I have a 18hp b&s inteck that want idle the govenor goes back and forth

HOW HIGH IS TOO HIGH RPM? (NO TACHOMETER?)
NO ENGINE TOLD OF MANY SO ANSWERS NOW GO GENERIC.
ALL ENGINES CARS, FAST IDLE LOCK IS BASED ON ECT SENSOR COOLANT TEMPS, AND COLDER IT IS THE FASTER IT GOES.
EVEN BY EPA LAW USA.
cold is 1500 to 2500 most cars. pcm logic hard facts. and tables.
fluctuates is super bad and dangerous A.K.A SURGE.
there is no pat answer to this, so many engines.
but here are some.

• IAC BAD. (IDLEAIR CONTROLEER VALVE) THERMAL
• ISC STUCK (IDLE SPEED CONTORLLER ELECTRIC.
• THERMOSTAT BAD, OR MISSING.
• ECT READS WRONG/ Most times see 1 line up.
• VACUUM LEAKS FROM VAST THINGS. (YES VAST)
• PCM IN LIMPHOME MODE NOT SCANNED AND IDLE CONTROLS 100% OFF LINE. AND SURGING.
• GROSS MISFIRING.
• GROSS LEAN BURN AND SURGE.
• other sensors failing hard and you not scanning PCM miss the boat.

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Under the hood, locate the throttle control (near or under the air filter housing, you can have someone step on the gas back and forth to see it move and identify it). Once you find it, you can manually move it back and forth (do all this with the engine NOT running, in the off mode). The idle can be adjusted usually with a screwdriver but you also might have a problem with the cable itself sticking. Run some oil or other suitable lubricator (like WD-40) back and forth, moving the cable to get it loose.
(there's a little screw to loosen or tighten the idle, it basically opens or closes the throttle more)

slave cyl on clutch or pressureplate

That CS-56 soil compactor is set up with a govenor to stay at a fixed speed! If you need to change the speed you need to reset the govenor bu adjusting it to tighten the spring. If the revs increase in that direction OK, otherwise turn the adjustment the other way!

Peck on it with something to see if it idles down, if it does you probably need a new idle positioning sensor.

Check another vac source in/around engine. When you load engine with brake booster, then lower vac pressure w/Up idle, it's amplifying another vac source that might be weak.

Hello: First check and see if you are low on coolant, check both the overflow tank and the radiator itself. If low put antifreeze in it the check for leak. Have unit pressure checked

is this the model with the vacumn operated govenor for the idle ??