Model ef6600de. How do I decrease rpm?

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the high speed needle is set first ( one closest to the air filter) to obtain smooth high rpm running then the idle needle ( next to the engine ) is adjusted as you slowly slow down the rpm
final idle speed is achieved by adjusting the throttle screw and should be just slow enough that the centrifugal clutch is not engaged

Older motors have two jets in the carb. A high and low speed jet. My guess is the high speed jet is clogged, and motor is trying to run on low speed jet.

Is the bike fully warm if so ya may have an intake leak. Check this by spraying starting fluid on the intake manifold where it bolts to the heads with the engine running and fully warm. If the RPMs either increase or decrease then you have an intake leak. It could also be that the bike is just running to lean(getting to much air) If it is a fuel injected model Id suggest taking it to a dealership and having them do the work. Carburated model you can simply put a bigger slow jet in it and adjust the fuel mixture from there. Good luck and thansk for using Fixya.com

You need to set the mixtures for the carburetor. You will find a Low and High adjustment screw. At idle turn the screw in or clockwise until the rps starts to decrease. Then start turning it out till you hit max idle rpms. Turn screw back in a quarter to half turn and it should be idling smoothly. Be careful here and get someone to hold saw if necessary but hold in throttle to full speed and do the same with high adjustment screw. turn it in till rpm starts to drop then back it out until you get to max rpm and it won't go any higher then turn it back in a half a turn. If you have to change the high rpm jet a lot you might have to go back and readjust the low rpm jet again and set the idle speed. Never idle it high enough that the chain turns while at idle.

We used to have to do this a lot in timber business. You go up on the mountain to cut logs and the altitude has high enough that you had to adjust carburetor and then do it again when you got back to the sawmill.

Your bike is carb or injected? Anyway Hd that are injected start at high RPM and after heating up the idle kicks down. You have a Temp sensor for this, which sounds like it is bad. I can not believe your bike would run at 100 rpm no way. harley runs at 950 to 1050 at an idle. It does not sound like you have a carb so Im not going to go into that fix.

The main jet need cleaning.(you should clean them all though)

Main Jet: Main jet comes into play at three-quarters open to full throttle conditions. CRITICAL to high-speed engine operation. Not only does it meter the gas into the engine, it can aid in cooling the engine as well.

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Are you sure your idle ports ( size of pinhole) in carb aren't plugged with something? What happens if it starts to die, and you give it a quick squirt of WD-40 , down throat of carb?