Well, no good. Motor quit after take off and a couple of lazy loops. Same symptom, everything fine for a couple of minutes then it would not throttle up after the backside of the loop.

So I'm baffled . . . . . something is fundamentally different between the 140 and the 160. When I initially started flying this plane I had a used-but-fresh back from YS service 140DZ engine in it. It would quit in exactly the same way. I was told I was running too rich, so I ran it on the ragged edge of lean, only to have the big end of the crank break off. YS made me a deal on the 160, and it ran like a clock, never missed a beat. The 140 won’t run reliably, the 160 never quit. Same tank, pickup, throttle servo. Since I have had the opportunity to thoroughly go over the YS parts list, I know the pump assemblies are the same, so that is probably not the problem, the throttle body and related parts are the same. I find it hard to believe any other internal part would cause this problem. The only obvious difference is the injector check valve on the 160. The 140 does not have one.

Why does the 160 have the check valve, and could that be the issue? Would the 160 check valve fit the 140?

Back to the bench.