Troubleshooting a Motorcycle

There are all sorts of things that can cause your motorcycle to run poorly or not start at all.

This page will cover the common problems your motorcycle might have and how to troubleshoot and fix them.

My Motorcycle Won’t Start

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.

1. 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.

2. 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 is 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).

3. Check the Fuel Delivery

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.

Why Does My Motorcycle Backfire?

A backfire describe the loud bang or pop that comes from your motorcycle’s exhaust.

Motorcycle backfire can be caused by:

  • Improper timing.
  • Fuel mixture is too rich.
  • Fuel mixture is too lean.
  • Compression is too low.
  • Poor spark.
  • Improper exhaust pipes for the bike.

What are the Symptoms of a Bad Carburetor?

Your carbs likely need to be cleaned and tuned if you experience any of the following:

  • Rich conditions.
  • Lean conditions.
  • Backfires.
  • Sputtering.
  • Bike won’t start.
  • Loss of power during acceleration.
  • High ideal.
  • Gas leaking from the float bowl.

Why does my Motorcycle’s Oil Leak?

Oil leaks can be caused by:

  • Bad gaskets/uneven gasket surfaces (most common).
  • Loose engine bolts.
  • Worn engine case seals.
  • Oil pan drain plug.
  • Cracked engine case.

Why is my Throttle Sticking?

Your throttle is likely sticking due to either:

  • Improperly adjusted throttle cable.
  • Broken throttle cable.
  • Improperly lubricated throttle cable.

My Motorcycle Clicks, but Won’t Start

If your motorcycle is clicking when you hit the starter, but the bike won’t start:

  • Your battery might be dead (most common).
  • Your starter might be bad.
  • Your engine may be seized.

The first step will be to charge up your battery and try again.

What Happens if I Run out of Gas?

If you run out of gas, here’s what will typically happen:

  • The bike will begin to sputter.
  • You’ll lose power.
  • The bike won’t start.

If you’re lucky, you’ll still be able to switch the petcock, or fuel valve, to the “Reserve” position and get yourself to a gas station.

Why Does My Motorcycle Battery Keep Dying?

If your battery keeps dying, you may have:

  • An old/dead battery.
  • An undercharged battery.
  • Bad connections.
  • A bad stator.
  • A broken regulator/rectifier.

What Happens if my Engine is Flooded?

If you flood your motorcycle:

  • Wait before trying to start it again.
  • Turn off the fuel valve and the ignition.
  • Remove the spark plugs.
  • Crank the engine.

You’ll flood the engine if there is not adequate spark to ignite the fuel mixture. First, make sure the bike is on. Next, check your spark plugs for proper plug gap and a good spark.

My Motorcycle Stalls When I Accelerate

If your motorcycle stalls on acceleration, you may have:

  • Clogged or blocked carburetor passages.
  • Improper fuel level.
  • Improper timing.
  • Vacuum leak.
  • Broken throttle cable or spring.
  • Too rich or too lean a mixture.

My Motorcycle is Leaking Gas

If your motorcycle is leaking gas, here’s what you should check:

  • Fuel hose fitting and connections.
  • Floats, float needle, and float bowl gasket.
  • Float bowl height.
  • Petcock/fuel valve.
  • Gas tank.

My Motorcycle is Overheating

A motorcycle that overheats may be caused by:

  • Too high of an idle.
  • Too lean of a fuel/air mixture.
  • Improper coolant levels.
  • Too hot of temperatures with no moving (i.e. air-cooled engine stuck in traffic).

Is My Clutch Bad?

Clutch baskets that have been sitting for a long time tend to bind together and not function properly.

Your clutch might be bad if:

  • Your engine revs unexpectedly.
  • You gas mileage is bad.
  • The clutch lever is stuck.
  • Shifting is difficult, jerky, and loud.

Is My Bike Running Lean?

Running lean means there is too much air in the fuel/air mixture. Symptoms of a lean condition include:

  • Poor idle.
  • Poor acceleration.
  • High RPMs.
  • Higher heat from the exhaust.
  • White/colorless spark plugs.

Lean conditions can be fixed with a more restrictive air filter or adjusting or rejetting the carbs.

Is My Bike Running Rich?

Running rich means there is too much fuel in the fuel/air mixture. Symptoms of a rich condition include:

  • Black exhaust smoke.
  • Black or dark brown spark plugs.
  • Bogging.
  • Poor acceleration.
  • Flooded engine.

