I have a c500-55 that is acting like it is lacking fuel. Idles fine but when I give it throttle it dies. It is a propane model
I am not the most knowledgeable on propane systems but I have run into enough of them that I have several things I do when having issues with the systems. Not everyone is going to agree with me but I do this based on all my experiences with them. I would replace the regulator first off, and the propane carburetor (especially if they are old). It seems like every time I tried to rebuild them I could never get them to work right. There is also an air plenum under the carburetor that adjust the air flow intake to gain the proper mix of propane. You may want to tweek that adjustment a bit in both directions and see if it makes a difference.
Is your LP tank full? Is it properly positioned (eg: withdrawal tube on bottom) to draw liquid? Is the machine new to you or are you familiar with it?