Hey I need some guidance, I have a GMS .47 ABC. The motor has about four or five tanks of fuel run through it. It idles perfect with the low speed slightly off of rich. It will transition from low to high with no stuttering. The only problem is when I hold the plane straight up the engine slowly begins to lose RPM's and finally dies off. I have checke the fuel lines, replaced the glow plug and pulled the tank and replace the fuel line and tightened the plug on the front of the tank with no luck. Am I looking in the right place with the fuel tank or could my problem be elswhere. The plane is a Hangar 9 Twist and the tank is installed with 1/2 inch foam arond the tank. (foaming in tank???)

Chuck