Typically, pick-up tubes are welded to a pipe fitting which then screws into the top of the fuel tank
. An anti-siphon valve screws into the fitting which in turn connects to the fuel
line. The fuel
pump's suction action is sufficient to pull open the anti-siphon valve and allow fuel to flow out. You should be able to unscrew both the fuel line fitting into the anti-siphon and the pick-up tube into the tank to examine.
I had debris in the tank that would be sucked into the pick-up tube and restrict the flow of fuel. Once I slowed down, the debris would "fall" away from the pick-up and the motor
would run normally for a while.
Your symptoms seemingly indicate a restricted flow of fuel. Could be debris in the tank at the pick-up, debris in the anti-siphon or even a collapsed fuel hose that may have been softened by E-10. All of these would appear to recover and then repeat. The best test would be a fuel pressure gauge mounted before the fuel pump
. If there is a clog between the tank and the pump, this instrument would show it. Should the pressure remain steady in spite of a drop in RPM
, then the problem is from the pump aft.