Small Engines (Lawn Mowers, etc.)/throttle fluctuation


throttle linkage
throttle linkage  
Eric, my Mountfield lawnmower throttle fluctuates up/down. Starts ok, but never runs smooth. i noticed the throttle control lever pivot spindle is rocking back and forth, causing the butterfly value to rotates slightly. you suggested it was likely to be due to a dirty/blocked carb, but after stripping this down and cleaning all the jets, there has been no improvement. this mower is a fixed throttle model, and i understand there is some type of balance mechanism that adjusts the throttle position as the engine rotates and warms. maybe this mechanism is faulty and causing the choke problem?
any ideas how i check this? i've taken the engine off the mower but it seems the only way to get at what’s behind the throttle spindle is to take the bottom of the engine off. i can do that, but would first like to know if i'm going in the right direction, and if so is this repairable.
engine RS100 - 100103651027 on a Mountfield SP414.

A few engines have a simple bi-metal sensor on the exhaust.  The parts diagram for your engine did not show any sensor.

Have the valve clearances ever been checked?  I can not find the valve spec clearance for your engine but I suspect it in the .004-.006 of an inch range.

Take a look at the link below, Overhead Valve Adjustment:

Most all 4 cycle OHV small engines use the same procedure for adjusting the valves.

DO NOT adjust the clearances unless you know for sure what the correct clearance is supposed to be.  Just measure the clearance and let me know what the clearance(s) are.

Are there any local small engine shops near you who might have the valve clearance specifications?


Small Engines (Lawn Mowers, etc.)

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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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