Jumpy Tach

Changed the bulbs in my speedometer and tach yesterday and developed problems. The speedo has always been a little jumpy but acceptable and the tach was always smooth as silk. After changing bulbs now I have a SEVERELY jumpy speedo and a tach that will only go to 4000 and if I ride long enough will eventually ease its way (erratically) down to 2000-2500 rpm's. I made sure the bulbs were the same physical size so I dont think they are up inside hitting or touching anything they werent before...went back and checked the archives on WiredGeorge's tach cleaning and disassembly but it didnt really show me what was inside there that could be messed up. Cables on both are the same as they were before I did this (1978 KZ 1000). Do these things work on magnetism or is it more of a friction/ clutch type operation?

Update: Took both instruments apart and couldn't find anything obvious. Hooked up the tach without mounting it in the holder and it worked smooth. Re-installed both gages and tried it out again. Back to normal (except now the glass is cleaner, thanks Wiredgeorge)and my theory is I MAY have tightened the mounting nuts too tight and possibly distorted the tach and speedo housing in the process causing the little bell shaped deal that the spinning magnet 'moves' and advances the needles in the gages , to rub or hang up just enough to cause problems.