Small Engine Diagnostic Introduction:
Small engine diagnostics in its simplest form is checking the three main systems that a small engine requires to run.
- Ignition: does the engine have spark?
- Fuel: does the engine have fuel?
- Compression: does it appear to have sufficient compression to run?
Lets go over these three systems more in depth and in what order you would test these from importance and order of importance:
- Ignition: You may check for spark by removing the spark plug from the engine and grounding it against the engine block and cranking the engine to see if it sparks. If you have a spark tester that can be installed between the ignition wire and spark plug then you can test without removing the spark plug from the engine.
- Keep in mind, you may have a weak orange spark or nice blue spark that sparks with the spark plug removed from the engine and yet while spark plug is installed and under compression may not have spark and you’re not able to see that because spark plug is installed in the engine. Which means your spark plug is fouled or the ignition coil is weak.
- Fuel: you may check if it’s got fresh fuel in the fuel tank and if it’s equipped with a primer bulb pressing that several times to see if you hear and see fuel being dribbled into the carburetor. Another method would be putting the choke on full choke and seeing if the engine fires or pops. If either of these methods fail, you could grab a carb cleaner spray or some fresh gasoline in a spray bottle and spray them into the intake of the carburetor and see if it pops or fires when you crank the engine.
In the event that you have checked and verified strong spark from the ignition system, You’ve verified that its getting fuel the normal method or your spraying fresh fuel or carb cleaner into the intake side of carburetor and its not firing or popping as it attempts to start, you’re third system to check and verify if its good is “compression”.
- Compression: The reason being, that without spark, fuel and compression, the engine wont pop or fire. So now we test the compression of the engine. The primitive and basically redneck version is with the spark plug removed, place your thumb firmly on the spark plug hole of the engine and cranking engine. Does it appear to have adequate compression when you do that? I prefer to test it with an engine compression tester gauge because it gives you an actual measurable indication in PSI how much compression it’s generating. For a 4 cycle engine 30-130psi is sufficient for it to start. For 2cycle it requires 100psi minimum to start.
So now that you’ve verified the three most important aspects of diagnoses and they’re all proving to be proper and correct, only then does it require moving to testing things further. I’m going to CAUTION you here that not adequately properly testing the above mentioned systems first, will most likely result in a waste of time diagnosing other aspects of the engine and will lead to great frustration in you diagnosing a small engine. It will also lead to forming bad habits of your diagnostic process as you establish yourself in becoming great at small engine repair.