My 912ULS with Airmaster CS prop (WarpDrive blades) was stripped and rebuilt at 45h from new after being in a fire. It had 1 h test running when I got it a few months ago. I completed another 1h ground test as specified by the rebuilder (Conrad Beale, UK readers will know of him as very experienced) and since then have done another 10 hours test flying the newly completed Europa it is in. This was all between May and September. Through all of that it ran very well, just a tiny bit less smooth around 2800rpm than in the rest of the range, not enough to be a concern. I put it down to a resonance.

Since September I've been grounded by Covid lockdown and waterlogged airfield. A couple of days ago I did a ground run after about 9 weeks stand. It started well and ran nicely warming up at 2000-2500. However from 2500 up to about 3200 it was really, really rough. By 4000 it was very smooth again. Once thoroughly hot it was much better but still not like normal. Mag drops at all speeds were normal, equal and rough running wasn't worse on L or R.

Fuel was alcohol free 99RON Mogas. The engine has carb heat from the coolant (the Skydrive system).

The ambient temperature for this run was around 3 deg C as compared with 18-25 C for all previous runs. That shouldn't be enough difference for a serious effect should it? I did think it had the feel of weak mixture but you can't adjust in that range can you?

Any explanations and suggestions will be welcome!


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