The needle valve is held on its seat by spring pressure transmitted via a' seesaw' lever and spring.
When you use the 'pop off' tester to find the point at which the spring allows the needle to unseat and allow fuel through, this is known as the pop off pressure.
You pump the pressure tester and observe the dial and note the point at which it suddenly drops as the needle unseats.
Do this test with a 'wet' seat, ie a drop of WD40 on the seat.
Once it has popped off, the pressure will then 'hold' as the needle re-seats itself.
This is known as the holding pressure and should stay steady if the seat and needle are in good condition.
There is quite a wide variation in these two figures and they can be adjusted by forming the lever and different springs.
A standard kit should put you in the ball park anyway.
Sounds good anyway, could be talking pitlane ********?
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