I have a linkbelt 330 with a hydraulic thumb. This machine bogs down almost to a stall only when using the thumb, it also overheats the hydraulics. I have changed the fluid, filters and even checked and cleaned the check valves. Anyone have any idea on where to turn to next?
My question would be how is the thumb actuated, is it an extra spool in the main valve, or a separate system, maybe a solenoid setup? I'm assuming you have buttons or a rocker switch on a joystick handle to operate it? It sounds to me like you're getting full pump flow when you actuate the thumb, and there are some restrictors or flow controls to limit the speed to a useable level. The excess flow has to go somewhere so it flows across a relief valve, either main relief or a thumb circuit relief. If it's a main spool that's shifting with pilot oil, you can maybe add a couple valves to limit shift pressure.
Buttons to operate it and it is an extra on the main valve
Is this something that's just started, or has it done it forever, or have you just bought it? Wondering if it's designed this way or something has recently malfunctioned. Thumb holds okay when it's set and the bucket clamps back against it? Do you get the same bogging in both directions of the thumb cylinder? Should be a solenoid valve somewhere sending pilot oil to the spool ends. If all the relief valves are working correctly and the cylinder is not bypassing, you could reduce the shift pressure to the spool ends. Might already be a way to do that, just depends on how it's set up.
It would help to know the model year and maybe a serial number.