For Background I have about 1500 hours on Rotax 912ULS engines in three different airplanes over the past 15 years. I have taken the Lockwood maintenance courses and have a Light Sport Repairman license for my airplane and still ran into this problem which was somewhat self-inflicted.
Two years ago, on departure from my airport with about 450 hours on my airplane and about 200 since carb overhaul I had a rough running engine after takeoff at about 450 ft and running at WOT for a minute or two. I reduced the throttle and the roughness reduced but did not entirely go away returned to the airport checked everything externally again and attempted another takeoff with the same results. After checking plugs, both ignition systems and fuel systems I could find no problem so assumed it was a carburetor problem, so shipped my carbs off to Lockwood for overhaul. Replaced the carbs and sockets at that time and the problem went away.
Two years later, this month, l had the exact same problem departing another airport and on climb-out rough running with some vibration and loss of RPM. Only 155 hours from the last incident and once again thought it was a carburetor problem or something getting into the fuel system and so checked fuel system, ignition, plugs etc and found no problem but wanting to take the carburetors totally out of the equation because I had never really found the problem when the problem first started, I replaced both carburetors.
Initial checks were fine BUT on departure once again the problem came back. Working out of an unfamiliar airport with limited tools and busy traffic pattern was difficult to debug, BUT I finally determined the following:
The problem was AIR leaking in past the carburetor sockets due to not exactly following the installation recommendations of the socket clamps and probably tightening too hard on the clamps even with the newer space that has been installed. After determining that the only thing different between fixing the problem the first time and the second time was that I had replaced the sockets the first time along with overhauling the carbs but did not on the initial fix the second time. Further examination of the carbs and the sockets clearly showed some discoloration of the area where high velocity air was leaking past the sockets (see pictures) and causing a much lower fuel air ration than required on at least one carb.
Replacing the carb sockets and installing them per the installation manual with the clamp at the bottom and ONLY tightening the clamp to the point where it touches the spacer SOLVED the PROBLEM.
