Hi,
Welcome to the forum.
I have no "magic bullet" fix for you, just some things you might consider.
When the problem occurs, you should take off the hood and see if you can physically manipulate the carb linkage to lower the idle. If so, then it is going to be something hanging up slightly. My first guess would be the throttle cable going bad if it is a remote unit. If it's a tiller control it could also be the cable for the twist grip. Either way, you will likely need to do some jiggling and shaking to try and narrow it down to a single component.
Another thing that can raise idle is an intake air leak. Intake air leaks usually occur more frequently when the engine is cold and then tend to seal up a bit as the engine warms and the metal expands. But, because the metal does change dimensionally with heat, it could very well develope a leak when warmed up.
Checking for an intake leak is fairly easy and most anyone can do it. The cheapest way is to spritz purified, distilled water from a spray bottle towards likely areas like carburetor mount gaskets and intake manifold gaskets. If the "cadence" of the engine changes while spraying a certain spot, then that is the probable leak..
You can also spray carb cleaner in the same manner to locate intake leaks. The engine will speed up if you spray a leaking area because the cleaner is flammable and the engine will use it as fuel. Because it is flammable, you must use care and caution if you decide to use this method.
Other, less likely, causes would be carburetor idle screws backing out and affecting the "synchronization" of the carbs. However, I'm not sure it would idle as well as you say it does when cold if that were happening.
Engine timing changes can also cause the idle to fluctuate. Check your belt and make sure it is still good and "taught" when the engine is warm. If it's old and stretching with heat, it should be replaced.
Hopefully others here will have some good ideas for you about things to check as I'm sure I haven't thought of them all.
Good luck and let us know if you find a solution or want to talk about it.
