I had similar problems using an electronic tach (multiple wraps around spark plug wire) the RPM's jumped all over the place...AS WELL AS...it would kill spark on that cylinder every now and then. I ended up going with a mechanical tach to keep it simple. I am now using the electronic tach on my generator to gauge hours used...single cylinder, no issues. I'm sure there's a solution or these wouldn't exist...but I gave up after trying various #'s of wraps and alternate wiring methods.