First, Bill has a good idea. Won’t argue with your shop’s logic but it should have given you a place to start. I’ve had this happen to me. After downloading the data from a MyChron4 at the end of the day, noticed that I had one more session stored than the number of times on the track. Found that the gauge thought it saw zero RPM’s during a 25 minute race. The only change made for the next race was to wrap the RPM sensor tighter around the plug wire. Ran the rest of the season without a problem.
You could try to tighten the lead around the plug wire and you could add an extra wrap of the lead around the plug wire. There is the possibitity that the RPM wire is damaged or theere is problem with the guage. Call AIM. Their service is excellent.
Don’t think it could be the on/off switch unless your engine is cutting out. From your description your engine is running and just the gauge is giving a false RPM reading. The spark plug wire is part of the coil. Now you’re talking bucks.
LAD Specialties customer / tony kart / rotax / kt100