Small Engines (Lawn Mowers, etc.): Carb Adjustments for a 5HP B&S engine, briggs stratton, internal passages

Question
I have a Briggs & Stratton Model 14FB Type 202540.  This is attached to a 2KW generator and purchased in 1966  The engine is a 5 HP, horizontal shaft, 14.2 cu inch, with L head (side valve).  According to B&S it's an antique and they have no listings of parts etc.  The fuel system, was badly gummed up and clogged and I have cleaned it thoroughly. The carburetor was totally disassembled and cleaned.  After assembly it works almost perfectly.  

When an electrical load, like a refrigerator, is applied the governor advances the throttle to maximum but the engine begins running rough and slows.  It sounds like it's starving for fuel.  If I enrichen the mixture with the high-speed needle valve it runs better but just as soon as the engine runs better it runs too rich and I must lean the mixture to get it to run smoothly again.  The range of settings are very narrow.  If I slowly advance the speed, it runs well but if I advance it quickly, it does not, it sputters but does not die.

Does this carburetor have any provision for getting an extra shot of fuel when it gets the maximum throttle setting? I suspect that there may be a narrowed or partially clogged passageway somewhere. I have the owners manual but no technical or parts manual.


Answer
I also suspect there is still a blockage inside the carb.  With the mixture screw around 1 1/2 turns out from lightly seated, it should be sufficiently rich to provide adequate power.  There is no feature to provide additional enrichment at maximum load as the high speed circuit in the carb is designed to accomodate the fuel demands.  Make sure you use a piece of tag wire to push through all of the internal passages when you clean the carb.  Hope this helps.