I recently picked up a small mower on craigslist as a fun side-project, but I have had only a little experience with small engines, and limited limited funds for troubleshooting.

Could someone give me their thoughts/comments on my thinking?

Here's what I know:

  • Spark plug was wet when I inspected it but has spark after changing it.
  • I don't smell any combustion
  • Gas might be a bit old (6-months to a year)

My conclusions / assumptions:

  • If spark plug is wet, this may rule out carb issues (please verify this logic).

  • If spark and fuel exist, this can only mean a weak spark (please verify this logic)

Before I buy an ignition coil or a tester, can someone help me understand anything else I may need to test or rule out?

Update: Upon further inspection, I discovered more. What seemed like dirt was actually a significant amount of oil buildup around the circumference of where the motor attaches to the body of the mower. I'm guessing oil leaks are bad news?

  • Welcome to Motor Vehicle Maintenance & Repair! If you pull the plug, leaving it attached to the lead, ground it so you can still see the electrodes, then pull the cord, you can then see if there's spark going on. If no spark, then you should consider the ignition coil. There are tests for these using a multimeter, but you'd need to know the exact engine model in order to do the right tests and know what the readings mean. Commented Dec 13, 2017 at 19:03
  • I second Paulster here. The spark shouldn't be too small; it will already be smaller in the cylinder, since fuel saturated air is harder to ionise to support a spark.
    – Bart
    Commented Dec 13, 2017 at 19:26

2 Answers 2


Will it run using starting fluid? If so, follow the following. If not, check compression on the engine before anything else.

  1. Change the spark plug - always a good thing to do at the start of a season (or during any major work)
  2. Change the gas for fresh gas.
  3. Check that the fuel flows freely when you remove the line from the carb inlet. If not, new lines are needed, and you may need to clean the fuel tank.
  4. Check the carb bowl - if there is varnish in there, the rest of the carb is probably plugged up as well. You can look for a carb service kit for that exact carb, or you can buy a replacement carb off Amazon - the prices are very similar for most common carbs.

You may find a service manual for the engine and/or carb online - they're great sources of information.


Your assumptions could very well be wrong. They could be, but aren't necessarily the causes of your problems.

As already mentioned, but i second it because it's important:

  1. Put new gas in there, old gas may have lost the easily vapourising components, making it harder to start.

  2. Clean the fuel line and float bowl. Check the jet orifices for blockages.


  1. Buy a can of brake cleaner, it'll fulfill all your needs here. It'll cost you only a buck or two, and it makes for an excellent detergent, degreaser and starting spray. It combusts and vapourises extremely well, so you can use it to get your engine going. Don't be too sparing when spraying in the carb for a start, or it won't help you. Spraying for a full 2 or 3 seconds should be enough for a reaction from a small engine. Pull the throttle while trying this, it'll increase the chances of success.

  2. Check the choke for proper working, the colder it is the more important that is. It constricts the air inlet and so creates a partial vacuum in the carb. This pulls more fuel so the engine will start easier when cold.

  3. Check the cylinder for leaks at the headgasket. Spray some soap on it so you can see any bubbles emerging if there is a leak.

  4. Check the ignition timing if you can. It's not probable that it's tuned wrong, but it could be. The spark must occur a few degrees before the piston hits top dead center on the compression stroke.

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