ORIGINAL: rcpattern
On new engines the backplate screws tend to loosen up and will give you the exact problem you are having. Pull the engine and retighten all the screws. Once you tighten them again they will stay tight, but I would bet you have an air leak somewhere,
Arch
I just went through this exact scenario and it turned out to be 3 of 4 backplate screws were loose. The gasket was wet under the area where it should be sealed 3/4 of the way around. I suggest using some locktite on the bolts when reinstalling. Everythingis now A-OK after tightening it all up. Thanks to Ken Velez for helping me with getting everything torn down & back together while I was on the road to get this done too!