Dirt Bike Start With Low Compression? Mechanic’s advice …


There may be many reasons why a dirt bike won’t start, and questioning engine compression is right up there on the list. Engine compression issues of course as you know can be serious, but I’ve seen a ton of compression issues in my workshop and they don’t all spell bad news. I’m a mechanic for more than twenty-five years and in this post, we’ll get your dirt bike figured out.

A dirt bike will not start with low compression. Three common causes of low dirt bike compression include:

  1. Worn piston rings, damaged bore
  2. Head gasket failure
  3. Valve seat fault

In this post, you’ll learn how to quickly troubleshoot a no-start dirt bike from top-down. You’ll learn the symptoms of low compression, how to check compression including a few mechanics hacks.

Dirt bike won't start on track

Troubleshooting No Start Dirt Bike

A dirt bike requires three systems to work in harmony in order for the engine to start. It needs a healthy fuel supply, a spark to ignite that fuel and it requires a cylinder capable of being compressed. When these systems work together we get rotational crankshaft movement we can harness and convert into linear movement.

When a dirt bike won’t start, one or more of the systems is at fault. Troubleshooting is the process of finding the root cause. If your dirt bike is a two-stroke engine it won’t have valves so that section below won’t apply.

Understanding a little about each system will help speed up the process.

1 Dirt Bike Fuel

Fuel isn’t gas alone, fuel in this sense is air (oxygen) and gas combined. Since gas won’t burn without oxygen, both ingredients are equally important. That said they are not equally proportioned in the fuel mix. Engines have a preference for 14.7 parts air to 1 part gas. This is referred to as the AFR (Air Fuel Ratio).

And as said, an engine is very particular to this ratio. Two problem AFR conditions are common, they are:

i Lean (Skinny) – Too much air or not enough gas in the mix is known as running lean

ii Rich (Fat) – Not enough air or too much gas in the mix is known as running rich

The symptoms of both these conditions largely differ (no-start is on both lists), and as you can imagine, vary in severity proportional to how out of shape the AFR is.

An AFR may be off for several reasons, some are easy to fix and some aren’t. The gas shot test below will go some way towards diagnosing a fueling issue.

Note – Stale gas is a common cause of small engine no starts. If your bike has been lying idle for a couple of months, it’s likely the gas is stale. Draining the gas and refilling will solve the problem. Using a gas stabilizer will keep gas fresh for up to twelve months.

2 Dirt Bike Spark

Hey, you know the deal here. The spark plug needs to produce a strong spark and crucially at the correct time. A dirt bike ignition system employs the following components:

  • Spark plug
  • Coil cap
  • Coil wire
  • Coil
  • Alternator
  • Pick-up
  • CDI
  • Kill switch

Any of these components are capable of causing a no-start and some are a common cause of trouble. The gas shot test below will go some way towards diagnosing an ignition issue.

3 Dirt Bike Compression

And so we get to compression. Compression is as you know crucial, but what is compression? Without getting into the weeds here, compression is an engine’s ability to squash (compress) a cylinder’s contents, (fuel-air plus gas) into the cylinder’s combustion chamber (area at top of the cylinder) without that compression escaping.

When the compression escapes, compression is said to be low. And of course, there may be many reasons for the loss of compressions, some are bad news for the engine but not all. And we’ll cover that a little later. But first, we’ll run a mechanics hack – the gas shot test.

Dirt Bike Gas Shot Test

The gas shot test is fast and simple to perform, that’s why mechanics perform it regularly when faced with a no start. This test isn’t conclusive but it does usually point to the system which has failed – Fuel system, Ignition system, or Engine compression.

Note: this test must be performed with fresh gas, old gas goes stale, and using the same to diagnose will lead to incorrect conclusions.

And here it is:

  • Remove spark plug
  • Add small amount of gas (about oil cap full) (gas must be fresh)
  • Refit spark plug
  • Attempt to start engine

Top tip – A funnel or syringe makes adding gas to the cylinder easier.

With the test out of the way now we’ll need to analyze the results, and the two results likely, are as follows:

1 – The engine started, or fired (fired – meaning attempted to start). Firing identified as the engine not running but smoke seen at the tailpipe when cranking.

This result tells us that your dirt bike suffers from a fuel issue. Bad gas, choke not working, Faulty fuel pump (if fitted), blocked gas filter, carburetor fault are high on the list.

2 – The engine doesn’t start and critically makes no attempt to fire. This result tells us that you may have an ignition system fault or indeed a compression issue but an engine timing issue is possible too.

Generally, a lack of spark is a more common no-start cause than a lack of compression. I’ve covered checking spark previously and you can check it out here.

