If the bike only runs for a few seconds then check/clean/replace the pilot jet. Just went through it with my bike. There's enough fuel in the bowl to let the bike run for a good minute so if it is shutting off right away there's something wrong past the float assembly.
 Does it run any better with the choke out or does it only start when you use the choke.
 Check for a vacumn leak by spraying the outside of the carb with carb cleaner while cranking the engine, you will need three hands for it or tie the clutch handle in while trying to start it.
 Keep the stock petc0ck in prime if you are still using one, it bypasses the diaphram.
 It sounds like a fuel problem but it could be electrical which is another can of worms.

troyrr wrote on 03/11/16 at 11:28:13:


Most likely your mechanic is not familiar with the Verslagen modifications to the cam chain tensioner.  It allows you to move the tensioner to the second hole, and get twice the mileage out of your cam chain and a considerably lower cost.

And.....as others have said....a loose cam chain is noisy and potentially harmful if the tensioner comes apart - but it does not cause the bike to run poorly.

It looks like the guy who worked on this engine before I bought it used an orange sealant over the cylinder gasket. My mechanic thought some of this sealant might have gotten into the engine. He described a banging noise right before the motor shuts off. Maybe something is stuck?  

You are right about him not knowing about the cam chain adjustor.

I haven't tried replacing the Rectifier. I've replaced the Starter and Igintion Coil. The wiring looks right, though.