For all Yamaha guys:  Sometimes it has happened that oil pressure will not pump up because of air in the system.  Although this most frequently occurs when installing an engine after pulling it out of a sled, it has also been reported on Totally 4-stroke Yamaha forum on sleds, so it CAN happen on your aircraft!  After finding out about this, I am definitely going to installing oil pressure sensors and gauges on my kits going fwd.   

The solution is as follows:

1.  Remove spark plugs

2.  Remove the large, top breather hose from the oil tank

3.  Place a compressed air gun in the tank inlet, and seal with a rag

4.  While cranking the engine pressurize the tank with just 2-4 PSI.  Too much air will screw things up. 10-15 seconds of cranking should do the trick.

5.  Replace the oil breather tube and spark plugs. 

6.  Start the engine and either check for oil pressure, if you have that ability, or feel for the oil tank heating up - which indicates oil is circulating freely.