Gary- 'pop-off' is the pressure it takes for the metering diaphragm to actuate the inlet needle lever and open the inlet needle. How you 'set' the pop-off pressure is by using a different rate spring that goes under the lever itself.
Walbro kits in the late '70's and '80's had several weights of springs in the kits.
Ya. Another way is to bend the tang on the metering arm, which in turn would have the effect of pre-loading the spring, and chainging spring rate.
I think of it this way...
The pop-off can make the saw super rich or super lean, you can play with pop-off and get it close....
Imagine pop-off like this.,
Take a yard stick, 1" is super rich, 36" is super lean.
You can set your pop-off to say, 12lbs, which may put your air/fuel mixture around 23" on the yard stick.
Your needles will only give you 3" of adjustment in either direction before they become un-effective in holding a tune.
Your goal is say...18" on the yardstick, dead center. You cannot tune the carb as it sits to run properly, you can get it darn close, but it will never be correct.
Likewise, you can set pop-off too low, and it will flood as soon as you pull the rope, and will only run with the needle cracked off their seats.
These are my findings...pop-off is very important in a diaphragm carb, be it a Zama, Tilly, Walbro, TK, whatever.
Here is a thread made by ther Wizard of Carbs himself, E.C. Birt, read it 3 times, and after a while it all starts to make sense, this guy can build and tune carbs in his sleep. His words are absolutely invaluable.
He is like the Smokey Yunick of Carbs...haha
