For all engines to be sold in areas over 4000 ft. in altitude, a system has been installed to automatically lean out the fuel mixture by supplying additional air. This also results in lower emissions.
Low atmospheric pressure allows the bellows in the system to expand the close a port, allowing more air to enter from different sources.
In the 20R and 22R engines, this also results in a timing advance to improve driveability.
All parts in this system must be replaced. The only adjustment available is in the timing.