Cessna 150 Flight Instructor and student reported experiencing a rough running engine at cruise altitude; resulting in a diversion and precautionary landing at a nearby airport.
Synopsis
Cessna 150 Flight Instructor and student reported experiencing a rough running engine at cruise altitude; resulting in a diversion and precautionary landing at a nearby airport.
Narrative
Was flying at 3;500 Southbound down the coast when we noticed our engine start to skip and then drop from 2400 to 2100. Checked that fuel was on went mixture rich; throttle full; carb heat on and verified the primer was in and locked. RPM'S came back to 2400 before dropping again to 1900. Notified ZZZ approach on XXX.XX that we were diverting to ZZZ [airport] which was roughly 3 miles SW of our position. When asked why we said we were experiencing engine issues but I chose not to declare with the engine still making power. They said we had ZZZ1 as an option as well but we were already pointed towards ZZZ and it was the closer of the 2 options. With the plane still creating some power and us being well within glide range of the runway we switched to UNICOM when instructed to do so; maintained the altitude that we could while positioning ourselves on a final for runway XX and slipped the plane to lose the altitude when we were comfortably we lined up on final. Engine never shut off and was able to maintain power that fluctuated between 1700-2000 RPM's. Once we were clear of the runway and had taxied to the ramp I ran the engine through it's normal magneto and carburetor checks at 1000; 1600; 1700; 2000; and 2;200 RPM's with all mag drops being less than 50 rpm. Carb heat seemed to be working on the ground as well. We checked the quality of the fuel from all 3 sumps and ensured the fuel vent was clear and it all came back normal just as it had when we departed from ZZZ2 [airport] 30 minutes prior. Chose to leave the plane tied up until an A&P can have a look at it rather than trying to fly it back because neither of us have the knowledge; experience; or certificates to diagnose what it could be whether it's a coil in 1 of the magnetos; a stuck valve; or something else.
Second reporter narrative
Aircraft X ZZZ Divert ReportAltitude: 3500Attitude: Straight & Level (15 min)RPM: 2500Mixture: Leaned for Best PowerFuel: FullOil: 5 quartsTemp/Dew Point: 28/M03Sky: ClearLast flown: Same day for 2.4 hrsExperienced engine roughness & sporadic RPM (17-2400). Diverted to ZZZ [airport] & on ground within 10 minutes of noticing issue. Set mixture full rich; throttle full power. When carb heat was applied; immediate increase in RPM (no temporary drop). Got back to 2400. Carb heat seemed to have opposite effect of how it would be expected in a carb ice scenario. Couldn't maintain RPM or altitude while approaching ZZZ. Engine dropped to 1900; and again to 1700 on final.In-air trouble shooting- Mixture to full rich- Carb heat on- Fuel selector on- Primer in & locked- Oil temp/pressure: in the greenOn ground trouble shooting- Engine run up @ 10/17/20 RPM:- Mag drops were around 100 - Carb heat drop were around 50 - Could not recreate issue- No water/sediment in fuel
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.