Rotax912 Sputters upon leveling off

[fiferphil]

I had about 35 hours on my Highlander powered by a Rotax 912 when I decided to see how ehe would do when fueled with LL100. After fueling run-up, takeoff and climb were all normal. Upon lowering the nose after level-off, the engine started running roughly and losing power. I reduced power and the engine started to run normally. Thinking I had fuel contamination, I returned to the field,re-checked for water and found none. Upon checking the inline fuel filters, I found a little trash in the right filter.I shut that side down and had an uneventful ride back to Toccoa.

I then drained the tank and filtered the fuel and found no debris. I also checked the carb float bowls and found no debris there, either. I drained all the avgas and loaded up with 93 octane auto GAS,The flight that followed did not exhibit any of the problems of the previous flight. Picked up a load of 100 LL and the same problem popped up.

Any ideas?

[HS-JAT]

Why run 100ll if you don't have to?

Too many negatives running AVGAS in a Rotax. AVGAS stores better. Thats about it.

[fiferphil]

My intent is to run 93 octane ethanol free whenever I can get it. Most of the time going cross country, I have to rely on what's available at my destination airport, ie 100LL.

[Johnny C!]

I ran 100LL exclusively while I had Lima Charlie
at Clemson for a few months during my early
test flights with Scott. I had no issues with
it or when I switched over to 92 NE here
at home. I have ran a mix of Mogas &
LL since I have had her home a few
times, but the last 10 or so hours
have been on the Mogas No issues
on either fuel.

Let us know if you find anything else.

Be safe!

John

[fiferphil]

I'm going to drain the tanks and use 92 octane ethanol free mogas see if there any issues. Letcha know what i find.

[User GDS]

I've always used mogas with ethanol in mine, avgas when on a trip. No problems like you describe. It could be a coincidence that it occured when using avgas? Is the avgas source a high quality operation? Tanks can build up sediment, water or biological contamination if not maintained regularly.

[fiferphil]

After further flight testing with pure 92 octane ethanol free gas, I found that the engine reacts to both the mogas and 100ll the same. On climb-out st about 500 feet agl the engine will act like it is being starved of fuel (full power) Upon reducing power to below 5300 rpm, the engine runs normally. I suspect the fuel pump and maybe the carb floats might be the culperts.

[danerazz]

No Rotax experience, but lots of float carb experience. If the floats are not adjusted right and "ride high" this could be an issue with those symptoms. More likely the float needle sticks and the carb needs a thorough cleaning and readjustment, especially since it is kind of intermittent.

Are you sure it is acting like fuel starvation? Is it possible that it is actually running over-rich when you level off? Could be caused by sticking float needle or sunken floats. Actual float issues generally aren't an issue any more with the plastic floats, but the needles are common problems.

[fiferphil]

That could possibly be, but this is no longer an intermittent condition. It is now very predictable. The main thing that gets my attention real quick is the engine at full power acts like its going to quit on climbout at about 500 ft agl until I reduce power to below 5300 rpm. Whether it be too much gas or too little gas -- it's perplexing. The bird is now at Shamrock Aviation, a Rotax shop at DLZ (Delaware, Ohio) for diagnostics and repairs. They replaced the fuel pump based on service bulletins calling for that and re-synced the carbs . I need to get down there to test fly it to see if what they have done so far have cleared the problem.

[danerazz]

The fact that it "got predictable" over a few hours could support the float needle issue, but it is a guess. Hopefully they actually find the issue, I hate it when a problem "fixes itself!"

[xpflyr]

I think it would have to be a coincidence. In my experience, everything runs better with 100LL.

[fiferphil]

Good News! Solved the fuel starvation problem by exchanging the fuel pump and re-syncing the carbs. Ran as smooth as glass flying from the Rotax repair station at DLZ to Wynkoop airport at 3500' msl ( 20 min flight) Thanks everybody for your responses .

[Johnny C!]

Sounds like that wasn't too expensive.

Good deal!

John