Small Engines (Lawn Mowers, etc.): mountfield lawn mower engine poor running, cable spring, fuel tank

Question

throttle mech  

carb   Hi,
My Mountfield lawn mower engine is not running smoothly. it starts ok, but the throttle/engine speed 'fluctuates' revving up and down. not enough to stop the engine or effect the overall performance, but i guess it will gradually get worse. I've dismantled the carb, and checked the air filter. this all seems ok. on closure inspection during running i could see the air intake/butterfly value fluctuating. the engine has a fixed throttle which controls this value. it appears the spindle that the cable/spring lever mechanism rotates around is rotating back and forth slightly. i assume this should gradually adjust as the engine warms. the spindle is located at the bottom of the engine but can't see what or how it adjusts the throttle.
hope that all makes sense. any ideas on what the problem might be and how to fix it?
Engine RS100 #100103651027#on a Mountfield SP414

Answer
The symptom you described is very common with a dirty carburetor.  To repiar we remove, dis-assemlbe and clean the carb by blowing compressed air from an air compressor through all the carb holes.  Next we spray carb cleaner through all the holes and once again blow air through all the carb holes.  Re-assemlbe and test.  Cleaning the carb fixes 99.9% of all surging problems.

We also drain the fuel tank and clean with compressed air to make sure the entire fuel system is clean.

There are lots of You Tube videos on cleaning mower carbs.

Eric