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Carb icing moment?


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My enjoyment of an early evening flight yesterday was interrupted by the sound and feel of my rough running Top 80 engine. Was at 800ft on a very hot & humid Hampshire day c.28C, had just come off 8-10mins of full power throttle on full speedbar, down to 60% throttle for a min or so before applying full throttle and speedbar again only to encounter rough running/vibration and no throttle response beyond 75%.

Yikes, so......trims to normal, back on brakes, turn back to t/o point 5 miles away, look for a suitable field for a likely forced landing. Tried full throttle again, no joy, eased back to 60% to maintain level flight, seemed a little bit smoother. Shall I continue and risk damaging engine (possible fuel contamination/blown head gasket/holed piston??) or make a precautionary landing miles away from t/o field? Thought crossed my mind it may be carb icing, had a similar prob on microlights a few times in the past, was maintaining level flight so opted to fly a little longer. Sure enough, a few minutes later I got back throttle response, vibration stopped, all seemed normal again. Took it easy getting back to field and when overhead, gunned the engine up and down the throttle range for a while and all was well, was relieved to land safely after the unexpected bit of excitement.

Conclusion? Most likely carb icing I guess due the venturi cooling effect in the carb particularly after a period of high revs - very humid air, happened during a flight transition when light aircraft would normally apply carb heat to counter carb ice risk, alas the Top 80 doesn't have that luxury :D. Next time I'll blip the throttle a bit more during wind down.  Last thing I was expecting on a boiling hot day, useful lesson learnt though!

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