I have three Stihl Combi engines: KM130R, KM55, and KM56R. The KM56 I only bought a month ago, and unlike the other two the coupling gets really hot in use. It is so hot that when you take the attachment off after even short use, the drive shaft is the same temperature as the exhaust; so hot it burns. It doesn't matter what attachment I use, the coupling overheats every time. The couplings on the other two Combi engines only ever get a bit warm, no matter how hard they're used. The machine is only a month old and still under warranty. I have had the dealer look at it and he says there isn't anything wrong with it, but surely it shouldn't be like this. Can anyone suggest what's going on here?