One more test - on the Mk2 models you have a diaphragm on the left side of the carb bank that allows the fuel to flow to the carbs when starting - if there are any leaks in that area it will take longer for the fuel to be pulled through from the tank.
Also, you may have small leaks around the carbs (float bowls) which are not visible when riding but may over a period of 2-3 weeks allow the fuel to drain out of the carb bowls and need more cranking.
My bike had this for a long time , and that was what I found........
Also, don't let go the starter button until the engine is actually running - if you do, current goes to the headlight and reduces what's available to start the bike. You won't do the bike any harm, just maximises the current available to the plugs.....

 

The mk2 has a vacuum operated petcock (at least on mine), the diaphragm on the carb wall you refer to is a fuel enrichener circuit. It says it is an air cut off but I conclude it adds additional portion of fuel when the throttle is closed under high rpms so as to avoid exhaust popping. It might be the other way round so it may lean the air in the pilot circuit. There is a spring there so even when the diagraph is shot, the only effect is that you just don't get this fuel/air because the valve remains closed (as it is in its neutral position), it will only work on the strong vacuum. And I am not sure how you got it in the US, but here in EU not every Mk2 has it. German ones don't for example and mine is from Switzerland and she does have it...

 

thanks for the advice on batteries and carb cleaners etc
battery:
since i went to gel it cranks much better and battery life is x 3
for the carbs they are squeaky clean inside and had a in line filter to keep out the ****
for the vac gas tap its been re-sealed and tested , no flow problems


so far with the adjustment of the valves its much much better
now going to re check the air bleed screws with vac gauges to sort out the last bit of wobbly idle
again my point
check the valve clearance 1t
reg
paul

 

My experience with my '90 as well. The battery may have enough snoose to spin the motor plenty quick enough, but the ignition is weak. It will fire right up bump starting. Over the years, that has been the indication that the battery is getting toward the end of its days.

 

Ha.. Today I was doing the valve job and had to unplug the coils plug... don't ask me what weakens the spark, just look into the plug ha:d