Idles fine but under power it runs fine, then stumbles then runs fine over and over. It was in storage for 2 years with an empty gas tank, and has fresh gas now. Wondering if the 15HP has a fuel filter, and if the siphon tube might be gunned up. (How to get it out of the tank?) and of course also wondering if it just needs a carb cleaning or kit. Thanks for any thoughts and advice.