After getting the first 90 hours on my -9A, I decided to tried the 91 Octane autogas that is sold at my home airport. Being overly cautious, I completely drained the left tank of 100LL and filled it with autogas. On the next flight, I took-off on the right tank (100LL) and once leveled off and with everything stabilized, I switched over to the left tank. About 30-45 seconds later, I felt a noticeable loss of power and the engine began surging from cruise power to lower power. I had the engine page up on the PFD during the tank switch just in case and the fuel pressure was dropping, accompanying the power surge. My IO-320's fuel pressure usually runs at a steady 18 psi, but it was dropping to 8-9 psi for a second, then back up to normal, then down again (again with a power loss).
Flipping on the boost pump (Andair) instantly brought the fuel pressure up to 29-30 psi and the engine ran normally. Turning it off, same symptoms returned. Going back to the right tank, after the 91 octane got run out of the lines, fuel pressure stayed at 18 psi like normal so it would seem the 91 octane was the cause? To make sure it wasn't some problem I induced while draining the tank, I ran on the left (91 octane) tank with the boost pump on, way ROP to burn as much as I could on this leg so I could refill with 100LL.
Upon refilling tank with 100LL, I had approx. 1/4 tank of 91 left so it was much more 100LL than 91 octane. On the next leg, I performed the same test. The fuel pressure still fluctuated, but nowhere near as much. I've since refilled 2 more time so it should be back to nothing but 100LL. Now the fuel pressure is running 18 psi on both tanks, just like before this experiment.
I'm just wondering what is going on with this? I've read countless posts about people running auto gas with no problems at all. What could be causing a problem specific to this plane?
Flipping on the boost pump (Andair) instantly brought the fuel pressure up to 29-30 psi and the engine ran normally. Turning it off, same symptoms returned. Going back to the right tank, after the 91 octane got run out of the lines, fuel pressure stayed at 18 psi like normal so it would seem the 91 octane was the cause? To make sure it wasn't some problem I induced while draining the tank, I ran on the left (91 octane) tank with the boost pump on, way ROP to burn as much as I could on this leg so I could refill with 100LL.
Upon refilling tank with 100LL, I had approx. 1/4 tank of 91 left so it was much more 100LL than 91 octane. On the next leg, I performed the same test. The fuel pressure still fluctuated, but nowhere near as much. I've since refilled 2 more time so it should be back to nothing but 100LL. Now the fuel pressure is running 18 psi on both tanks, just like before this experiment.
I'm just wondering what is going on with this? I've read countless posts about people running auto gas with no problems at all. What could be causing a problem specific to this plane?