I have a FZ63 that's about 2yrs old but with only about 1 1/2 gallons of fuel run through it. I have this engine in a u-can-do, using PowerMaster 20/20, OS F plug, APC 14x4w prop. This engine was running great a couple of weeks ago then I wanted to change my CG so I had to pull the engine out, reinstalled it in the plane & I can't get it to run now. At first I thought I might have crossed the fuel & pressure lines but I checked & it's connected as it should be. Cleaned /inspected the regulator & still will not run. A couple of times I did get it to run for about 30 seconds by adjusting the HSN to about 3 turns open but this was only a rough idle & wouldn't take any throttle at all. When I turn the prop by hand to prime it I can hear a hissing coming from the carb(never heard this before) & it's leaking fuel from the carb.
Any thoughts on what this problem might be ?

Regulator installed backwards?

Obviously you have something hooked up wrong. Engines don't stop working properly just by removing/installing.

Did you take the engine apart?

All I did was pull the engine out then reinstall, after the reinstall I could'nt start it. I never even touched the regulator till after I started having this problem. I double checked the fuel & pressure lines & they're hooked up as should be, the check valve is in the pressure line so as to hold the tank pressurized. The last time I ran this engine(couple weeks ago) it ran great, I shut it down after a ten minute flight, drained the tank & ran it dry, did'nt notice anything unusual.

I have no idea, but since the only real difference is a new tank, that's where I would look.

I've got the same tank installed, no apparent leaks, no fuel residue in fuse when filled, just leaking fuel from carb when I try to start it. Also it doesn't seem to be pressurizing like before. Before this problem I could turn the prop over by hand 10 - 15 times & I'd get a good hissing sound when I disconnected the pressure line, now I get a weak hissing when I turn it over 30 times.
Think I'll just pull it out again & try running it on my test stand with a different tank, maybe I'm just missing the obvious.

The regulator sounds like it might be stuck open.