The diaphragm on a Briggs & Stratton 550 lawn mower engine is part of the carburetor. A carburetor's job is to gather the fuel and mix it with air before delivering it to the engine. It delivers different ratios of gas and air, depending on the running speed. The diaphragm helps to regulate the fuel in the mixture. When the diaphragm is bad, the lawn mower will not run smoothly.
Signs of the End
The symptoms of a bad diaphragm are similar to other mower problems. Refusal to start or cutting out during running may be the first sign of a problem. The mower may smoke excessively or sound sluggish. At high speeds, the motor is more likely to give out or seem like it is starving or flooding. Conversely, the idle speed may be higher than normal and the mower may rev up and down on its own. The symptoms are not exclusive and a mower may have any combination of them.
What Causes It?
The diaphragm is made of a rubbery material. With constant exposure to fuel and heat, it can stiffen or dry rot. It needs to remain flexible to do its job properly. Going bad may not have anything to do with something you've done wrong or faulty parts, but is likely the result of regular use. If the diaphragm becomes stiff or cracked, it will put too much fuel into the engine. The excess smoke that accompanies it is fuel burning off, while the revving and stopping is caused by flooding in the engine.
How to Fix It
Fortunately, a diaphragm can be replaced without replacing the entire carburetor. To access it, you must remove the air filter and the bolts holding the carburetor onto the mower. The carburetor is either on top of the fuel tank or right next to it, depending on which series of the Briggs & Stratton 550 your have. Screw off the bottom of the carburetor to reveal the rubber gasket and diaphragm. One corner of the diaphragm has a spring. When you squash it in, it should bounce right back and have good spring. There should be no cracks and the rest of the diaphragm should be pliable. If it is rotting or has some damage, pry it off and replace it with a new one. Carburetor rebuild or repair kits will provide a new diaphragm as well as other replaceable parts.
A few other lawnmower problems may have some of the same symptoms as a bad diaphragm, particularly the difficulties with starting and sputtering. A bad spark plug can cause many problems. This is one of the easiest fixes, because all you have to do is unscrew and replace the spark plug on the outside of the mower. The starter assembly may also have some complications. The rope should pull smoothly and begin the internal spinning within the first few inches of rope. If it does not, the rope may be tangled or the spring may be broken. Poor compression may also be at fault. Briggs & Stratton recommends visiting an authorized service center for compression testing on its engines.
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