Renault 19 1.8litres engine problem

Hi all, My wife's got a Renault 19, with the F3P engine, 1.8 litres, 8 valve, monopoint fuel injection. The engine feels somehow weak, more like it were just having 3 cyl operating. The sound from the engine is like only 3 cyl is ok. Also, it's rather thirsty (this may be the result, since you're giving more gas) I've done all "first step" service, like: Compression test OK , with 10kg all cylinders. Ignition system is ok. Sparkplugs ok, the electrodes look the same in colour. Airfilter, fuelfilter, oil & filter replaced. I was thinking of maybe a vacuumleak somewhere could do this. I've also heard sounds as misfire through exhaust during heavy engine loads, but it's not common. Any advice is appreciated.