Rich conditions can be fixed with a less restrictive air filter, adjusting the carbs, or rejetting the carbs.

My Bike has a High Idle

A motorcycle with a high idle is typically caused by:

  • Carbs that are out-of-tune.
  • Idle screws out of adjustment.
  • Lean mixtures.
  • Vacuum leaks.
  • Stuck throttle or throttle cable.

Why is my Motorcycle Smoking?

Smoking can be caused by a number of issues include:

  • Lean mixture.
  • Rich mixture.
  • Leaking valves.
  • Worn pistons.
  • Oil leaks.
  • Electrical issues.

You can typically diagnose the cause of the smoke by it’s color and where it comes from:

  • Smoke coming from the engine cases may indicate an oil leak.
  • Smoke from anywhere other than the engine or exhaust may indicate an electrical issues.
  • Dark, grey, or blue smoke indicates too rich a mixture or burning oil.
  • White smoke may simply be normal condensation burning off, coolant leaking into the combustion chamber (sweet smell), or excess oil.

My Motorcycle’s Exhaust Pipes are Blued

If you notice that your motorcycle’s exhaust pipes are blue, it’s an indication of too much heat.

This can often be caused by a lean condition.

Why Does my Motorcycle Wobble at High Speed?

If your motorcycle wobbles at high speed you may have:

  • Worn tires.
  • Improper tire pressure.
  • Worn wheel bearings.
  • Worn steering bearings.
  • Loose axle nut.

If you experience this while you’re riding:

  • Ease off the throttle.
  • Hold the handlebars firmly.
  • Lean forward over your tanks.
  • Gradually slow until you regain control.

My Motorcycle Pulls to One Side

Your motorcycle may pull to one side if:

  • The tires are uneven.
  • Alignment between forks and handlebars is off.

My Motorcycle is Sputtering

If your motorcycle is sputtering or feels jerky, you may have:

  • A lean condition.
  • A vacuum leak.
  • No gas or improper fuel level.
  • Bad spark plugs.
  • Improper timing.

Do I have a Vacuum Leak?

A vacuum leak is caused by extra air entering the combustion chamber. Leaks are often caused by hard, cracked rubber carb/manifold boots or bad o-rings/washers.

Signs of a vacuum leak include:

  • Racing RPM.
  • Loss of power.
  • Dies with no choke.
  • Sporadic idle.

You can identify the exact source of a vacuum leak by running the engine and spraying some carb cleaner around the carbs and carb boots.

Are my Spark Plugs Bad?

Your spark plugs might be bad if your motorcycle:

  • Misfires.
  • Backfires.
  • Engine floods.

Spark plugs do deteriorate with age and should be replaced periodically.

Improper plug gap can also cause issues.

Fouled plugs or plugs that have been run too lean and have burnt out should be replaced as well.

Lean and rich conditions will wear down your spark plugs faster.

Generally, spark plugs should be replaced every 5 years.

My Motorcycle Engine is Seized

Your engine might be seized if the starter clicks, but doesn’t rotate the engine or if the kick starter feels stuck in place.

Engines can sieze:

  • If they’re oil starved.
  • If they’ve been sitting for too long.
  • If the oil is contaminated.

A seized engine warrants a full engine rebuild.

To free a seized piston, allow it to soak in penetrative lubricant or diesel fuel.

A piece of wood can be used to strike a stuck piston when the head is removed.

Cylinders may need to be bored and honed as a result of a seized engine.

My Bike Dies When the Choke is Off

If you’re motorcycle only runs with the choke on, it might be time to clean your carbs.

Motorcycles that will only run with the choke on typically indicate a clogged pilot jet.

There is a Knocking Noise in my Engine

Engine knocking is typically caused by one of the following issues:

  • Improper timing.
  • Warped cylinders.
  • Worn bearings and guides.
  • Valve-piston contact.
  • Broken transmission.

If your engine is knocking, perform a full tune up and see if it fixes the problem. If not, you’ll need to take the engine apart to find the problem.

Why is my Motorcycle Vibrating?

Unexpected vibrations may be caused by:

  • Uneven tires.
  • Sticking brakes.
  • Bent rim.
  • Improper timing.
  • Worn bearings.
  • Worn pistons or cylinders.
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