Check the Mixture working and also will burn the carbon off the plugs which I suspect might be a problem if they are taxing around fully rich in high temps.
I have had a C172 that when you pulled the mixture back absolutely nothing happened. The boss thought I was talking bollocks and went to do a trial flight with it. He appeared an hour later with 6ft of cable in his hand after ripping the whole lot out trying to pull it because "the cable must of stretched"