It was making a strange noise so I took the clutch cover off and found a broken gear tooth.
The engine had to come out and I split the crankcase to get to the broken gear. Luckily I had a spare gear and crankcase gasket in stock.
It took me 6 hours to remove the engine, strip down, replace the gear, reassemble and put back in the frame. Running great now, no weird noises anymore ?