Will an ATV Run With Low Compression?


Have you been experiencing trouble starting your ATV’s engine? How about some weird noises coming from the engine? If you answered yes, your ATV could have low compression. At times, the ATV might run well for a while, then suddenly quit or fail to speed up as much as it should. All of this can be quite annoying since it could mean time away from your beloved trails.

An ATV will run with low compression so long as the engine’s compression is at a minimum of 100 PSI. Depending on the engine, most ATVs have a compression ranging between 100 and 250, while others could go up to 300 PSI. However, an ATV whose compression falls below 100 PSI will not run well.

In this article, you’ll discover whether an ATV can run with low compression. You’ll also find more information related to the topic, such as:

  • Signs of ATV low compression
  • Causes of ATV low compression
  • Ways to fix low compression
  • How to test ATV engine compression

ATV Low Compression: The Signs

When your ATV has low compression, you may see or experience some of the following issues:

  • Low engine power
  • Noisy sounds coming from the engine
  • Problems when trying to start up your ATV
  • The engine might have intake valve leaks
  • The spark plugs might have oil residue – they should be completely dry

ATV Low Compression: Causes and Fixes

Low compression leads to loss of power or poor performance in your ATV. In most cases, problems arise either from wear and tear or the improper maintenance of certain parts in the internal combustion engine. As such, low compression in your cylinders could result from any one of the below issues:

  • Broken valves
  • Bad head gasket
  • Worn out piston rings
  • A cracked spark plug

Other Possible Causes

But that’s not all; other problems such as bad gas, a clogged fuel filter clog, or failure to get a spark could also cause power loss issues. So, you may need to rule these other culprits out first. However, if your low power issues persist, the first thing you may want to check out is the cylinder compression.

But first things first. Is the pressure gauge working correctly? If it is, run a compression test to try and diagnose the engine problems. When running this test, remember to warm up the engine first and have the throttle wide open.

A pressure gauge can fail to work properly if affected by dust, liquids, or improper care. It’s also important to point out that some pressure gauges might be unreliable. Such gauges bear tapered rubber ends, so you may need to hold on to the rubber plug to retain the seal. The best type of pressure gauge to use is one that threads perfectly into the spark plug hole, providing a tight fit.

To ensure that you’re using proper working equipment, test the accuracy of your pressure gauge by:

  • Testing it against a recognized compression source
  • Comparing the gauge readings against an alternate gauge
  • Taking the gauge for a professional test and adjusting if necessary

Check out this video to see how to do a compression test:

Now, let’s explore the common causes of low compression a little further. One thing to note here is that pinpointing the low compression’s exact cause usually entails disassembling the engine.

Broken Valves

In an ATV engine, intake and exhaust valves help control the flow of fuel-rich air and exhaust into and out of the cylinders. As such, an improperly seated valve could hamper the cylinder’s ability to hold the correct compression. In addition, a cracked or broken valve can cause the piston to create inadequate pressure in the cylinder, thus failing to operate properly.

Other valve impairments include:

  • Worn out valves that fail to seal tight (could be intake valves as they wear out fast)
  • Cracked valves that allow air-fuel leaks
  • Too tight valves due to wear on the valve train
  • Valves that are out of adjustment (no gap between the cam and valve bucket)
  • Broken valves that prevent compression from occurring in the first place
  • Failed valve seats that prevent cylinder compression and whose pieces could fall inside the cylinder and damage the engine

Tight valves can mess up the valves and seat pretty fast. You see, whenever the engine fires, hot combustion gases get pushed back via the partially open crack in the tight valve. If not closed, these gases can erode the valve and seat incredibly fast. The valve can only cool down by returning to its seat momentarily and sinking off some heat. But since it’s not closing completely, it doesn’t get to cool off.

Performing valve adjustments can help sort out a noisy engine, but you need to ensure that you don’t make the valve clearance too small. When hot, the valves will not sit, which isn’t ideal for exhaust valves.

To repair or even replace spoilt valves or valve train assembly components, you need to remove the engine block’s cylinder head, followed by removing the valves from the cylinder head. But since the valve seats in the cylinder head need lapping or polishing before installing the new valves, it’s best to have a professional with the right tools perform the job.

A Leaking Head Gasket

Head gaskets take a lot of abuse given that they endure high pressures, extreme temperatures, and compounds that weaken the gasket material. What’s more, even the tiniest imperfections in a head gasket are enough to hamper engine performance. In case of a head gasket leak, you’ll notice:

  • Overheating
  • Compression loss
  • Fouled spark plugs
  • Leaking coolant (from the engine)
  • White smoke emanating from the exhaust

Any one of the above symptoms points to a bad head gasket. However, replacing a head gasket calls for unique skills, so it’s best to have a trained technician do this for you.

Worn Out Piston Rings

A loss of ATV performance accompanied by exhaust blow-by (smoke puffs) might indicate a problem with the piston rings. Piston rings tend to wear out but more so in high-performance ATV engines. Unfortunately, they can cause engine problems since:

  • Combustion gases can enter into the crankcase, cause lubrication issues, and compromise seals and gaskets. 
  • Excess oil can enter the cylinder and lead to compression loss through soiling the spark plugs, forming valve build-up, and obstructing proper combustion of the fuel and air mixture.

If your issue is well-worn or broken piston rings, take your ATV to the dealer for professional repairs as this requires an engine rebuild.

A Cracked Spark Plug

A hairline crack in a spark plug’s ceramic component can lead to low compression or lackluster performance. But if there’s no pressure loss in the compression cylinders, the most likely culprit could be the spark plugs. To fix the problem, the right thing to do is to replace all the engine spark plugs. 

Below is a video explaining more on how to go about diagnosing low compression:

Final Thoughts

An ATV with low compression shouldn’t cause you too much worry – now that you know how to diagnose and sort the problem. However, if you’re not very confident in your mechanical skills, it’s best to take your ATV to your local dealer for any professional repairs. Also, it’s advisable to conduct bi-weekly or monthly compression tests so that you’re always in the know regarding whatever’s going on inside your ATV engine.

Rob

That's me sinking another ATV. I love to ride no matter what it is, snowmobiles, four wheelers, dirt bikes, and anything else off-roading. I've experienced my fair share of machines, and like to share that experience here.

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