What causes an engine idle to increase briefly?

That is probably a air leak n the intake after the carb. Probably the gasket. Airs sucked in and the idle goes up. Then the mixture causes the engine to start to die and the idle goes down. Repeats over and over. Check gaskets, hoses, head warpage.anything that would allow air sucked in after the carb.

Sputtering out the carbs. The fuels not being regulated as it enters the carbs. Check the power needle, float level adjustment, regulator.

In addition to the carb gaskets, double check the rubber intake boots for cracks that can open and close as the engine vacuum changes, causing occasional lean running, making the RPM increase.

A cheap diagnostic is to warm up the engine, then when the idle is hunting like that run a garden hose or other smooth stream of water over each gasket and rubber intake boot, one at a time. When the water goes to the crack it will prevent it from sucking air and you will either hear it sputter, see it start blowing steam (if the leak is bad) or stop hunting (if it's just a crack the water fills it in for a few seconds). In either case, that's your culprit.

Another test: If you take off the rubber boots, go in a dark room and put a bright flashlight in each, flex the rubber and see if any light comes through the cracks.