What causes a briggs and strattin motor to speed up and slow down

These carbs are kind of touchy. If you are handy, remove the air cleaner, Pinch off the fuel line. pull off the fuel and move it out of the way. Remove the nuts that hold the carb and and remove it, being careful when unhooking the linkage.Drain the gas in the bowl and remove the cap screw at the bottom of the bowl clean the bowl with spray carb cleaner, lay it aside, on som carbs the cap screw has a very tiny hole, pull a wire from a wire brush and clean the out. next look up the brass part hanging down and see if there is a slot for a screw driver and being very careful unscrewing the jet. Sometimes you need to put the carb in a vise and tap on the jt and turn it at the same time. After you get the emulsion tube ou there will be some small holes that will need to be cleaned with your small wire. Then spray all the carb holes with carb spray, remove the float and needle and spray this area and on the inside at the bowl base there may be a small round thing sme smaller than a dime you willsee a small slot that needs to be sprayed also. put the emulsion tube back being caRFUL NOT TO TIGHTEN IT TO TIGHT. Put the needle and float back in and then the bowl being careful with the gasket install the cap screw. and replace the carb being carful of the linkage, and attach the fuel line.. You may have to pull the plug and put about a squirt or two of gas or carb cleaner in the plug hole to help it start. Good Luck. if you have any trouble let me know.