Engine Surge when both Rotax fuel pumps are on and then I turn one off

Eric:

This sounds similar to or the same problem I've been having for the past year. See "fuel pumps" topic below. Zlin Norden airplane.

Happens only #1 pump at any power setting. Sent pump assembly to Lockwood (twice); can not duplicate. Lockwood tests them with Acetone; doesn't run fuel through them. I've had the #1 check valve replaced. Will run it soon; more on this later.

I'm beginning to believe it's a design issue. With both pumps running, outlets of both check valves are pressurized, keeping them closed. Inlet to check valve 2 is pressurized; inlet to check valve 1 is not pressurized except for head pressure. When pump 1 is turned off, it probably takes a finite amount of time to drop the outlet pressure of check valve 1 and restore flow momentum. Turning off pump 2 is no issue since check valve 2 is pressurized by flow from pump 1 and check valve 2 opens instantly.

I'll know more soon. What type aircraft do you have?

Steve G.

Given you are talking about a type 915 perhaps the pumps are just not enough flow.  The reason for the type 916 having the Bosch pumps is to get the correct flow.  This is just my opinion. I know that Rotax would prefer that all the engines, particularly the 915 and 916, used the bigger Bosch ones.  (yes I know they are expensive) 

Cheers

Ram the aircraft with new #1 checkvalve. Same problem; momentary fuel flow drop, engine power reduction, then recover. Any power setting.

No issue turning pump 2 off.