Sounds like the primer plate is bad or the check valve in the carb is bad. Its cheaper to just replace the carb. George
Sometimes even if the diaphram and gaskets look ok they may not be. If the diaphram is stretched or distorted even a little they wont work properly,same for gaskets and other parts of the carb. sometimes the littlest things can make a big difference. I would try a complete rebuild kit and while everything is out of the carb. spray the carb real good and then lite-ly blow out the carb. with an air hose also if you take a twist tie from bread and take off half of the paper from the wire and carefully probe all holes in carb. that might do the trick for ya. After you put the high and low adjustment needles back in (if your zama carb. is set up with them) you'll want to set the low needle (after turning all the way in till it just bottoms) at 1 and 1/4 turns out and the high at just about 1and 1/2 turns out, then while running make final adjustments.
Yes, as you push on the primer it pushes the air into the tank but is also pulling fuel up into and thru the carb which is also causing the diaphram to flutter which alows the fuel to pass by the needle to supply the engine with the fuel it needs to run. After the engine is started the engine takes over and creats a vacume in the carb to make the diaphram move and the excess gas is sent back into the tank to cause presure in the tank to supply fuel to the carb. If the diaphram gets streched out then it doesn't alow fuel to pass by the needle enough to supply the engine with fuel. If one of the several holes or ports through out the carb gets plug in any way the fuel cant pass thru the carb to cause pressure in the tank so then the circulation is stopped and the engine wont run. If a gasket is worn out to where it alows any air to pass thru, it will not allow the engine to run right or not at all because the air - fuel mixture is off. Bottom line is yes it causes pressure at first till the engine takes over and the whole process is simple yet complex. I hope this gives you alittle better understanding of the way the carb works. Any other questions feel free to ask.
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This may sound very dumb. !!!.. but I am not sure of the fuel circualtion within the carburerttor.... even the action of the primer bulb. My primer pumps air out a tube into the fuel tank... this would indicate that the increased pressure in the fuel tank forces fuel into the carburettor via the other tube.
Form your understanding of the machine is this the correct interpritation?
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