I may be wrong but it sounds like it could be an air leak if it's revving up high when it starts.
Look up on this forum how to do a leak test. You can make a gauge pretty easy, or there is a member here who makes and sells them. Even if I'm wrong and you don't have a leak it's still good to check it periodically to make sure the engine is healthy.
Basically you use the gauge to pump the engine full of air (7 psi) and see if it holds pressure. If it doesn't then you have a leak somewhere. At that point I guess you could use some soapy water to find the problem and then go from there.