My liquid-fuel stove lights, but has a very weak flame.

There are several reasons your stove may have a weak flame. The most common one is that the jet and fuel line are clogged from use or from old degraded fuel. 


First, try cleaning your jet. Shaker Jet stove owners simply shake your stove vigorously up and down. Non-Shaker Jet owners use the jet cleaning wire that was provided with your stove to poke out any debris that may be blocking the passage of fuel. If that does not resolve the problem, you will need to clean the fuel line. 


It may also be that the fuel you are using is old. Over time, white gas degrades. The type and quality of fuel, and its exposure to air and heat will affect how fast this degradation happens. Old fuel will clog fuel lines and should be avoided. 


Try searching "Cleaning the Fuel Line" for the WhisperLite, WhisperLite Internationale, SimmerLite & XGK EX stoves.