Small Engines (Lawn Mowers, etc.): Robin EC10 idling problem, lawnmower shop, loose particles

Question
Hi there
Recently I began to experience idling problems on my Robin EC10 engine(on lawnmower). Since the engine has worked from 1978 until now without any problems, I didn't expect something serious. I first suspected a dirty carburetor. I cleaned it, blew it out with compressed air, but to no avail. The fuel tank has got rust and loose particles, but it has three fuel filters. The fuel reaches the carburetor easily. I then enquired from a lawnmower shop what they would advise, and told me to check the governor setting. When I dismantled the engine, I removed the cylinder head and barrel. A piece of the top ring on the piston had broken off. Suspecting this condition to be the culprit, I had it rebored to oversize .25(never been rebored before) and refitted it to the engine. Its compression is now excellent, but the same idling problem: At idle, engine speed rises and falls, with the engine nearly cutting out. Low speed performance is also not as usual. The air filter is clean and in good condition, new spark plug fitted. The spark plug's condition seems to indicate a lean mixture: White grey tip, with a brick red surrounding colour. Could it be the idle mixture screw, the governor/governor setting(I had fiddled around with the setting only on the shaft, but haven't opened the gearbox), or something else?
Thank you
Edwin  

Answer
The idling problem definatly sounds like a carb to me. I noticed you said your tank had some rust in it. It is very possible that rust has accumulated in the carb and made into the low speed circuit on the carb. Getting this rust out of the carb may be tricky if even possible.
I would suggest removing the carb, cleaning it up real good, then take to a shop that has an ultra sonic cleaner. This is your only hope for that carb. If at that point the carb doesn't clean up, then its time for a new carb. And do your self a favor and replace the tank. All those filters won't catch all the very tiny rust particles.