Chainsaw Repair

Hi Joe,
Be careful with the B12.  That stuff is known to melt the check valves inside the high speed nozzle. Walbro HDA-268 doesnt have a replaceable nozzle fwiw so if you melt it you will need to make some other arrangements.  I may be telling you something you already know?  There are two kinds of check valves in the H side of the walbro carbs.  i. is a capillary seal type and ii. is a mechanical seal.  The reason that the check valve is needed is to keep the baro pressure from getting down inside the metering chamber of the carb.  Symptoms of a leaker is an idle mixture that wont tune.  One moment lean the next rich and so on.  Wont accelerate from an idle (bog).

edit:  taken from bob's post above.     

...  wanted to post this very informative link, (the best i have found)..... for basic carby operation, so folks can get a visual understanding of what you are referring to.......  http://helifreak.com/showthread.php?t=326599