So originally you had neither the check valve nor a filter?

YS engines don't like crud in the fuel and a filter is MUST or the regulator will get junk in it and will not operate properly. This can and will cause the engine to hydro-lock with fuel which is what I think might be happening since the starter cannot turn it over until the plug is removed.

First thigs first though....where are the high, low and hover needles set? If they are in the running range of the engine as noted in the manual then you might want to make sure the regulator is clean.

If you don't feel comfortable working on the engine then box it up and send it Richard at YS, if you feel confident then you might want to pull the regulater body off the engine and flush it out fresh fuel to make sure it is clean. If you look at the exploded view in the instruction manual this would parts 35-39. The screw on the side of the regulator right under the fuel nipples is FACTORY SET...do not mess with it unless you are careful to make note of exactly where it is set.