Curious starting issue

Hi all.
I have a starting issue that I feel sure someone on this site will have an answer to.
I have a KZ750 ltd, 1982 that starts and runs fine but it will take longer to start depending on how long it has stood. For example: If I park the bike after a ride and go back to it after 24 hours, it will start instantly. If I go back after a 48 hour break it will take about 3 seconds on the button before starting. After 72 hours I am looking at 10 seconds on the button and a full week will result in about 30 seconds before starting. Good job my battery is good! Could the fuel be evaporating off while standing? I have no carb leaks.
Once started it runs absolutely fine but is never keen on holding a steady throttle. She seems to prefer to be either on or off throttle. Not a major problem though.
Anyone got any idea what might be causing this strange starting issue?
Thanks.
Gary.
PS. Forgot to mention that a friend also has a 750 ltd (1981) with a very similar problem.

Try putting the petcock on Prime for a few seconds if the bike has sat for a day or more. If you have a clear fuel filter you will probably notice it filling up. If I leave my 750 for a few days or a week it will take maybe 5 seconds to start but I put that down to the battery being about 3 years old and while the dash will say 12 V I believe it does lose power and the motor doe not seem to spin as fast if bike sitting for a few days or a week.

30 seconds is not right. Probably evaporation

Thanks for that JR.
I tried priming for a minute or so and that seemed to reduce the starting time quite a lot although I didn't watch the filter filling up. I will try that again tomorrow as it has been standing for over a week. Will let you know how it goes.
I will also check the fuel level in the bowls before priming and starting.
Thanks again.
Gary.

Hi JR. Tried priming again like you said and seemed to work this time. Started after just 2 or 3 seconds and has stood for over a week. What do you think would cause so much evaporation in just a week of standing? Thanks again.
Gary.

The need to prime the carbs is normal. Keep in mind that there may be just a small amount of evaporation; just enough to lower the fuel level in the bowls below that which is best for starting the engine. I'm sure the bowls are not actually empty after a week. Ed

Thanks for that Ed.
That makes sense.
So the rule of thumb would be to use prime any time it has stood for more than a day or 2.
Thanks again.
Gary.