you need to seal carb and exhaust ports then introduce compressed air into the engine and see if and where it leaks out. I put the sealed engine into a tub of water and look for bubbles. I've found leaking crankshaft seals, a bad cylinder gasket and leaking rotary valve covers.

The blanking plates are made from aluminum plate with rubber gaskets from an old inner tube.

To introduce the compressed air I first installed a bicycle tube valve JB welded into the exhaust blanking plate but found that the access was difficult if I tried to turn the engine in its bath water to see various potential leak areas. So then used the base of a spark plug with a more sophisticated bike inner tube (Presta) valve that I got for free from my local bike shop... they have lots of old tubes that they throw away. The valve is an easy fit, injected epoxy to fill the cavity between the valve and the spark plug and put a washer and nut on to hold it in place. Since I don't have a Presta air pump I added an adaptor for a regular air pump.

 

Here is a tip on how to "disassemble" an old spark plug.
https://youtu.be/ziWcLDL-g_E