I had the same problem today. I was getting too much fuel to the engine even before turning it over. It hydraulic locked on me and I had to clear the fuel before asttempting to start it. I pinched the fuel line and tilted the heli on it's side to let the fuel run through the muffler port and sure enough a lot fuel ran out of the muffler. I did this a few times and then left the glow igniter on for a couple of seconds before attempting a start. The engine fired right up and I let it run out of fuel to clear it out. I took the clip off of my fuel line and fired her up. She ran great the rest of the day. Do not attempt to start the engine if it locks up, you can cause serious damage with hydraulic lock.

Since you bought the heli used I would go to the engine manufacturer's website and make sure the low and high speed needles are set properly. Good luck