You stop for gas on your way to work.
You pump the fuel, run inside for a coffee and muffin, scarf it down and jump back on the bike, but now that it’s hot, it won’t start.
It started when it was cold this morning, but why won’t your motorcycle start when hot?
Numerous electrical components could be the culprit. As there are a few other potential parts that’ll impair the ignition process, we’ve put together this list of X Reasons a motorcycle won’t hot start.
Here is the short answer to why a motorcycle won’t start when hot:
If a motorcycle won’t hot start, it may be due to a weakness or failure with an electrical component. As electronics become hot, resistance accumulates; and a faulty electrical component will have a harder time functioning when it’s hot.
1. Weak Battery
A weak battery may have had enough voltage to start the motorcycle when the bike was still cold, but even modern stators need at least 15 minutes to charge the battery.
There are a few reasons a battery might die. If the bike sits inactive for an extended period without a battery tender hooked up, your battery capacity will plummet.
Your bike undergoes a process called parasitic drain. Many modern motos use CPUs, and the units sap bits of energy from the battery while the bike is off to keep themselves functional. Or maybe there’s corrosion in the ground wires. Regardless, if you’re not riding and you’re not tendering, your battery isn’t charging.
You may have had enough juice to get you started, but were you on the bike for at least 15 minutes (or much longer on a bike with an older charging system)?
If not, you may have killed the last of the juice you had on ignition.
Check your battery with a voltmeter. If it’s just dead, give it a charge. If it’s all the way bad, it won’t hold a charge, and it’ll need to be replaced.
2. Tight Valve Clearance
If your valve clearance is smaller than the spec outlined in the service manual of your motorcycle, your bike will fail to start when hot.
The valve-metal expands when it gets hot. Therefore, if your motorcycle’s valve clearances are less than spec, there’ll be even less clearance once the bike gets hot.
If the valve expands enough to tighten up and hold itself open, you’ll lose engine compression, and the engine won’t start.
Valve adjustments are a bike-specific process to be done in adherence with service manual specs and feeler gauges. If you don’t have the tools and know-how, it might be good to get professional help on this one.
3. Worn Spark Plugs
Once a spark plug dies, a motorcycle will crank over but never startup. If the spark plug isn’t completely dead, it might fire up when you first hop-on, only to weaken from engine heat during the ride and fail on your next hot start.
A spark plug that’s at the end of its life causes a bike to run more poorly than usual.
Heat can also crack an old, worn, and torn spark plug.
Once cracked, the spark recognizes the crack as the path of least resistance and grounds out through the engine block before it travels to the gap and ignites the motor.
4. Loose or Cracked Spark Plug Wires
If your bike started cold just fine, and your spark plug wire loosens or cracks while you ‘re riding, your motorcycle won’t hot start or ignite at all until you replace the wires.
A loose spark plug boot is an easy fix; slip the boot back onto the plug until the connection is secure.
A cracked spark plug wire is another story. On a long rough timeline, engine heat can wear down the wire-rubber, and today’s ride may have been the rubber wire straw that broke the moto camel’s back.
If the heat of your ride fried your spark plug wires, your bike wouldn’t start again until they’ve been replaced, and you may as well replace them all at once.
5. Bad Ignition Coil
A worn ignition coil won’t spark when it’s hot, and therefore the motorcycle won’t fire up. If your motorcycle won’t hot start, carefully touch the ignition coil; if it’s too hot to leave your finger on, a bad ignition coil could be the problem.
Your bike might turn over fine when it’s cold, but it builds resistance as the coil heats up with the resonating engine heat. When you come back for a hot start after being in the saddle, the coil will be too weak to support the spark.
Heat can also degrade the coil’s windings. It’s possible that your coil was still ok at the cold start, but the heat of the ride finally did your old coil in.
Replacing an ignition coil is an easy job for anyone with basic moto-mechanic skills and a quick-fix at any bike repair shop. Just test out your coil with a voltmeter to ensure that’s the culprit before you go through the trouble of replacing it.
6. Slipper Spring Grounds Points when Hot
If your motorcycle has points for older bikes, a worn slipper spring could interfere with your hot starts.
As the bike heats, the spring expands, arching between the slipper spring on the points and a nearby screw, and the arch grounds out the points before they transfer energy to the plugs.
