MZ/MuZ Riders

I wonder if anybody can solve my latest connundrum. With my 500R ticking over the tachometer works fine but rev the bike above about 2000rpm and the tachometer drops to zero. If I switch on the headlight or the brake light the tachometer comes back to life, behaving perfectly. I wondered if it was a high voltage issue, regulator/rectifier issue, the extra load of the lights maybe drawing down the voltage but it doesn't look like it. The voltage at the battery, engine running and lights off is 13.86v, with the lights on it drops only slightly to 13.64v. The battery isn't getting warm as it did on my Honda VFR when the reg/rec failed. So I'm at a loss as to what's going on.
Any suggestions gratefully received.
Ta,

check the big ground wire (minus) from engine to battery mine came loose at the engine

and caused a small BROWN wire to smoke when i was starting the bike

Thanks for the suggestion Dave but it's still misbehaving. I have undone, cleaned, replaced the main earth on the engine and similar with the common earth point under the seat where about 6 small earth wires meet. I didn't find any problems anywhere. Ah well, more head scratching

the voltages in your original post look ok keep a check on battery water and use distilled water if it needs topped up

rex speed shop in uk lists replace ment stuff for rotax motors used on this bike

these bikes are getting older so do not be surprised at faults caused by bad wires
my rt125 is suffering from bad wire harness and its a 2002 at 23,000 miles + several years newer