Why and How to Provide Regular Compression Tests on Your Small Engine

In our last blog post on DIY New Year’s Resolutions, we emphasized the importance of performing regular checks on your small engine, just as you would a car engine. This includes oil checks and changes, checking the ignition system and ignition coil, and performing compression tests. As promised, here’s our run-down of the why’s and how’s of compression testing. It may seem daunting if you’re new to performing your own maintenance, but the compression test is one of our favorite “essential” tests for two reasons:

1. It’s of the utmost importance, and
 2. It’s easy!

So, let’s start with the basics.

What is engine compression testing? 

In short, this is the first question mechanics ask when they’re brainstorming with other mechanics about why an engine isn’t running: “Does it have good compression?” An engine requires a certain amount of compression to drive the piston and crankcase. This test, which involves checking for leaks in essential components like around the piston or valve, will give you an immediate picture of whether each part is wearing evenly. Any problematic part can then be replaced so your cylinder can do its job properly.

When should you check your small engine’s compression?

As mentioned above, if your engine isn’t running, compression is the first thing you should check. However, there are other, less catastrophic signs that it’s time for a test. If you start your engine and find that it’s spinning unusually fast, that’s a tell-tale sign. Other signs include loss of power under load and erratic idling. Common causes of this occurance could include a blown head gasket or a stuck/bent valve. Even if you’re not experiencing any causes for concern, regular compression testing is the key to proactive maintenance.

Here’s how you do can perform your own compression test:

1. Obtain a gasoline engine compression tester that is specifically suited for the small engines used in chainsaws, cut-off saws, and lawnmovers. Ours is fitted with a 65 millimeter gauge and provides two scales of reading, psi and kPa.

 2. Access your spark plug by unscrewing the cylinder cover on your engine and gently slipping a flat-tip screwdriver under the rubber boot, maneuvering as necessary until you can get the rubber boot off the spark plug.

 3. Loosen the spark plug with a socket wrench and remove it, then set the choke to “off.”

4. Make sure there’s no remaining fuel in the cylinder or crankcase by cranking on the starter cord a few times.

5. Insert your compression reader’s hose into the spark plug hole. Pull on the starter until the compression gauge needle hits its maximum – it usually takes about 10 to 15 pulls. As a general rule of thumb, compression shoud hit at least 90 PSI if it’s hot or 100 if it’s cold, but check your engine manual; some engines require anywhere from 80 to 150 PSI. These readings are such great indicators of performance because if there’s a leak somewhere in the engine, there will be a drop in compression. If you’re more of a visual learner, check out this great video that details a lawnmower compression test:

Now what do I do?

If you noticed a poor compression reading, it’s time to do some trouble-shooting and repairs. If you have the expertise you can do this yourself, and we have all the parts and tools you’ll need. But when in doubt, take it to a mechanic; it’s not worth breaking your engine further.

And that’s it! You’ve just saved money on performing your own compression test, while keeping your engine happy and healthy in the process.

Here's a good piece on choosing lubricant for small engines, if ya need Lubricant.