As others have stated it's a bad idea; wastes fuel, dilutes lubrication. If you need to relax your throttle hand on long rides I would suggest changing the grip by using different fingers occasionally and for a complete rest using a throttle-holding device. There are various commercial clamp and resistance types you could buy but I find the best one is one I made myself by out of heavy gauge wire. Get a piece of heavy gauge solid core insulated wire about 1 foot long. center it on the inner edge of the throttle then twist the 2 tails tightly so that it forms a stiff coil extending from the throttle past the front brake lever. It should be wound tightly enough that that it will hold the throttle open when rotated forward to rest on the brake lever. To disengage, just roll the throttle forward. Simplicity itself!
