How to diagnose small engine problems

If you're a handyman sort and want to do some of your own repairs to equipment I've got some tips I'd like to share from years of work in this field.

First a little background. I recieved my first ASE (Automotive Service Excellence) certifications after taking automotive repair courses through Umpqua Community College in 1990. Later I took advanced engine performance and electrical training and then took all of the (then available ASE tests and became Master Certified in 1996. I've also been trained and received Master Certification from Kohler and Briggs & Stratton in the mid 2000's. I spent most of my mechanic years employed by the local school district where I repaired and maintained everything from late model cars and trucks to mowers, floor care equipment, handheld power equipment and a host of miscellaneous stuff. The neat thing about that experience is I always got to see how the repairs worked out. It was kind of a lifetime warranty situation where if I didn't do it well it meant more work for me. Working in retail, you frequently don't get the chance to see the stuff that didn't work, it just doesn't come back.

Here's my take, the K.I.S.S. method if you will;

Pretty much everything is powered either by an electric motor or a gas powered motor.

It powers something that either goes up and down or round and round.

The power is connected between these two by either belts, gears, or hydraulic pumps and motors.

For now lets focus on that pesky gas motor, it seems to cause the most grief.

All gas motors have had the same basic technology since its inception. Four Strokes motors have the following cycle. Intake > Compression > Power > Exhaust .... then repeat....frequently.

For a motor to run there needs to be just 3 things basically. Fuel, Spark, and compression (or a mechanically sound motor). If these 3 are present the motor will run, perhaps poorly or for a short amount of time, but it should run.

Fuel and spark are the quickest easiest to verify so I start there (once I have verified there is correct oil and that the motor in fact doesn't run). I like to start with spark. I'll remove the spark plug and hook the plug wire back up, firmly ground the threaded part of the plug and turn the motor over. If I have spark I'll move on to fuel, if not I'll try the same test with a known good spark plug. If I still have no spark I now know I need to diagnose the ignition system. Most small engines have a magneto style ignition and you can remove the ground wire and retest. If you have spark now, it means you need to diagnose what safety switch or other point is grounding out that wire. If you still don't have spark chances are good the ignition coil/module is bad. The important points to remember are firmly grounding the plug, turning the motor over quickly enough, and KNOWING you have a good spark plug.

Fuel is easy, but somewhat dangerous. I like to add fuel either by dumping it into the spark plug hole before I put it back in, or after it is reassembled, dump about 1 1/2 tablespoons right into the carburetor (have to remove the air filter for this), quickly put things back together and try to start it. If the machine start and then dies, you know it is a fuel problem. Usually this is a plugged main jet. Some quick research on the web should get you the info you need to clean the jet on your particualr model of engine. If you don't know what a main jet is, you may want to take it to a shop for this repair. 

PLEASE use safe practices and common sense if attempting any of this. If you get hurt doing it,you're going to spend a lot more money on medical bills than it would have cost to just take it into a repair shop. 

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