Sport Pilot Talk

A Rotax 912ULS with 655 hours is exhibiting a serious issue with "stumbling" during take-off. Here's what happens:

• No problems on startup.
• Engine comes to temp, smooth operation on taxi to runway, and runup goes fine (90 RPM drop on each side).
• Perform short-field takeoff procedure because there are trees at end of 3000 ft runway: flaps first notch, hold brakes, aux fuel pump on, WOT, release brakes, rotate, level off in ground effect to gain speed, raise nose to climb at Vx.
• Somewhere around leveling off in ground effect and pitching up to climb, the engine suddenly "stumbles". It doesn't quit, but it is much less than full power. This has happened twice now and both times I was able to abort, settle on the runway, and get it stopped before doing any damage.
• When the throttle is pulled back to idle to get back on the runway, the engine idles smoothly and doesn't die.
• Taxi back to hangar is uneventful (purrs like a kitten).

The Ignition modules were replaced (new soft-start modules) a few months ago. 39 hours on them with no issues.
Rubber replacement 10 hours ago, fuel pump was replaced. No issues with it during those 10 hours.
Fresh mogas, 93 octane, lead and ethanol free.
Carb floats were checked at last annual.
I can't decide if it's ignition or fuel. But biggest mystery is: why always when just starting to climb out, but no engine problems at any other time?

First, check for debris in the float bowl, not uncommon after a hose change.

Second, verify that fuel pressure is not dropping off when power is added.

Third, not sure how your set up is, but check that the vent tubes are pulling air from the same source.

Fourth, if you can reach a finger in the filter side of the carburetors and make sure the slide moves up and down easily.

Also the fresh to you auto fuel may not be as fresh as you think. If you don't find any of the above to be the culprit, try adding some 100LL or Swift fuel if it is available.

One more thing. If you have dual EGT see if the temperature changes on one side or the other when this happens.

In my case it was debris is in the carb bowl - when full power was applied on take off and it would stumble and “roll back”.

Check fuel pressure and that your carb heat is functioning properly. I had a similar problem of stumbling on full throttle when my carb heat was stuck open.

Disclaimer: I am not a LSRM or AP

I appreciate all the input! I thought I'd give a quick update of what I've found so far.
The carb vent lines are fine. There was no contamination in the float bowls. The floats weighed out fine. The pistons in the carbs move freely.
So, I'm scratching my head while putting carbs back together when I notice something: there is discoloration on the 2-4 intake manifold around the little M6 screw that plugs a port that can be used to connect vacuum gauges during carb synch. Sure enough, that screw was not tight - probably not torqued correctly during previous maintenance. I'm sure it was leaking and affecting the fuel mixture reaching cylinders 2 and 4. Maybe only noticeable to me during max mixture flow (lowest manifold pressure)?
I'm doing other maintenance now that I've got everything cracked open, so I haven't been able to test yet. I'm hoping I found the root cause.

I'm the O.P. of this thread, so I thought I'd write the conclusion to the story. While I did find a few "unordinary" things while poking around (see post above) I eventually found the real cause of the symptoms. Way up under the instrument panel, where one can't see without standing on their head inside a two-person cockpit, the primary fuel line had an attachment point to the firewall. That attachment had come loose, and the fuel line apparently began twisting, causing an unusual bend in the tube and a fuel flow restriction. Not enough of a restriction to notice in startup, warm-up, taxi, or run-up conditions, but enough to cause the engine to lose power during heavy climb.
All fixed now and the test flight was excellent. I learned my lesson: when faced with a mysterious engine power problem, make it a task to inspect every inch of every fuel component, even the ones in "can't see" places.

Good find! And thanks for sharing the end result with us. Never know when it could help someone else!