With my other bikes, I can twist the throttle with just 3 fingers. However, with my 1987 K100RS, I need to use my full hand to turn it. Is that normal with the K100 bikes? I can move the throttle body/housing with my thumb, the cable is properly routed, and throttle returns fine to origin when I release it.

Hi Your K is working normally. The (drag) that you feel is caused by the chain/crown wheel/pinion system,make sure you use a light grease not some treacle stuff. your other bikes are a cable only system hence the different Feel 

moriarti wrote:The (drag) that you feel is caused by the chain/crown wheel/pinion system. Make sure you use a light grease not some treacle stuff. your other bikes are a cable only system hence the different feel.

Thanks moriarti for confirming. Is Honda Moly 60 okay for this?

Personally i use Copper Slip 

Paste is thicker than treacle.

Maybe so but treacle tastes better

Check the teeth. Right about cruise position I'm willing to bet the teeth are chipped, hooked or both. My RT still has the stoopid chain arrangement on it and it's turnable with a finger grip. My RS has a pure cable setup, but that's a homebuild job.

I use aerosol silicone on all my K75/K100 throttle tube gears. They not only twist much more easily but they also return much faster.

If you don't have heated grips then pulling the throttle tube off and doing the handlebar with silicone also helps.

If'n you're Type A you can pull the perch apart and clean out all of the 30 year old way too thick dried out grease that came from the factory but even if you don't the silicone will still make a world of difference. I used to get out the old grease but don't even bother now when rehabbing an old K. I just spray the gears down with silicone and all is good.

You should also be sure that the grip rubber is not rubbing against the perch or, if you have it, a bar end.

Thanks duck, I'm going to try this aerosol silicone. The grip rubber is not rubbing against anything and the teeth are neither chipped or hooked. So, as moriarti's pointed out, I now believe that the chain/crown wheel/pinion system is causing the drag.

BTW, can lithium grease be used as well?

Thanks everyone for all the answers/suggestions.

I've been using spray lithium grease on my gear set for over twenty years. I also lube all the cables with PTFE lubricant called Tri Flo and lube the throttle linkages. When I lubricate the cables it also flushes out any debris that promotes wear.  Don't use WD40 or oil based lubricants that can degrade the Teflon cable liner. It's important  to make sure the cable routing doesn't have any sharp bends that can cause drag. I'm still running my original 30 year old 180,000 Km cables. 
Regards Martin.