**Always follow the instructions in your repair manual when doing repair or maintenance work on Outdoor Power Equipment. Manuals can be found at the manufacturers website.*
Checking the fuel system, and vacuum fuel pump on a V-twin 25HP Kohler engine involves checking the fuel lines that attach to the pump, and checking fuel flow. The first step is to check to make sure fuel can flow from the gas tank to the fuel pump. To do this, disconnect the fuel line from the vacuum inlet fitting on the vacuum pump. Take the fuel line and lower it to a point that is below the fuel tank and see if gravity will cause fuel to flow from the tank through the line. If fuel flows, than the line is clear. If fuel will not flow, than check the fuel filter to see if it is restricted. Also check the fuel tank shut off valve if the tank is equipped with one. The valve may be in the off position, or dirty and restricted. Check the fuel tank cap to see if the vent on the fuel tank cap is plugged. If the fuel tank cap vent is plugged than fuel will not flow out of the fuel tank.
Next, check the pulse line on the vacuum pump. Make sure it is connected correctly to both the vacuum pump and the engine crankcase. Also, make sure the engine is not overfilled with oil and the pulse line is not filled with oil. If the engine oil is overfilled with oil, than oil can make its way into the pulse line and block the pressure necessary to operate the fuel pump.
The next thing to check is the fuel line from the vacuum pump to the carburetor. To do this, take the line off and make sure the line is in good condition, and there are no restrictions in the line.
The final check is to make sure the vacuum pump is pumping fuel. Disconnect the vacuum pump outlet line where it connects to the carburetor inlet, and turn the engine over. If fuel flows from the vacuum fuel pump, than the pump is working. Kohler does not provide a more precise way of checking the vacuum fuel pump, and they say if you get fuel flowing through the pump when the engine is turned over, than the fuel pump is working correctly. Kohler does say the vacuum fuel pump should pump a minimum of 2 gallons of fuel per hour with a pressure at 0.3 psi and a fuel lift of 24 in, but they do not give you a method to check these specifications.