You probably don't want to hear this, but you need to pull Stupie's engine.
You need to take all the tin off to get to the top bolts and retorque all of them. Do you have 8mm studs or 10mm studs? If they are 8mm, 20 pounds is too much. If they are 10mm, 20 pounds isn't enough.
Don't know how to tell the size? Stick a gage on them. Before you say you don't have one, yes you do. At least you should if you are working on a bug. Your 8mm gage is an opened end wrench with an 8 stamped on it. funny thing about metric wrenches . . . the ones 8 stamped on the have an opening across the open end that is just about 8mm. The one with 10 stamped on it, guess what? . . . Yep, it's about 10mm across. Now if your wrenches were made of inferior Chineese metal, that might not always be true, but it works most of the time.
Anyway, if you're having trouble with the lower studs backing out, I'd bet the top ones are loose.