Small Engines (Lawn Mowers, etc.): 3.5hp Briggs surging, miss, internal passages, recoil starter

Question
Hi,
I tried to read the model, type but the decal is too damaged to make it out.
This vertical shaft, B&S 3.5hp is on a 18" mulching mower from the early 1970's.  The recoil starter pulls straight up, not out the side.  The carb. has only one mixture screw.
There seems to be a strong spark and the motor starts easily.  It has a "miss" which sometimes vanishes for 10 seconds after immediately after pushing the handle of the mower to the ground, thus tilting the motor, and then returning it to the normal position.  I tried cleaning the carb and fuel tank.  No help.  I notice that there isn't much suction at the air intake.  In fact if I place one finger partially over the opening, the engine shuts down.  The old foam air cleaner was replaced with a new one.  No help.  When it first starts it runs like a new motor for only 10-15 seconds....  I also tried turning the single mixture adjusting screw and now don't know where it should be set.  Can you help?

Thanks,
Tom

Answer
That's about the same B&S style that's on my 1972 Toro pushmower.  The mixture screw should have a base setting of 1 1/2 turns out from lightly seated.  How you have described that it starts right up and runs nice for a few seconds and then dies, it sounds like the mixture adjustment may correct this.  If it doesn't you will have to re-check your cleaning job on the carb and tank.  While cleaning the carb, make sure you push a piece of tagwire through all of the internal passages.  Avoid the temptation to remove the screen filters on the pickup tubes to the carb but make sure they are clean.  You might also want to check the carb mounting area (on the carb and tank) with a straight edge and correct with a coarse oil stone if necessary (make sure to re-clean everything).  Hope this helps.