I recently dismantled, cleaned and re-assembled a Rochester Quadrajet carburetor which is on a Mercruiser 228 engine. No trouble starting engine and after it had been running steadily at 900rpm for about 10 minutes whilst waiting for the temperature guage to rise to normal level, the engine revs suddenly rose to almost peak revs and stayed there. I had not been revving the engine it just suddenly happened. I switched off and tried it a couple of more times but the same happened after a moment or so.. I checked the various linkages, etc but couldn't see any problem. Any advice gratefully received.