Replacing the spring or rigging it, so it doesn’t ground out the charge, could be enough to fix a failing hot start.
7. Weak or Failing Charging System
A weak stator can stop functioning once the bike gets hot, leaving your battery drained below the voltage necessary to hot start your motorcycle.
As mentioned earlier, electronics get hot and quit working. When you go to hot start your bike back up, your battery might not have the juice it needs to turn over.
For example, if the stator cracks, the engine may expand the metal and widen the crack enough to fry the stator.
To make sure it’s the stator and not your battery, use a voltmeter to test the stator directly.
Knock out what we refer to as a “static stator” test first.
- Flick off the ignition switch, and keep the bike off.
- Disconnect your stator from the Regulator/Rectifier.
- Set your multimeter to “Ohms” (or Resistance), as low as the scale goes.
- Stick a probe into one of the pin sockets on the stator and ground the other probe somewhere onto the chassis of your motorcycle.
- Your display should read either with an infinity symbol or the word “open,” indicating an open circuit. If it reads anything else and your circuit is grounded, your stator is fried, and it’s time to swap a new one onto your motorcycle.
If the battery and the above stator test check out fine, the problem could be heat damage to the rotor that spins around the stator. If the motor overheats and fails, it’ll affect your stator’s output. With the stator still unhooked from the Regulator, perform the following dynamic output test:
- For this one, you want the bike’s motor running. Set the multimeter to AC voltage and hook the probes up to the stator sockets.
- Hit the throttle until your engine revs at 3,000 RPMs.
- Check the meter reading. If it’s less than 60 volts, your rotor is shot and needs to be swapped out for a new one.
If the stator passes both tests, it could be an issue with the Regulator/Rectifier:
- Kill the bike, flick off the ignition switch, and hook the stator back up to the Regulator. Set your multimeter to check amps on the low-scale setting.
- Start the motorcycle and make sure all electronics are flipped on—every accessory.
- Uninstall the negative battery terminal and connect the meter probes to the negative battery post and the negative cables.
- Check your reading; if it’s less than four AMPs, the Regulator/Rectifier is your culprit and needs to be replaced.
8. Defective Starter
If your bike has an electric starter, a defective starter could be building resistance with heat and stopping your motorcycle from starting when hot.
A defective starter is easily diagnosed by the sound it makes while your bike isn’t starting.
For example, a high-pitched buzzing noise that lets you know the gears aren’t even engaging indicates a bad starter. Or you may hear a clicking sound or no sound even though you’ve tested the battery, and the battery is charged (told you to check the battery first).
All motor parts wear out eventually, and your starter is a complex working little motor on its own. Engine heat can wear out its wires or expand its components into an already tight space. If your starter is the culprit, the only feasible solution is to buy your bike a new starter.
9. Failing CDI Box
A faulty CDI may work alright when the bike is cold, but like all the other electronics mentioned, a weakening CDI will build resistance as its temperature rises and may fail to start when the bike is hot.
What Is CDI?
A Capacitor Discharge Ignition, or CDI, is like a battery because it deposits energy. The CDI box has the potential to discharge its stored energy almost instantly.
So putting a CDI on an ignition circuit is a smart move. You’ll often see it as a TCI system, but all CDIs use similar fundamental parts—a coil, a sensor, and a box.
Troubleshooting a CDI system on a Motorcycle that Won’t Hot Start
If your bike starts when cold but not hot, and the other electrical components all seem to read fine, think about any other unconventional behaviors your bike has exhibited lately:
- Bizarre tach behavior,
- And dead cylinders all related to how smooth the motor runs are all symptoms of a failing CDI.
If it gets bad enough, you may notice your bike wouldn’t low rev without dying.
CDIs tend to withhold heat a little better than some of the other components, so check the other things on this list first.
The CDI box isn’t indestructible, though, and if you have an early CDI-equipped motorcycle, it may be time to look at replacing your CDI box.
Eliminate all other variables first, though, because this is the most annoying troubleshoot on the list. And remember, you can always take your motorcycle to a trusted mechanic for a job that affects your bike’s computer unit, and messing with your CDI definitely does that.
At the end of the day, there’s no easy way to test a failing CDI unless your bike’s OEM gave you pin-by-pin CDI testing specs. If you suspect your CDI is bad, and every electrical component thus far has checked out, just cut your losses and replace the whole CDI box with a fresh one.