Hi Rod,
All electronic devices, especially those employing digital techniques, are highly likely to suffer abnormal operation when operating close to a running, spark-ignition engine. The radiated electrical energy from an operating ignition system can be very strong and it covers a wide frequency range. It will disturb the normal operation of meters, timing lights and even the electronic ignition that is producing the spark - if one is fitted. Turn on an AM radio (off station) while your bike is running and you will see, or rather hear, what I mean. Actually experts use this very technique to diagnose intermittent misfires in engines. It often answers that thorny question - "Is it ignition or carburetor?" (almost always ignition in my experience).
While most electronic equipment manufacturers include design features to limit the impact of external interference some don't try at all.
It is always best to try to minimize the troublesome radiated signal at its source anyway.
With so many electrical devices around these days vehicle ignition systems are fitted with devices at manufacture to reduce this problem.
Modern vehicles are all fitted with suppressor-type spark plugs (resistor plugs) and suppressor-type spark plug wires which together dramatically reduce radiated interference. Fitting these to your bike will stop your problem I expect. The NGK R range of spark plugs are very good and spiral-wound spark plug wires are the best to get (MSD or Moroso's Blue Max are both super). Even though our spark plug leads are very short replacing them with these special wires will help reduce the problem with your multimeter.
There is a slight loss of spark energy when these devices are fitted (they are mandatory for electronic ignitions!) but this loss is small and you will not notice the difference with a properly working battery & coil ignition system as employed on our GTR's. Magneto ignitions (e.g. Bridgestone 90's, etc.) may not work so well with suppressor devices fitted due to this loss.
Hope this helps Rod - sorry for so many words but this subject is huge & complex (just like 2 stroke engines!).
Best Regards,
Mike (in sunny Australia)