With the carbs off , spin the motor for a second or so and place the palm of you hand over the intake as you do this.
You will feel suction and as you do this momentarily release the starter and pull you hand away.
You will hear a pop and also feel the amount of suction.
Do this a few times across the cylinders in fairly quick succession and once you have the knack you be able to differentiate both audibly and by feel if one or more is not holding the depression or not pulling like the rest.
Finally delay pulling your hand off each inlet and you will feel how quickly each one loses suction and it will be obvious if one is normalising quicker than the rest.
This test puts way more suction on the boot joint etc than would ever be seen in normal use and if you can't detect a significant difference between them doing this there is no way there is a leak there under normal running conditions.
Also a static test for the diaphragms and carb top leaks.
Simply fully lift two slides at a time with your index and middle finger and let them drop.
Observe the speed at which they both close which should be at equal rates, smooth and damped.
Do this across all four to compare all equally.
If one falls rapidly or snaps shut you have a diaphragm split or leaking carb top etc.
If one falls much slower ( which is what you are really looking for ) then you possibly have some slide stiction or some other issue such as a bent jet needle binding in the jet etc.
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