Anything that affects the temperature of the bimetal strip inside the unit will affect flash rate. Too much current, as you'd get with bulbs that draw more power than stock, higher-than-spec voltage, or both will result in rapid flash rate. Too little current draw, low voltage...or/and low ambient air temp...will slow flash rate, of stop it altogether.
That finickiness allenp mentioned, is worse even than it sounds. Everything in the system: switch contacts, voltage, current draw, battery condition, ambient air temp has to be within spec,as it was when everything was minty-new, or you can have problems. Even when new, these relays left a lot to be desired on these bikes. Now, four decades later, there's a good chance of something being oxidized and that could be anything from a ground, to the switch assembly, a bulb socket contact...even the contact points inside the flasher relay. The bimetal strip can be affected by oxidation, too. IMO, the foolproof testing method is beyond the reach of virtually everyone; that would be a dedicated tester with precisely the right voltage & current draw. Thus, the best anyone can do is make sure that all of the contacts, throughout the system are clean, the DC power source can deliver 5a and 6-7v and the bulbs are of the specified wattages. Bulbs are another variable, as the nominal wattage can and will be different than the actual wattage. Unfortunately, bulbs are a dying technology, especially 6v flavor, which has 3-decade headstart toward obsolescence. Just to be clear, I'm not saying that 6v bulbs are going to disappear; but, the number of choices and quality are both in decline. It only takes a slight variation in bulb draw to affect these mechanical relays.
FYI, oftentimes the turn signals will begin working better...if not normally...after the bike has been running for some time. I'm talking about 30-60 minutes, or more. The combination of charging the battery, engine heat and vibration seem to have a positive impact. It's just a damned slow test method.
If you can source an electronic relay, that's the best solution. For those running 12v electrical systems, it's easy; solid state relays can be had for less than $30 and they work the same regardless of current draw. Those running 6v electrical systems have very few choices and the few that I've seen, so far, tend to be unavailable much of the time...that will likely change for the better.
