I think my tach is ok. I check it with the flourescent lighting like the directions said. I was losing rpm because my valves had enough clearance to shove a toothpick in there.

I still would like to see a 91FX turn a 16x6 at 10,000 rpm. My Webra works pretty hard to turn a 16x6 APC at 9,600. With more nitro I've seen 9,800. If I let it whail like a banshee it might get to 10,000. If your 91FX does that, you got a good one.