^ Had this issue on an Onan powered ( that`s what is in your 318 right?) Ingersoll, carb disassembly and cleaning made it disappear. Would not hurt to "run through" the entire fuel delivery system YMMV
The propane, when sucked into a vacuum leak will change the idle.re: " -Check for manifold leaks with an unlit propane torch "
What's this all about?