Yamaha Dt 125 Engine Diagram
The long-running Yamaha DT 125 motorcycle is a charming option for anyone looking to buy a no-frills, rock-solid off–road trail bike.
Compared to modern dual-sport bikes, the DT 125 is simple and down-to-earth.
No bike is perfect, however, but problems are easier to manage when you know what to expect, and this article aims to help you get ahead of some of the issues DT 125 owners have had in the past.
Based on our analysis of multiple customer complaints, here are the most common problems associated with Yamaha DT 125s and how they were solved:
1. Erratic Engine Performance
The engine is the heart and soul of a motorcycle, and nothing inhibits a bike's performance quicker than engine issues.
We've encountered a few Yamaha DT 125 owners who have complained about engine issues, who've shared their troubleshooting.
The most prominent of these problems is engine-stalling or "cutting out." If your engine is stalling, your day of trail ripping becomes a day of wrenching pretty quickly.
When you start your bike, you expect it to keep running. Some owners of the DT 125 found that their engines often acted erratically while they were riding.
In most cases, the stalling was a result of worn spark plugs or a bad ignition coil.
Like most motorcycle motors, the DT 125's motor runs on combustion. Combustion requires three things:
- Ignition
- Fuel
- Oxygen, or Air
Your spark plugs are the ignition spark that start's the whole process. Just like anything, they wear out over time. If your spark plugs are loose or burned out, they need to be replaced. Fortunately, this cheap, simple job is just a part of routine maintenance on any motorcycle.
The other common culprit for a stalling engine is a bad ignition coil.
The ignition coil increases the low voltage your battery cranks into the high voltage required to spark the spark plug and ignite the combustion.
If you've checked the spark plugs and they're not bad, the next step is to safely test the ignition coil to see if it's the violator here.
To test your ignition coil:
- Hit the kill switch to turn off the bike's ignition. Keep it off.
- Detach the spark plugs from the ignition coil with your hands. With a wrench, detach the small primary wires.
- Measure the resistance between the coil connections with an ohmmeter. The ideal range would be 0.5 to 3 ohms if you got something else, the coils bad.
- Measure the resistance within the small primary wire connections with an ohmmeter. The ideal resistance range is 6,000 to 12,500 ohms. If you're out of this range, you've got a bad coil.
Stalling isn't the only issue that Yamaha DT 125 owners have had to rectify,
Like all engines, if the engine-tuning of the Yamaha D125 is out-of-whack, the motor might 'misfire' or 'backfire.'
Engine Misfire Symptoms:
- Rough idling
- Poor acceleration
- Excessive exhaust fumes
- Engine stalling
- Weird engine noises
Engine Backfires Symptoms:
-
- Loud popping noise from the exhaust
- Decrease in power
- Excessive exhaust smoke
Causes of Engine Misfiring/Backfiring:
Here are some possible reasons for the engine misfiring problem:
The first thing to look into on a bike that's popping like crazy is the exhaust system, especially if you bought the bike used and it came equipped with an aftermarket system.
If it's misfiring, the same could be true of the air intake.
If the bike is misfiring or backfiring, take the bike to a Yamaha-literate mechanic and have them tune the air intake and exhaust to specifications.
2. Problem with Transmission
The transmission is responsible for the bike's acceleration; any problem with it will affect a bike's speed, shifting, and momentum.
Some Yamaha DT 125 owners have expressed difficulty shifting gears.
Here are the most common transmission-related problems on the Yamaha DT 125:
– Bike Won't Rev Past a Specific Limit:
In this case, the bike's RPMs do not increase beyond a specific range.
For example, a particular DT 125 owner complained his bike had problems revving in the 5000-7000 RPM range.
Another customer faced with the same problem reported that his bike's RPMs never increased beyond 5000 RPMs.
The number varies across different users, but the problem remains the same: these bikes are not revving properly. This problem poses significant risks, as it interferes with acceleration.
– The Bike is Stuck in Gear:
Another frequent concern involves the transmission being stuck in a particular gear.
When you change gear, the transmission has to disengage briefly, disengaging from the current gear and re-engaging with the desired gear.
If the transmission can't disengage, then you won't be able to shift, sometimes while firing the bike up, other times while already in motion.
Either way, you won't be able to increase or decrease your bike's speed.
Here are some possible causes for this problem:
- Faulty clutch cables or linkage
- Worn clutch plates
- Low/Improper/ Dirty Oil
- Faulty gearbox/transmission
- Faulty shift pedal
- Clutch drag
- Faulty clutch lever
If you hear clunking noises or feel some jerkiness when switching gears, or if shifting seems clunky, something's up. It could be a bad clutch or a damaged clutch cable.
The latter is a much cheaper fix, and if you're fortunate, it might just be a lack of free play in the clutch cable that's making things stiff, and all you need is a cable adjustment.
If it's not the clutch-cable, it might result from a stiff/stuck clutch or that the plates themselves are worn out.
Sometimes it's as simple as cold oil. Anytime something feels stiff, check your oil first. If it's clean and the bike has the proper amount, run it for a few minutes to warm the oil and thin out its viscosity.
If you have the proper amount of the appropriate oil type and your bike is warmed up, it's time to look into adjusting your cable or inspecting your plates.
