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Small Engines (Lawn Mowers, etc.)/mountfield lawn mower engine poor running


throttle mech
throttle mech  

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

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.


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Eric A. Jones


Lawnmower Repair . Certified Master Service Technician from B&S. Have 23 years experience on B&S, Lawn Chief, Weed Eater, Echo, Peerless, Wheel Horse, Snapper, Atlas, MTD, McCulloch, Homelite and many other numerous brands. Specialize in electrical repair.


Born and raised in the midwest. Started tinkering with engines when I was about 14 on my Suzuki RM-80. I began lawn mower repair at a small hardware store. I knew absolutely nothing. I read lots of repair manuals and met an older fellow who taught me many lessons. I continued working on small engines through high school and paid my way through college working on mowers at the same hardware store. Decided to get away from the midwest and mower repair so I joined the Air Force. I repaired air traffic control electronic equipment and ended up in Hawaii where I got a part time job at Small Engine Clinic. I gained a lot of experience from the Small Engine Clinic and had a blast repairing small engines. I then took the Briggs and Stratton Master Service Technician test and earned my MST. I then traveled to Wisconsin where I attended the factory update training seminar and received formal training. Continued working on mowers part time as I completed 20 years of military. Retired from the military on a Friday and continued in the lawn and garden industry the next Monday.

MAS Aerospace Operations BA Mathematics AAS Electronic Communications AAS Electronic Technology

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