MildBill said it right. I tried a check valve in a Cap with a Mag 1.08 2C in the nose and there was nothing but problems once the throttle was brought back from WOT. The engine WILL NOT have consistent fuel metering below WOT. I thought the pressure would be constant with a check valve but it wasn't (I was using a Pitts style muffler with little back pressure).

The fix I found was to REMOVE the check valve and add some stainless steel window screen in the outlet pipes of the Pitts muff. This increased the back pressure slightly and toned down the exhaust note a little AND solved the fuel pressure/feed problem. Give it a try. It might work for ya.