ORIGINAL: rangerfredbob

I've got a carb question, it's still in weedie form and I need it for that purpose... it's an Echo 21 with a Zama two needle carb. For one, the tank vent seems to cause a vacuum in the tank, I have two of these engines and have tried both of the vents I have with the same result. Sometimes it'll lean out then if I open the tank cap it'll run fine again for another few minutes. Then when it gets to half tank (always half tank, clunk still fully submersed, no bubbles in fuel line) the carb seems to suck air and it's not through the fuel line or fuel inlet. Pushing the primer bulb you can hear the air being sucked but it's hard to tell from where. I've put a different metering diaphragm in and checked the fuel pump diaphragm with no noticeable change. the other echo 21 I have didn't come with the carb, so I can't steal it. fuel lines are routed right.

the thing runs great when it gets fuel... any ideas are appreciated, thanks!