Rpm

1st thing to look for is a vacuum leak, listen for a whistling sound, if your mechanically inclinded, take a propane hand held torch, unscrew the very tip, not separate the brass from bottle, but the torch tip find a section of hose, put it on the brass where tip used to be, crack open the valve just a little, trace around vac. hoses, head gasket ect. Reson being at idle engines tend to run about 28 in. Hg. Vac, as rpm rises vacuum reduces, so at idle if you come across a vac. Leak, the propane will increase rpm even more, so listen to your motor for incress/decress of rpm, when u do, theres your vac leak, now, there are other things that could cause idle prob. but start with simple things first, (it is usually that) and what ever you do dont over think the problem

You may need new spark plugs they may not be able to compete the fuel roatation to the engine