Small Engines (Lawn Mowers, etc.): Briggs and Stratton fuel mixture too rich, briggs and stratton, starter rope

Question
Eric,
A friend gave me a 3 HP B&S horizontal shaft engine off his small tiller because it smoked and would not run smoothly.
Its the model with the deep fuel tank, fuel pump diaphragm on the muffler side of the carburetor, horizontal rectangular paper air filter on top, and has the throttle and choke levers right on the engine.

I've fixed several dead mower engines, and know that some tiller engines don't get a lot of hours on them.  Just pulling the starter rope on this 1990 vintage engine tells me that the compression is quite nice.

I found that the fuel mixture is way too rich with the mixture screw backed out about one turn. But gently screwing the mixture screw ALL the way in (until it won't turn any farther) only smoothes out the engine a little, reduces the exhaust smoke about half, and reduces RPMs very little.  Blowing compressed air into the intake leans it out some, as it should.  It just WANTS to be leaned out!

Close examination of the taper on the fuel mixture needle does show any wear or damage to the taper.  I'm thinking maybe the mixture needle seat got damaged by a previous owner, or something is keeping the needle from seating.  The spark plug is fine.

My engineering background has enabled me to successfully modify several small engine parts rather than replace them.  So I'm wondering what rework (if any) can be done to this carburetor so it will lean out the mixture enough to run smoothly and stop smoking?
Many thanks in advance.
George  

Answer
It's possible that the carb or tank, or both are warped allowing fuel to enter where is shouldn't.  Same goes for the diaphragm cover.  This is not very common but it is possible.  Sometime you can use 2 gaskets on the tank/carb mounting point to rectify the problem.

A flat file will show if parts are warped.  Just run the file across the parts in a couple of different directions.  High spots will be shiny where material is removed.

If the seat is damaged, and it very well could be from what you described, you may not be able to it.  If all else fails and your sure nothing else is wrong, you may be able to take an ice pick or something pointy and lap the seat with some fine valve lapping coupound.  This works well on large tapers but can be difficult to do on small jet tapers.

Let me know what you find.  I'm interested in what is wrong.
Eric