What Causes Low Compression in an ATV and How to Fix it

Have you ever got on your ATV and tried to start it only to have the engine spin like crazy but not start.  Or, have you encountered your ATV losing power and performing more like a golf cart than your fun ride? 

You may have a a low compression problem!

What Causes Low Compression in an ATV? Low compression can be caused by several issues. Most of these problems stem from wear and tear and improper maintenance where low compression results from one of the following:

  • Bad or Broken Valves
  • Worn Piston Rings
  • Cracked Spark Plug
  • Bad head gasket

Things to rule out before your look at low compression

There are other things can cause similar issues to low compression. You probably want to look at these first to rule them out.

The problems we find more often than low compression that cause power loss include the folloing

  • a clog in your fuel filter clog
  • bad gas
  • not getting a spark

If you have ruled these common problems out and still have issues, it is time to check out your cylinder compression.

Have you checked your gauge?

Before we go any further, we should eliminate the obvious.  Have you checked your compression gauge against a known cylinder?  Pressure gauges are sensitive pieces of equipment and are susceptible to:

  • Dust
  • Liquids
  • Abuse

Before you take any other action, it is always good to make sure that your compression gauge is accurate and working properly.

Are you using a pressure gauge that threads into the sparkplug hole and fits tightly?  The types of pressure gauges that have the tapered rubber end that require you to hold the rubber plug to keep the seal just don’t work reliably so we recommend swapping them out if you have one.

There are a couple of ways to test the operation and accuracy of your compression gauge.

  • Test your gauge against a known compression source
  • Compare the readings of your gauge with another gauge on the same engine
  • Take your gauge to a professional that can test it and adjust it if necessary

Once you are satisfied that your compression gauge is working properly, you can begin diagnosing your compression problem.

The Usual Causes of Low Compression in an ATV

There are only a few things in an internal combustion engine that can lead to low compression in the cylinders.  Unfortunately, diagnosing which culprit is the cause of the low compression almost always requires disassembling the engine to some degree.  

Cracked Spark Plug

This one is usually easiest to diagnose.  You may notice that your ATV seems to have lost power or isn’t performing up to its usual levels.   When you get to the point of checking the compression levels, you find that there is no loss of pressure on any of the cylinders.  This can leave you scratching your head.

Before you grab your tools and start tearing into the engine, take a moment, and look at the spark plugs.  Even a hairline and almost microscopic crack in the ceramic part of a sparkplug can be enough to cause a loss of compression and a noticeable loss of performance.  

If your compression gauge confirms that you have good compression in all the cylinders, the first suspect should be the spark plugs.  The easiest fix is to replace all the spark plugs on your ATV engine and see if it fixes your problem.   

Spark Plugs for your ATV are cheap so this can be a good place to try and get lucky. For a few bucks, see if you can find the right spark plug on Amazon.

Bad or leaking Head Gasket

The gasket that fits between the heads of your engine cylinders and the engine block takes a lot of abuse.  They must endure:

  • Extreme temperatures
  • High pressures
  • Effects of compounds that work to weaken the gasket material  

The tiniest imperfection in a head gasket can be enough to seriously affect the performance of any engine, not just your ATV.

To check a head gasket leak, look for some of these common signs and symptoms:

  • Coolant leaking from the engine
  • White smoke from the exhaust.
  • Overheating
  • Water or oil that has a milky white color in the crankcase
  • Fouled spark plugs
  • A loss of compression

If one or more of these symptoms is found, you might have a bad head gasket.  While replacing a head gasket is possible for a weekend mechanic, it is not something that most people have either the tools or the skill to perform.   This is one of those repairs best left to the trained technician at the dealership if you are not confident in your abilities.

Bad or Broken Valves or Valve Springs

The valves on your four-stroke ATV engine control the flow of fuel enriched air and exhaust in and out of the cylinders. A valve, either intake or exhaust, that doesn’t seat properly, can prevent the cylinder from holding the proper compression. 

Likewise, a valve that is cracked or broken will prevent the piston from creating enough pressure in the cylinder to operate properly.  

The valves sit against the cylinder head, allowing the piston to compress the fuel-air mixture in the cylinder.  Many high-performance ATV engines are fitted with valve seats that aid in the sealing to allow higher compression in the cylinder.   Several things can go wrong with the whole valve assembly:

  • Valves can wear or become encrusted with carbon buildup that can prevent them from sealing tightly.
  • A valve can crack, allowing leaks to occur as the piston rises to compress the air-fuel mixture in the cylinder.
  • A valve can break, which will prevent any compression from happening in the cylinder.
  • A valve seat can fail.  This will not only prevent any compression in the cylinder, but pieces of the valve seat can fall into the cylinder resulting in more catastrophic damage to the engine.

Repairing or replacing bad valves or components of the valve train assembly requires removing the cylinder head from the engine block and then removing the valve or valves from the cylinder head. 

Most valve repairs require that the valve seats in the cylinder head be lapped or polished before new valves are installed.

This is a job beyond most amateur mechanics and should be left to the professionals with the right tools and training. There are several common ATV services that include valve repairs as well that may be worth looking into.

In this repair article, we looked at the prices you can expect to pay for a valve job and other repairs.

Worn Piston Rings

If you have noticed a loss of performance in your ATV and you are noticing blow-by from your exhaust, you may have a piston ring problem.  Blow-by is those puffs of blue smoke that happen when you first start your ATV.  

Piston ring wear is normal, even in the best-maintained engine.  In high-performance ATV engines, piston ring wear can happen quicker than you think, especially if maintenance protocols aren’t followed closely. 

Worn piston rings can cause several problems inside your ATV engine.

  • Combustion gases can get into the crankcase.  Hot combustion gases can cause the oil to break down, leading to lubrication issues.  Seals and gaskets can also be compromised as the pressurized hot combustion gases are leaked.
  • Too much oil gets into the cylinder.   Oil leaking by the worn rings can enter the combustion chamber.  This excess oil can foul the spark plug, create build up on the valves and valve seats, and interfere with the proper combustion of the fuel-air mixture, all of which can lead to a loss of compression.

If worn or broken piston rings are the culprit to blame for your loss of compression and performance, it’s time to take that ATV to the shop for some professional attention, installing new pistons or rings is a job that requires knowledge and specialized tools.  An engine rebuild of this complexity is usually expensive and not a job many of us would undertake.

Good Luck Moving Forward

We are bummed you are having ATV problems and hope we were able to help you figure out and fix the problem. Hopefully, you are back on the trails soon!

Brent Huntley

Brent Huntley is the owner of ATV Man and is responsible for almost all the material on the website. He also runs photographyandtravel.com and loves to travel and ride ATVs with his family. When he isn't playing, his day job consists of owning Huntley Law.

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