What is the box on the side of a Yamaha twin cylinder riding mower motor it has gas line from tank on one side and gas lone going to carb on the other

May not be the carb at all. You have somehow managed to suck gas into the oil. Take the plug out and crank it a few times. It will likely throw excess gas and oil out of the cylinders. I had the same problem with a brand new (5 hrs.) DR Field and Brush Mower that was equipped with a "carbon recapture unit" - a ridiculous new EPA requirement. Rather than a simple vent on the gas cap, these geniuses have put a valve at the top of the gas tank with a hose running back into the carb. No problem until you fill the gas tank and try to mow uphill more than at 25 degree angle. Then, what happens is when the gas level in the tank makes contact with the valve, you wind up with an air filter full of gas and gas/oil in the cylinder. You'll need to replace and gap the plug, drain and replace the oil, install the CRU (if it has one) replace the air filter, and it should be ok. I'm going to bypass this CRU. The EPA, for all the good work they do, gags on a gnat but swallows a camel. Good luck. P.S. My DR is still in the shop.

Its your fuel pump or a fuel filter

Check your fuel cap vent

If it is a Briggs Twin it is part of the carb , if not it will be a vacuum pump somewhere close to the engine because it is driven by a vacuum hose hooked to the block . As the piston goes up and down the pulse of the vacuum moves a diaphragm in the pump to move the gas from the tank that is lower then the carb .

briggs and stratton fuel pump fix

it depends on where the fuel tank is follow the fuel line from the carb to the tank the pumps used on mowers look like a round cylinder with 3 lines going into it 1 to the carb one to the tank and one to the engine block/valve cover but sounds more like the carb needs a good cleaning pull the choke on a little and see if it runs smother

When the float in the carb sticks, it allows fuel to drain into the cylinder, that runs pass the rings into the crankcase. When the crankcase is full the pistons cannot go down, and the starter won't turn the engine. Drain all the oil gas mixture, and refill with proper oil. Replace float, needle and seat in the carb bowl. Throughly clean fuel passages, and bowl. If motor won't turn, remove spark plug and turn over engine a few times to remove gas oil mixture from the cylinder too.

on weedeaters,chain saws. most are the same.
line with little filter in gas tank goes to carb. line out of carb to primer bulb
line from primer bulb to gas tank

THE V-TWINS HAVE AN EXTERNAL FUEL PUMP AND IS GRAVITY FED FROM THE TANK TO THE PUMP. MAKE SURE THE FUEL LINE IS IN GOOD SHAPE AND THE FUEL LINES ARE BELOW THE FUEL TANK LEVEL ANT TIME THE FUEL LINE GETS ABOVE THE FUEL LEVEL IN THE TANK FUEL WILL STOP FLOWING TO THE FUEL PUMP CAUSING THIS PROBLEM

There is still either water or debree contaminating the fuel system. Surging is usually debree, empty and flush the tank with gas or Iso alcohol, disassemble carb soak in solvent for 30 minutes blow out the carb passages, reassemble and if the problem persists run engine at top rpm with air cleaner off and block off carb intake with hand, this will stall the motor restart and repeat 4 - 5 times this wiil draw out any debree or other blockage in the carb.

Not so many reasons she won't start. 4-Strokes are Simple!

Here they are:

Motor:
Fuel Tank-> Fuel Line-> Fuel Filter-> Carb-> Intake (Valve)-> Piston-> Exhaust (Valve)-> Muffler

Carb (Old Style):
Fuel Side:
Fuel Bowl-> Float-> Needle Seat-> Fuel Ports (Venturi)
Air Side:
Air Filter-> Carb Air Intake-> Butterflies (2)
(Clean 'em ALL!)

Hardware (Motor):
Rotor (Magnets)-> Magneto (Ignition Coil)-> Spark Plug Wire-> Spark Plug-> Cylinder Head (Spark Plug Ground)

Ignition:
Kill Switches (Seat, Clutch, etc.)-> Spark Plug/Ignition Coil-> Ground
Key Ignition-> Battery->Starter/Generator

If one of those fail.......= Non-Starting 4-Stroke!

Fixes-
Fuel:
You are either getting too much, or too little. Too much = Carb.
Too little = Carb. Therefore, tear down the Carb (COMPLETELY-Careful!) and clean it up.

Electrical:
Grounding out the System (Kill Switches)
Battery-> Ignition-> Starter.
Magneto->Plug Wire->Plug (Check 'em all!)