RE: Range Check Problem
I'd fix whatever is rubbing on your spark plug lead.
I think it's the spark plug lead. If it's rubbed through the outer sleeve--then it could be bleeding out electrical energy. I wouldn't trust electrical tape to provide a shield against that RF leaking out.
Something as simple as a Zip tie can be used to pull the plug lead up and away from the cowl, or muffler--whatever is causing the rubbing. If your worried about the Zip tie rubbing through the plug lead--just slap a piece of 1/8" heat shrink tubing around the Zip tie before you snug it up. The heat shrink will act as a cushion between the plastic Zip tie and your soft plug lead.
You can put a very small EYE bolt in the firewall if you need an anchor point for the Zip tie. I'm always a bit shy about Zipping my plug lead to a bolt or bracket on the engine. Too hot near that cylinder head. Just screw a little EYE bolt right into the firewall and run your Zip Tie through that.