My Lawnmower Won’t Start: A Troubleshooting Guide – Part 3

You have already done the ignition systems check outlined in article #1 and the fuel system check outlined in article #2 and still your mower doesn’t start. So what else could be the problem?

In order for an engine to run it needs ignition, fuel, and compression. Now it is time to check the engine’s compression.

Checking Your Mower’s Compression

Compression problems can be caused by the valve train, piston rings, or the head gasket(s).

First with the spark plugs installed but the plug wires detached from the plugs crank the engine and listen carefully. It is sometimes possible to hear compression leaks from the engine.

Listen at the carburetor, if the intake valve isn’t sealing, you will hear air rushing from the carburetor throat through out the complete cycle of the engine’s rotation. You can place the palm of your hand over the carburetor throat and feel the suction as the engine goes through the intake cycle, but if you feel it pushing your hand away most likely the intake valve isn’t sealing.

If you hear air escaping from the muffler, likely the exhaust valve isn’t sealing. Bear in mind that during the exhaust cycle air will escape through the muffler, but if air is escaping through all the engine’s cycles, this would show exhaust valve problems.

Note:
Further inspection and repairs will require disassembling the engine enough to get to the head gasket area. If you aren’t comfortable further disassembling the engine, consult a professional small engine technician.

Head Gasket

One of the main areas of compression loss in a small engine is the head gasket(s) of the engine. To check these requires some disassembly. As you proceed, pay close attention to the head bolts. Are they are torqued (tight)? Are one or more head bolts looser than the rest? If so, you likely have a blown head gasket caused by loose head bolt(s).

Remove the head and check the head gasket. If you see a definite discolored area on the head, cylinder or head gasket, this is the blown area of the head gasket. If a head gasket is blown, replace it and reassemble the head making sure the head bolts are torqued properly.

Valves

If the head gasket checks out, it is time to check the valves. Look at the valves and see if anything is obvious. In overhead valve engines you can check the valve adjustment and set it to spec. Any thing beyond valve adjustment usually requires special tools and experience and is better left to professional. The other main compression item is the piston rings. It is necessary to remove the pistons from the cylinder to check the rings, again this is best left to a professional.

So beyond checking and replacing the head gasket(s) compression problems are usually internal problems and as such should be taken to a professional small engine technician for repairs.

At this point, we’ve considered the three reasons why your mower doesn’t start; ignition, fuel, and compression. By now your mower should start like it’s new. And, as always we’re happy to help you with all your lawn mower parts needs.