Usually a dirty carb ,but an inproperly adjusted carb can do it also .
******* it can also be the govenor setting in engine they are usually an oil bath govenor check your oil and if no luck you will have to adjust your govenor linkage*******A load on motor cutting high grass to short.
Check your vacuum lines this usually the cause for a surge in engine speed when idling or running.
It is possible it is one of the speed sensors.
A failing throttle position sensor is a common cause for idle speed fluctuations on a Jeep 4.7L.
Engine surge in a 2004 Kia Sedona is often caused by a clogged fuel filter or dirty injectors. When insufficient fuel reaches the engine, it will exhibit these symptoms.
My guess is it will causes the engine to surge and change engine speed up and down over and over because the ECU (computer) is searching for the right idle speed, but the bad sensor isn't sending the signal. It could also possibly cause the engine to just idle at a higher speed than normal.
we have a gas engine jenbacher 1006kw.we face a black shutdown due to exhaust back pressure high.
Mass Air Flow MAF sensor could be your, problem check that!
a surge of testosterone in males and it causes ovulation in females.
probably the governor going bad
as in a surge of energy or power or short bursts of speed or energy.
engine torque and small brakes