Air. Fuel. Spark. Compression.

“What is wrong with this thing?!” exclaimed my cousin for around the 20th time as we stared at his 400EX. “It runs like a scalded dog one minute, and then out of nowhere just loses power!” At this point it seemed like we had tried everything: changed out the fuel, installed a variety of carb jets, heck, he even borrowed a CDI box off of another bike to see if that was the problem. No dice. We were about to go insane.

At times like these, it’s nice to have a fresh perspective. Lucky for us, our fresh perspective’s name is “Uncle Roger.” More than once we have had to give him a call when we faced mechanical issues, and he never fails us. He has the uncanny ability to diagnose issues through a series of methodical tests he has developed over the years, and we knew he could help us out. 

We dialed his number, and after a couple rings he answered with a welcoming, “Hey guys! What can I do for you?” We gave him a quick overview of our problem along with the remedies we had tried to that point, and that is when he gave us a knowledge drop that I won’t soon forget: “Guys, always keep in mind the 4 basic components that internal combustion motors need to run: air, spark, fuel, and compression.”

This is a simple statement that speaks volumes, and it is an excellent framework for diagnosing internal combustion motor issues. Sometimes the complexity of all of the parts working together can be overwhelming, and this is exactly why thinking through individual, high level components of the combustion process is so helpful. Let’s run through a scenario:

Your bike has trouble starting, and if it does happen to run the motor misses and has little power. Where do we start?


The first thing to check is your air filter. Internal combustion engines need oxygen to burn the fuel, and a clogged air filter will not allow this process to happen. Paper air filters should be replaced entirely, and washable filters should be cleaned with filter cleaner, dried, and have filter oil reapplied.


Does the bike have fuel in it? Once again, seems simple, but it happens. If the bike does have fuel, is it old or does it have a low octane that the bike doesn’t like? It is also a good idea to check your fuel lines. Old crusty fuel lines won’t allow fuel to get to your carb, and this starves the bike. If your bike is fuel injected, make sure the fuel pump is working.


Is your kill switch in the ‘on’ position? (Don’t laugh, we’ve all been there haha.) Once you’re good there, check your spark plugs out to see if they need to be replaced. If your spark plugs look good, your cdi or coil may not be working properly, and these can be checked with ohmmeter.


Finally, the top end should be tight enough for the piston to compress the air and fuel before ignition. Use a compression tester to see if the motor makes and holds the compression it should.

Again, these are high level components, and, while they may not solve for every situation, they will at least give a structure to eventually get to the root cause of the problem.

Back to my cousin’s 400EX. After going through the framework, we found a loose electrical connection. Sometimes the connectors would make contact and the bike would run great, and other times the connection would break and cause the bike to lose all power. Firmly pressing the connectors together did the trick, and the frustration was finally over.

What are some issues you have had and how did you fix it? Let me know in the comments!

Back to blog

Leave a comment