If the pressure is a little low on one side of the fuel delivery when you switch then it will miss. Maybe the pressure is not equal on both sides for some reason?
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
The regulator takes a second to adjust to the slightly different pressure?
I notice the pressure is always different between the two pumps and the aux pump does have a different/longer route through the plumbing of the pump pack.
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Glenn I have not found a solution. I changed the fuel pumps and same thing happened. a mate who has the same engine has the same problem. my LAME says it is common and not a concern as the engine runs fine. I live with it and once I turn of aux in flight I have no problems.
I have experienced this and at least in my case, I think it may be related to vaporization of the fuel. Occasions when the condition is experienced or not experienced is temperature (and fuel) related. When doing an engine run up for the first flight of the day- normal. All operations during cool or cold weather- normal.
After a long cool down period in cruise flight- normal.
I have only encountered this condition when 1. using unleaded fuel with ethanol. 2. during warm or hot ambient conditions. 3. during or after prolonged climb to cruising altitudes. 4. with somewhat restricted fuel flows.(aircraft manufacturer plumbed fuel hoses from tanks to fuel valve with 7mm hose (slightly less than that called for by Rotax).
Using "Both" on fuel selector provides increased fuel flow since I used 5/16: ID hose from the valve to the engine.
If the engine compartment is "hot" and the fuel valve is on either Left or Right the engine will react to deactivation of the AUX fuel pump. If I have the fuel valve on "Both" (the fuel flow is increased) the engine will not react to the deactivation of the AUX fuel pump.
Since I very rarely use 100LL my observations may only reflect variances.
Using the above experiences I have been able to completely avoid engine disruption when managing the fuel pump.
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