Zenoah G290RC primer bulb empty after being ran for 10mins.

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The pop Off mod is not something that is new. It is something that should be looked at for sure. These carbs were build for lawn and garden equipment and the metering requirements were not what we are needing. I would strongly recommend you at least look into that it is before you say that it isn't needed or has never been needed. I know a few major motor builders swear by this mod to make sure you don't starve the carb for fuel.

Jimbo1 you can do this by changing the spring to the correct tension spring inside on the diaphragm. The spring that should get you close without measuring it with a gauged pump is Walbro #98-344-7
 
we have been using the walbro spring # 98-3247 to get 13-15 psi pop off and a reseat of 10-11 psi and also using walbro's notched throttle plate to start the air flow faster over jet holes and make all the off idle lag go away. with some time in setup with these carbs we are now using 6000 rpm springs in all our clutches as the low end pulls at a instant off idle and pulls hard, Also our motor temps are also cooler ...............
 
Here are a couple of pics of the metering needle pump that @lorraine is talking about.
[url=https://flic.kr/p/TfNyQU]Metering needle pump by TDC, on Flickr[/URL]
[url=https://flic.kr/p/TfNyFq]Metering needle pump2 by TDC, on Flickr[/URL]
The gauge on mine has a 30psi limit. The carb I tested is a Tillotson from a demo saw. The metering diaphragm hooks on to the lever so it would never pop with the pump. If air made it by the needle the diaphragm would close the passage. I didn't want to fatigue my right hand (gonna need it healthy for later tonight) so I hooked up my leak down tester just to demonstrate whats happening. Of course the air compressor is keeping the needle open for a long duration. The pop and close would be instant with the hand pump.

This has to be the best write-up with pics that I have ever seen. This guy explains it so well, a cave man could understand it.
https://www.helifreak.com/showthread.php?t=326599

My theory is that heat is the issue. I believe the heat is causing a pressure increase in the primer bulb and pushing fuel back through the two way check valve to the tank. You can confirm this without a gauge. Run without a body and maybe fab some sort of heat shield with aluminum foil like the Heli guy describes in his post. If that doesn't solve the issue, then follow @lorraine 's advice on metering springs.
 
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