3. Exhaust Manifold Rattling
The exhaust manifold collects exhaust gases from the engine's cylinders for dissipation.
There've been a few cases where owners report either a rattling or ticking noise coming from the exhaust manifold.
These noises can be annoying and nerve-wracking and may make your ride very uncomfortable.
Possible Causes:
Bikes Vibrate, and it's not uncommon for bolts to vibrate loose. A common diagnosis for the DT 125's rattle has been that the bolts securing the exhaust manifold were shaken loose over time.
Should this happen, the exhaust manifold will rattle/tick from the vibration of the loose manifold.
Other potential causes include cracks in the manifold or gaps in the manifold seal.
If the bike is rattling and tightening the manifold bolts doesn't do the trick, you right look into the integrity of the manifold itself. It's not uncommon for parts to wear on a dirtbike, no matter how rugged.
Possible Solutions:
Check if the manifold bolts are not loose.
If they are loose, some wrench those puppies back down tight and see if the rattling stops.
Inspect the exhaust manifold area for any signs of cracks or holes, especially if the exhaust manifold is old. Do the inspection when the motorcycle is cold; you may see/feel steam or gas coming out of the holes.
If the crack isn't particularly large, you may be able to seal it with a sealant.
If the hole is large, you would be better off replacing the exhaust manifold. If you're unsure whether the crack is too big to seal, take it in and have a professional look at it.
4. Rough Idling Issues
Have you tried warming up your bike, only for it to launch into a series of wild revs?
You might also have noticed the bike shaking or vibrating excessively. We call that "rough idling," and it's common among DT 125 that haven't been routinely maintained.
Your engine should have low revs while you're idling, ideally, between 700 RPMs to 1000 RPMs.
Anything above this is called a "high idle" (another term for rough idling). High idles are not only annoying but can be dangerous for you and your motorcycle.
What causes rough idling on motorcycles? A motorcycle idles high when it gets excess air-fuel mixture while in neutral or a low-speed position.
The increased combustion forces the pistons and crankshaft to move faster, explaining the shaking and vibration.
Another cause for high idles on motorcycles is a poorly adjusted idle screw. This component prevents the throttle valves from closing fully and controls the idle level.
You can turn the idle screw in or out to increase or decrease the bike's idle speed. If you turn the idle screw too high, the idle speed will also remain at a high level.
Some also suggest a bad crank seal may cause a rough idle problem. A symptom of a failing crank seal is unusually high RPMs.
The bike will rev normally at first, but it will start revving high as it gets warm. Sometimes, the revs may eventually come down; in other cases, the bike sticks at high revs.
Other causes include an out-of-place throttle handle screw, bad throttle spring, or sticky throttle/throttle cable.
Possible Solutions:
Ensure your throttle and throttle cable are in good condition always.
Don't allow grime and dirt buildup in these components; otherwise, they could stick. If this happens, the throttle can be stuck at a high idle position, leading to some rough idling on your bike.
Use the choke to control how much fuel your bike gets while idling.
Adjust the idle screw to change the idle speed of your bike.
The service manual comes with a recommended idle level; we suggest you follow it.
General Pros and Cons of the Yamaha DT125
The following are some of the Yamaha DT125's top-selling points and problems:
Pros of Yamaha DT125
Here are some of the pros of the Yamaha DT125:
1. High Build Quality:
The DT 125 bikes have been around for literally forever.
This has given Yamaha the time needed to work out the bugs on these motorcycles.
Barring a few problems, DT 125s are extremely durable bikes.
Moreover, the lack of extra components means there're fewer chances of something breaking down.
2. Offers Great Value for Money:
The DT 125 is a low-cost bike that will prove attractive to any budget-conscious buyer.
Besides, it is cheap to insure and maintain and represents good value for anyone. Spare parts are widely available and are inexpensive.
Add to the fact that these bikes are extremely easy to service, and you understand why the DT 125 is a good investment.
3. Beginner Friendly:
Hardly any other bike screams "novice-friendly" as much as the DT 125.
Thanks to low power output, even newly licensed teenagers will find this bike easy to ride.
Wide bars also make it easy to maneuver for beginner riders.
Cons of Yamaha DT125
The following are some drawbacks of the DT125:
- Erratic Engine Performance
- Problem with Transmission
- Exhaust Manifold Rattling
- Rough Idling Issues
What Do The Reviews Say?
"Tall, upright and commanding, the Yamaha DT 125X is perfect for town riding and twisties. The long-travel suspension absorbs bumps while good, firm brakes will safely bring you to a halt. The handling's the best bit, though: throw the motorcycle around, and it'll respond brilliantly."
[Source: Motorcyclenews.com]
"The evergreen Yamaha DT 125 continues to offer new riders an enjoyable route to life on two wheels…While that heady 14bhp may seem impressive to begin with, it won't take long to outgrow."
[Source: Visordown.com]
What's The Resale Value On the Yamaha DT125?
| Year | Mileage | Price ($) |
| 2002 | 17,000 | 2,759 |
| 2007 | 23,000 | 1,630 |
| 2009 | 18,000 | 2,005 |
| 2013 | 1,392 | 1,254 |
Note: You may have to import these bikes into the United States.
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Source: https://motorandwheels.com/common-problems-yamaha-dt125/
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