Small Engines (Lawn Mowers, etc.): Cant regulate fuel - Nikki carb and B&S OHV, steel needle, valve clearances

Question
QUESTION: I have a Murray tractor with a 13.5 horse B&S Model Series 21B800 Type Numbers 0236 through 0448 OHV engine and Nikki carb.

The tractor died and wouldn't restart, so I pulled and cleaned the carb to no avail. I then pulled the valve cover and found that the intake rocker arm nut had come loose and both the nut and rocker arm were lying inside the valve cover.

I put everything back in place and set the valve clearances and fired it up. It runs just fine for a few moments, and then starts chugging and spewing black smoke out the exhaust - obviously over rich, but even with the mixture screw full in I can still see a good amount of fuel at the choke throat .

If I pinch the fuel hose, it eventually leans out and the engine runs fine until the bowl empties.

I've checked the float for binding and replaced the needle valve. I even pulled the cut off solenoid and replaced it with a plug to make sure fuel wasn't getting past the needle valve with the engine off.

With some careful tinkering of the mixture and idle screws, I can usually get the engine to idle away quite nicely, but just as soon as there is any load, it goes over rich again.

I'm stumped for where to look next :(

ANSWER: Hello Paul:

Is there Any peices of Casting on the Center of the Housing Where the Float Circles it? If Yes, File the Burrs Off. Check the Seat in the Carburetor Inlet Port. If you are Using the One Piece Needle with the Rubber Tip, You May have to Locate the 2 Peice (Steel Needle and Rubber Seat Insert) Needle and Seat. Simply Insert the Rubber Seat Using the Blunt End of a Drill Bit that is Just Smaller than the Rubber Seat. The Steel Needle is Shorted than your Old Needle to Account for the Insert. Check the Carburetor Bowl Vent. Is the Welch Plug Missing? Hope this Helps. I am here if you Require more Assistance. Let me know what Happens, Please. Thanks.

Good Luck

Respectfully

John

---------- FOLLOW-UP ----------

QUESTION: Hello John:

Thank you for your prompt reply.

In response to your questions:

No burrs or anything out of the ordinary where the float circles the plastic casting. No binding of any sort.
The needle seats properly - with the carb off the machine and the float bowl off, raising the float to level while blowing through the fuel inlet seals off the inlet to the bowl as it should. I've used both types of needle valve also - no difference.
Bowl vent is clear
Welch plug is in place and not leaking., I even put some silicone sealant on it to make sure there was no leak at that point.

I just ran it through a 1/4 tank of gas with no load and it purred away with nary a hesitation.  Any kind of a load and it goes over rich and dies.

Crankcase breather tube is also clear. It's as though any increase in vacuum just pulls too much fuel into the intake.

Replacing the entire carb would be cheaper than the 90 miles to the nearest repair shop plus the shop charges, but I'm not at all confident it really is a carb issue any more.

Strangely enough, if I put the float in upside down to limit the level of gas in the bowl, and fully close the mixture screw, I can get it to run under load at a very much reduced efficiency without it going over rich.

Paul
ANSWER: Hello Again Paul:

Sorry for the Delay. I was Recreating the Problem in the Shop. Recheck the Valves. If the Valve Adjuster Lock Isnt at the Proper Torque, it Will Loosen and this will Happen. Use Red (High Temp) Tread Lock on the Adjuster Lock Threads and Make Sure you Tighten them Properly. The Exhaust Valve Lock wasn't Tightened Properly on this Test. Yhat Should be the Cause of your Problem. Hope this Helps. I am Here if You Require More Assistance. Let me Know What Happens, Please. Thanks.

Good Luck

Respectfully

John

---------- FOLLOW-UP ----------

QUESTION: Good Evening John:

You Sir are amazing. The issue was in fact the valve timing.
The exhaust setting was off by barely a thousandth in relation to the intake.

I was always told to use the mid value if the specs provided a range, and it was better to err on the side of larger. I would never have believed that such a small difference could  have such a large impact on performance.

Many, many thanks for the assistance and a fascinating lesson.

Paul

Answer
Hello Again Paul:

Great!!! I am Glad I was Able to Assist You in Solving this Problem.  Thanks for the Good Words. Till Next Time. May All Your Mechanical Problems be SMALL ONES and May the All Mighty Bless You and Yours. Thanks Again.

Best Wishes and Good Luck

Respectfully

John