This is part 1 of what will ultimately be a 4-part series dealing with basic troubleshooting of a two stroke motorcycle engine. This, of course, also applies to quads and much of it applies to any engine of any type, but will specifically cover two strokes while skipping over 4 stroke related things such as valve timing, etc.

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Two Stroke Troubleshooting in 4 Parts (Part 1)

by Ron Smith on 01/25/2017

Part 1 The fundamentals.

Diagnostic Procedure

Before we get to the proverbial (and actual) nuts and bolts of troubleshooting it is important to understand a bit about proper diagnostic procedure. Scores of examples are available of people who have spent ridiculous amounts of money trying to fix a simple problem because instead of following a systematic diagnostic plan they changed random parts in the hope that blind luck would lead them to a workable solution.

It is important to develop a plan and stick with it. The basic idea is you are identifying the things that are not your problem in a structured and orderly fashion. As you identify things that aren’t your problem you will develop an ever shrinking list of things that could be your problem. While you will usually identify your issue long before you get to the bottom of the list, eventually you will identify the problem because it will be the only thing left.

Experience and a firm grasp on the fundamentals will help you be able to narrow down your starting point more rapidly by evaluating the symptoms, but even without that by following a structured plan you will be able to correctly identify the problem(s) that are keeping you from riding.

Where to begin

First let’s cover some quick and dirty ways of narrowing the search. Parts 2,3 & 4 will cover each of the related systems in detail and how to troubleshoot them, but for now while these methods are not in themselves definitive they should help you to settle on a starting point.

Fundamentally an internal combustion engine only requires three things to run:

  1. Correctly Timed Spark
  2. Adequate Compression (Primary and Secondary in the case of a two stroke)
  3. Proper Fuel/Air Mixture

If you are presented with an engine that refuses to start one (or more) of these things is very likely the problem.  There are a few other random strange things that can prevent it from starting such as a plugged exhaust or rag in the intake that prevents it from breathing for instance, but those are few and far between.

Always start with the basics, once you are sure they are good, then look for the strange things.

We will cover each of these three areas individually, if you already know, for instance, that you have no spark you can simply jump to that part for the specific steps needed to diagnose that condition.


More often than not your problem is going to be ignition related and it happens to be the easiest to do an initial test on as well so let’s begin here.

Remove the existing spark plug and grab a fresh one (even if the one in the engine was new you want a clean, dry one for the test), insert the new plug into the cap and ground the base of the spark plug against something metal (a head bolt / nut works well for this). I don’t recommend holding it with your fingers as a motorcycle CDI ignition generates somewhere in excess of 80,000 volts and it may choose to use you as a ground. Trust me, it hurts!

Using your hand or foot, whichever is more convenient, kick the engine over, with the plug out it will spin easily. You should see a bright blue/white spark. If the spark is yellow or there is no spark at all go to the Ignition section and continue on.

Compression (Secondary)

Since you removed the spark plug in the previous step it is simple to do a down and dirty test of secondary compression (more on what is primary and secondary compression in that part.)

With the plug out of the way so you don’t get zapped by the ignition system put your thumb over the spark plug hole so that is completely covers it and kick the engine over again (you can use your other hand if you can reach it easily.) If everything is good you will find it all but impossible to hold your finger over the hole when the engine is turned over.

If you find that it is no problem to keep the spark plug hole blocked, you have a secondary compression problem. Get ready for a tear down and skip on to that section.


If you have a spark (and we will assume it is correctly timed at this point) and you have adequate compression (we are still going to gloss over the primary side for now) all you need is some fuel mixed with air and you are roosting!

Testing for this is a bit more challenging but there are a couple of methods you can use to perform a quick and dirty test using either raw fuel or (and I don’t recommend this) starting fluid.

The best way I have found is to use an old perfume bottle, put a little bit of premix into it and use it to squirt some fuel into the engine. You can squirt a little down the spark plug hole (since you presumably have the plug out from the first two steps), or alternatively you can hold the throttle wide open and give it a few squirts into the intake tract from inside the air box (if you choose to use starting fluid you can use this technique, but be aware that starting fluid is a great fast dry solvent and can wash the oil film off critical parts.)

If you have the spark plug out put it back in and give the engine a couple of good solid kicks. If it fires, however briefly you likely have a fuel system problem and should proceed to that section.


If you figured out which system is your problem it’s time to move on to the next step and determine what exactly the problem is so you can buy the parts you actually need and fix it.

If the quick test failed to pinpoint the issue, it is time to check a couple of the more unusual issues.

Pull the air filter off and make sure that you don’t have any blockage in the intake tract. You may find that missing rag you have been hunting for.

If that looks good, pull the carb and reed block off and inspect your reed valves. Look for broken, bent petals or ones that don’t fully close. Replace them if you find any problems.

Finally pull the exhaust system off and make sure it isn’t plugged or recently been the happy home of a rodent family.

If you still can’t find a problem start with the ignition troubleshooting as it is the most likely to have a problem. Keep working through the systems one at a time in a systematic manner. Be patient and persistent. You will eventually find the problem.

Next Up: Part 2 Ignition Systems.