Ferris, you will definately need all the tins on the engine, with all the tins in place the mwsc flywheel will move enough air to cool the engine and everything will be fine. 400 degrees is a little warm for the cylinder head temp, on gas somewhere around 330 would be all you want to see on a lawnmower engine. On alcohol we could only get the temps as high as 180 or so. I wouldn't use the cylinder head temp to tune the engine, you should be reading the exhaust temp somewhere around 1200 would be a good place to start.