Symptoms Of Low Dirt Bike Compression

The symptoms of low compression vary depending upon how low compression is and those symptoms often change as the bike warms (assuming a bike runs). And so here’s a list of the most common symptoms, some of which you may recognize:

  • Engine no-start
  • High oil consumption
  • Blue smoke at tail pipe when running
  • Wet engine
  • Oil leaks under the bike
  • Back pressure seen at the oil filler with bike running
  • Engine misfiring
  • Oily plug
  • Engine flooding
  • Carbon on piston
  • Oily tail pipe
  • Overheating engine
  • Engine

How To Check Dirt Bike Compression

Checking compression is a definitive way to check if we have a mechanical issue. Obviously, we’ll need some tools and a compression test kit.

We have another option, a Leak-down test kit will work also and in many ways is a faster way to find a compression issue. I’ll detail testing using both types of test equipment and what their results mean.

If you don’t have a compression tester, check out the test procedure below.

Not all low compression issues are serious, a faulty spark plug that isn’t sealing the cylinder for example is an easy fix.

Checking Dirt Bike Compression MacGyver Style

This test method isn’t scientific or very accurate, it’s loosely accurate, and that’s why I call it the MacGyver compression test. Anyhow if you don’t have the proper test kit, give this try.

Test as follows:

  • Remove spark plug
  • Ground plug against cylinder head or use a jumper cable to remotely ground. An un-grounded coil wire will give you a jolt and is also bad for the ignition system.
  • Place your thumb over the the open plug hole (important to wear rubber glove)
  • Have helper crank engine

Two results are likely:

1 You’ve got some compression – You feel the effects of good compression on your thumb (strong vacuum and then strong propulsion). I go ahead and check the engine timing now.

2 You have no compression – You feel the effects of no compression as your thumb will feel mild or no effects of vacuum and propulsion as the engine is cranked over.

A lack of compression will likely require the aforementioned Leak-down test kit to diagnose the root cause.

Checking Dirt Bike Compression Like a Pro

In this test we’ll use a regular compression tester, you’ll need a plug wrench. This is a two-step test. We’ll run a Dry test first and then run a Wet test, after which we’ll compare results and diagnose. It’s best to run a compression test on a warm engine, but if your engine is a no runner you’ll need to test cold.

Dry test as follows:

  • Remove spark plug
  • Ground coil
  • Fit compression tester
  • Choke off
  • Throttle wide open
  • Crank over 6 – 10 times
  • Read and note the highest reading

Wet test as follows:

  • Add a cap full of engine oil to the cylinder
  • Repeat the dry test as above
  • Read and note the highest readings

Next, we’ll look at what the different readings mean.

What Dirt Bike Compression Results Mean

Comparing the wet and dry measurements means we can determine where if any, the compression problem lies – in the bottom end or in the top end of the engine.

Diagnosis of results as follows:

Readings are the same – If both dry and wet compression readings are unchanged (or largely unchanged) and the compression readings are below spec (as per your engine maker), then it is likely you have a top-end issue.

Readings are different – If on the other hand, the readings are different meaning the wet compression readings are significantly higher. Then it is likely your engine suffers from worn rings or piston or cylinder or combo of all three.

If your readings test above 100 psi and wet and dry tests are similar, congrats! Low compression isn’t your issue. I would now go ahead and check engine timing.

What Should A Dirt Bike Compression Read?

A reading between 95-115 psi is considered normal for most bikes, but it does vary between engine makers so I advise you to check your make and model. A reading below 95 psi means your bike may indeed have an issue.

What Dirt Bike Leak-down Results Mean

The Leak-down test won’t suit everybody as it means not only will you need a Leak-down tester but you’ll need a compressor also. That said, a leak-down tester is likely a faster and more conclusive way to both test and pinpoint the root cause.

In this test, we’ll fill the cylinder with compressed air and we’ll use the tool to measure the cylinder’s ability to hold pressure over a given time period.

But the real bonus of the Leak-down tester is its ability to point to the area at fault. In a problem engine, the compressed air will be heard to leak from the cylinder.

Diagnosis as follows

Air leaking from:

  • Oil cap or dipstick – worn piston rings
  • From muffler – Exhaust valve issue (worn seat or adjustment)
  • From carburetor /air box – Intake valve issue (worn seat or adjustment)
  • Between cylinder head and block – Head gasket failure

You may find the following posts helpful:

How do dirt bike kill switches work?

Will dirt bike start without kill switch?

How to start a flooded dirt bike?

Dirt bike won’t start when hot

Dirt bike not starting after winter

John Cunningham

John Cunningham is a certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty-five years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance & repair to troubleshooting.

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