The fuel pump is pulling a vacuam on the stystem. You have an obstruction somewhere. Primer bulb itself, anti-siphon valve, Fuel tank setting on the fuel line, (not unknown).
The engine runs fine up to 2500 rpms then stumbles and cuts out. If I keep it below 2500 rpm's runs all day. I reached down and made sure the primer bulb was full and it is hard and cant be squeezed. I was planning on replacing the fuel line/filter/plugs/and check the pick-up tube in the tank.
Replaced the primer bulb/ fuel line, and fuel filter. Emptied the bowls on the carbs and took her out yesterday. No more issues, ran great. Thanks for the help.