Fixing a Briggs and Stratton engine Lawn Mower (won't run)

This is sort of a continuation of the post I did on Ethanol Gas and the damage it does to your small engines. It is also another of my self-help posts that will save you money, a lot of money, which you can use to establish and maintain food security.

I often cut the grass at my dad's farmstead and have had to bring my own push mower to his place because his wasn't running. It would start after priming (3x in accordance with the instructions) but would quickly run out of gas and die. I did an Online search and determined that the Carburetor Diaphragm had probably failed. The cheapest labor rate in my area for small engine repair is $45 an hour with a one hour minimum charge. Add to that the cost of parts and my time to deliver and pick up the mower and it becomes a bill I'd rather not pay.

But, luckily for me, this is a very easy and cheap repair and can be done by anyone with the appropriate tools. What is needed follows: Standard slot screwdriver, a Phillips head screwdriver, a 1/2 inch and 3/8 inch socket wrench, and needle-nose pliers.

My mower has a fuel bowl with a float that regulates gas flow into the carburetor. There is a fuel line leading from the gas tank, which sits above, down to the carburetor. Gravity feeds fuel into the carburetor. But in my dad's mower, seen above, the diaphragm acts as a fuel pump lifting gas up from the gas tank and into the carburetor. The diaphragm sits sandwiched between the top of the fuel tank and the bottom of the carburetor.

How do you know which diaphragm to buy? You'll need the engine data to look it up on the Internet or at your local parts store. The engine identification data is stamped onto the front of the engine as shown below. I went to several auto parts stores from which I have bought small engine parts before but they don't carry these diaphragms. NAPA does but my local NAPA store did not have any in stock.  So I went to a local small engine equipment shop and bought one from them. I paid a dollar more because of their parts mark-up but even so it was only $4.83 for the diaphragm and the gasket that comes with it. You can also order these parts Online but then you pay shipping.

Once we have the replacement parts we then have to take apart and remove the gas tank and the carburetor.

The first thing to remove is the air cleaner. While you have it off give it a good cleaning or replace the filter element (they are cheap enough). The air cleaner is held on with just one long screw-type bolt. This is the only thing that you need the slotted screwdriver for.

Next you need to remove a retaining bolt on the front of the engine. This is a 3/8ths inch bolt. Remove it and set the bolt and 3/8ths inch socket to the side together.

Next remove the fuel tank hanger bolt with a 1/2 inch socket. This bolt is easy to see. Set the bolt and socket wrench together with the other bolt.

Next we have to disconnect the throttle springs. These are tiny springs that automatically adjust the throttle based on the engine load. Be careful not to stretch or otherwise damage these fragile springs. There are two and they come off easy with a needle-nose pliers. Underneath the springs is a thicker  throttle linkage wire. This is a little harder to disconnect and is easiest done once you pull the carburetor and fuel tank away from the engine. 

If you look on the side of the engine where the carburetor was there are two tubes coming out. The larger one has a white ceramic ring and a rubber O-ring on it. This is where the fuel/air mixture enters the engine. The other is about half the diameter and is to the left of the intake tube in this picture. This is some sort of vent and is attached to the carburetor with a short L-shaped rubber hose/tube. That also needs to be carefully removed.

The carburetor is attached to the fuel tank with five Phillips head screws. Remove those and set them aside.

When I removed the Carburetor from the fuel tank the gasket and diaphragm stuck to the carburetor. This is what the top of the fuel tank looks like. I took a clean paper towel and cleaned the surfaces being very careful to not allow any dirt to fall in the tank opening, the upside down teardrop looking port on the left of the silvery area. The larger hole on the right is sort of a well to allow dirt to settle out of the fuel before the fuel goes into the carburetor. There is a close up below:

You can see some of the accumulated gunk in the bottom of this well or sump. Carefully clean this out with a Q-tip or something like that.

This is the old diaphragm and gasket. The diaphragm was stiff and stretched out, a condition caused at least in part by cheap, low grade fuel with Ethanol. This mower is only four years old and should not have a problem of this type already. The long yellow tube with screen is the fuel suction tube. Make sure the screen is clean but be VERY careful if you have to clean it.

This is the new gasket and diaphragm setting on the fuel tank. I found it easier to place them on the tank and then mount the carburetor on top than to mount them onto the carburetor first and then mount all that to the fuel tank. The diaphragm must go on the tank first and place the gasket on top of the diaphragm. The carburetor then goes on top of the gasket.

Since I had the carburetor off the engine I gave it a good cleaning and wipe down with a paper towel. Carefully align the screw holes so the gasket, diaphragm, and the holes are all stacked properly. Then insert the five screws and tighten them each little by little going from one screw to the next so each screw has equal pressure on it. I used the same star pattern you would use to tighten the lug nuts on a car wheel. Once the five screws are in and evenly tightened you can reassemble the parts in reverse order of disassembly. I put a tiny bit of grease on the black O-ring on the intake tube to help it seat properly.

This whole process, even including taking the pictures, took me only 25 minutes. I wheeled the mower out to the yard, primed it three times; the mower started on the second pull and ran like a champ. I saved at least $45 dollars and had the mower out cutting grass the same day instead of three weeks later (the approximate wait period posted at the small engine shop I go to).

Not bad for a non-mechanic.