Carburetor high speed jet adjustment in the Walbro WG
by Had Robinson
It is rare that the WG will need any high speed adjustment unless the carburetor has been damaged, the ambient temperature is below 50F, you are flying at high altitudes, or you installed the FSM. There is no easy adjustment of the high speed system. That is, there is no needle jet that you can screw in or out to adjust the high speed fuel mix. This was done to keep pilots from burning up their engines.
Before attempting to adjust the high speed system, be certain that the idle and low speed systems have been properly adjusted, the metering lever value is correct, and the fuel delivery system is working properly. If the ignition system has any defects, starting will be difficult and the top end performance will suffer. If the carburetor is worn out, it will not idle or run smoothly. If the reed valve block is not torqued down properly it will leak air and the fuel pump will not work properly, if at all. Always check these things if carburetor adjustments do not work as expected.
I do not recommend changing the pop-off pressure as it requires a complete set of precision made shims or a spring kit. It would require a lot of experimentation, as well. Another problem is that when the pop-off is changed, it affects the air/fuel ratio over the entire throttle range.
On the other hand, changing the high speed jet is easy to do. We can custom make any size jet for the WG (minimum size is #108). The most common high altitude jet is the #112.
Contact us to purchase, if you wish.
It is unwise to make any changes to the high speed system unless a CHT is installed.