I extended the boom on my harbor freight engine crane, with a length of 2x2 (if I remember right) tube steel. Obviously lifting stuff with an extended boom risks putting the center of gravity past the front wheels and overloading the frame/welds/piston, but…
Its very useful for lifting stuff that weighs a decent amount (150-250 lbs), but less than the crane itself. Especially when you can’t get the wheels under the thing you’re lifting. For example, came in handy when I had to hoist my new wood stove up a few steps from my garage to my living room.
Where are you reading it is only until it hits a load?
If the reviews…I think it is still working as advertised, just slower because it has a load on it. I have been using 20ton air/hyd bottle jacks for 20+ years and they run fast until it hits a load then it does take another quarter second to pressurize the air to actuate the motor.
If you want a rapid raise, then you have to go to a straight up electric hyd pump. $$$
You could use the Harbor Freight 20 ton recommended by @jamesdhatch
Move the jack to contact the boom very close to the fulcrum point. With all of that power it would not take much movement to effect the moment arm. This would place tremendous force on the pivot bolt but may work.
Edit: I must have too much time on my hands right now. I went on the this Vevor website with the other air assisted jack and here is a picture description of how to connect to air. There is no way those two fittings make a connection. Am I right???
I don’t know how they connect air lines in China, but that male connector is what came on mine. I have never seen anything like it. It sort of looks like what we use, but there is no way it would connect to any fitting we use in the US.