First Steps When Troubleshooting a Motorcycle

First Steps When Troubleshooting a Motorcycle

Remember, all a motorcycle engine needs is fuel, air (compression), and spark.

If it has those 3 things, it should run. It might not run well, but you’ll be able to tune it once you’re able to diagnose what’s going on.

First: Check the Compression

When troubleshooting a motorcycle, it’s always a good idea to begin with checking the compression in the cylinders.

Remove the spark plugs and hook up a compression gauge. Turn the engine over and check the readings. Proper compression may vary slightly by bike, but it should read around 100 PSI for it to run.

If the compression readings are too low, you’re going to have a hard time getting the motorcycle to start.

If the compression readings are good, move on to step 2 and check for spark.

Low Compression on a 4-Stroke Motorcycle Engine

When the compression readings are too low on a 4-stroke motorcycle engine, check the valve clearances to make sure there are within spec as per the shop manual. If the valves are too tight, you may be loosing compression.

If the motorcycle’s valves are within the proper clearance, you’ll likely need to rebuild the top end.


Low Compression on a 2-Stroke Motorcycle Engine

If the compression readings are too low on a 2-stroke motorcycle engine, the piston rings may not be sealing properly. You may need to rebuild the engine.

Second: Check the Spark

With a brand new spark plug, set the gap to at least 1/4″ and set the plug on the cylinder head to ground it.

Push the starter button or kick the kickstarter and watch the spark plug.

If you see a strong blue spark jump from the electrode, your spark it solid and you can move on to checking the fuel delivery.

If the spark is weak or there is no spark at all, check that the battery is fully charged and go through all the electrical connections. When the motorcycle has good spark and a full battery, you may need to inspect the ignition system itself (points or electronic).

Third: Check the Fuel

With good spark and compression, it’s time to troubleshoot the carburetors.

Any old motorcycle that has been sitting for a long time is likely to have some gummed-up or varnished carbs. If fuel delivery proves to be the issues, a thorough carb cleaning can usually solve the problem.

If the motorcycle engine starts, but will only continue running with the choke on, the idle jets are likely clogged.

When the motorcycle won’t start at all, then either the fuel needle/seat or the petcock is clogged shut or stuck. If you’re able to start the engine with a little bit of gasoline or starting fluid down the spark plug hole, there is mostly likely a problem somewhere along the fuel delivery line – from the petcock to the needle valve.

Once you can start it up, you’ll be ready to tune it to a roadworthy condition.