RV Repair: Onan Generator wont run smooth, mixture control, fuel needle

Question
My Onan 4kw gen set in my RV will run but it belches out black smoke and runs
like it has the choke on.  I have removed the air filter and checked it for
cleanliness, it was good.  I only have about 28 hours on it.  The spark plug is
black with little black chunks of stuff on it.  It used to run real good. I am going
to change the plug and see where that takes me.  I don't think the plug is the
running rich problem.  Can you help me with this?  I am in Alaska also.  the
Genset is in a 1994 winnebago class C coach.  

Answer
Hi Wayne:
Sounds like the gen set is running chocked all right. With the air cleaner off you should be able to see, or feel, the choke butterfly valve. Move it around a bit to see which of the rods activate the choke. When you start the genset it starts full throttle and choked but the choke should come off quickly and the governor takes over the idle. You can overide both to manually control RPM and choke. If doing that does not clear it out then something is wrong in the carburator. There are two idle screws on the carb side, one is for idle the other for high speed mixture control.A good starting point is the idle screws all the way in (clockwise) and then out a turn or so. If you do not come out far enough it will run lean or will not idle. Once you get it started bring the srews out one at a time until the engine starts to choke then back in till it smooths. Be carefull running the engine too lean for very long could blow a piston. If all that does not correct it then there probably is something wrong inside the carb, the fuel needle valve is not closing or has some debrie in it. If that is the case the carb has to come apart and be cleaned out and reset. I have at times been lucky and able to dislodge whatever is in there by taping on the side of the carb - but not often. Good luck.
Bill