Diagnosing a bike that won’t start can seem overwhelming at first glance. There are so many components that could potentially fail. Where do you even start?
That said, diagnosing a no-start can actually be a straightforward process once you know how an engine runs, and better yet, have an idea of the bike’s history. From there, it’s a matter of brainstorming possible scenarios, determining how likely each one is and testing your theories to see which one is right. If you can get this down, I assure you that you’ll spend more time fixing the problem and getting your bike on the trails, as opposed to spending time sitting on your crate, scratching your head and wondering what the hell to do next.
Determine the History of the Bike
The first step to diagnosing a no-start is to get some history on the bike. This will help you to determine what problems your bike is more likely to have (over others). For example, you might ask yourself questions such as:
- Did the bike sit for a long period of time? Did it run before it was parked? Or did it start/run fine yesterday, but today it won’t start?
- Has something on the bike just been repaired or replaced?
- Was the bike laid down recently?
- Has the bike been making funny noises or running differently than usual (before it broke down)?
- Was it hot/cold outside? Was the bike hot/cold when it stopped running?
- Did you run the bike through (deep) water or mud?
Write down anything that you can think of (or are told in case of a customer situation). This will help you to come up with theories to test.
Coming Up With Theories
Once you have some history on the bike, you can then start to come up with theories as to why it won’t start. It’s slightly difficult to read, but Dan has a really good approach to it. Take the history and apply percentages to how likely it is that the problem is either electrical, mechanical or fuel related.
For example, say someone dropped off a bike that won’t start. The current month is March and the owner said the last time his dirt bike started was in September. He says that the bike started and ran fine, and nothing was done to bike after he was done riding it. It has sat since.
Knowing this, we can apply the following percentages:
- Mechanical (5%): The bike started and ran fine before it was parked. It’s unlikely that the piston and rings are bad.
- Electrical (5%): Same reason as above. Everything electrical worked fine when the bike was parked, so while possible, it’s unlikely that the main cause has anything to do with an electrical component.
- Fuel (90%): The bike ran when parked, but the customer mentioned that he didn’t do anything with the bike before it sat. So there is probably bad fuel in the tank and carb(s).
Taking these percentages, you’d come up with the theory that the problem is highly likely to be fuel related. This is a straightforward (easy) example, but hopefully you get what I’m trying to show you. You simply take the facts/history and apply your knowledge of how a motor works to come up with the most likely scenarios as to why it won’t start. Once you have your theories in place, you can then go on to proving your theories right or wrong.
This is a great approach because it’ll save you time from trying to find a problem in the unlikeliest of places. In other words, it doesn’t make a lot of sense to worry about compression, or worse, to start tearing down a top end when it’s more likely that you have a dirty carburetor. This is especially important if you’re working flat rate for a shop, or trying to maximize your hourly as an independent mechanic.
Understanding the Basics of an Engine
At it’s most basic, all an engine needs to run is fuel, spark and compression. Knowing this, and combined with the history of the bike and your theories as to what could be wrong, you would come up with a list of components to check. I’ve listed the basics to help you get started.
Checking for fuel.
Here are some things to look for when checking for fuel.
Check to see if the carburetor is getting fuel. You can do this by cracking open the overflow at the bottom of the float bowl. If fuel runs out, you know that fuel is getting to the carb. If it’s not, is your fuel (at the petcock) turned on? Stupidly simple, I know, but it is possible to forget. If it is, it’s possible your needle and seat are stuck. You can tap on the side of the float bowl to see if you can dislodge it.
If your carburetor is getting fuel, then you might open it up to see if there is residue or bad fuel. This can clog up your jets which can keep your bike from starting, especially the pilot jet. One way to determine whether the pilot jet is plugged or not is to start and run the bike with the choke on. If the bike dies when you turn the choke down (or off), then it’s very likely your pilot jet is clogged.
Checking for spark.
To check the spark what you need to do is pull the spark plug out, leave it in the cap and ground it to something (the motor). Have someone kick the motor over and see if the plug sparks. It should be bright blue and should be able to spark over a 1/4″ gap.
If you’re not getting spark, is the spark plug fouled (oily or coated with residue)? Is the cap on tight (on the wire)? If so, you then would want to test your coil. There are tools to do this, or you can just take your coil to a shop. If it’s not your coil, then it could be your pickup coil (on the flywheel) or CDI box (on new bikes). CDI boxes are difficult to test accurately, and they’re expensive to buy new, so be sure that’s your last resort.
Another possibility is your kill switch. To test this, simply unplug it and loop the connection (if you don’t loop it, it’s like the switch being turned off). Then check for spark again.
Checking for compression.
Checking the motor for compression is straightforward. Remove your spark plug and hook up your gauge. Open the throttle and kick the motor over until the dial stops moving. As a rule of thumb your cylinder should have 100psi, although some bikes can start with less and some will be higher if they’re high performance parts. But as a rule of thumb, 100psi is fine.
If you don’t have enough compression, or none at all, what you’re facing is a seized top end. At the very least you’ll want to have your cylinder bored out, honed and then reinstall it with a new piston and rings.
The ideas I gave you above should be more than enough to try in your no-start situation. However, that’s only half the battle. Once you determine what the problem is, you ought to figure out what caused the problem in the first place. Sometimes this will be simple, such as my example above with the dirty carb. The initial problem was letting the bike sit. What about a top end seizing? Did you run out of oil or maybe your valve clearances were off? Whatever the reason may be, the point is that if you don’t determine the cause of the no-start, there’s a good chance that you’ll find yourself in the same ‘no-start’ situation in the near future.
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