My Yamaha DT125 cut-out and died on me! Any Suggestions?

Question

I was traveling along the road and my DT125('99) cut out as if it had run out of petrol. I checked this and it is 1/2 full and the oil is 1/2 full as well. I checked the spark plug and it was dirty so I put my new spare in. There is a spark from the plug, and the battery is a new one - so I'm thinking that it is these two that are not the problem.
I put the new plug in and tried kicking (I had a few loud miss-fires during this!) However, once I removed the plug again there was a lot of petrol/oil mixture on the spark end of the plug.
So i dried the plug end off, cleaned the casing where the plug sits (and inside) and checked the throttle cable to ensure it was not stuck and opening the throttle - flooding the engine, but that was fine.I have kept the manual petrol OFF and the choke on and tried starting the thing but I have had no luck! I know it is hard to diagnose without looking at it! Please Help!

Answer 1

It sounds like your altenator has packed up..take it to any bike shop . ask them to check your electrics..it should be free...

Answer 2

Gas off , key off , plug out, Kick it 20-30 times to clear chamber of unburned fuel.
Pull your fuel line off the carb, crack the drain screw then close it again.
Fill the carb, through the fuel inlet ,with carb cleaner and drain a couple times.
Clean your air filter
turn fuel air screw out 1/8 turn
You could also try a hotter plug.
If what you say is true ( That there is spark) then you are loading up due to one or more of the below :

Too rich a fuel oil mixture
sticky floats
Dirty air filter
Improperly adjusted carb.

Answer 3

George,,
That sounds like either a Ignition CDI Failure,,
or a remote possibility of a piston/ring seizure

Maybe something simple as a flywheel key shearing,,throwing ignition timing way off???

...........................................
Ok,,,You are cruising along and all is well.

A sudden fault with little or no warning can occur just as easily with ANY of the systems that support engine running.

FUEL:
If fuel was Blocked,,,you would not be getting a wet plug

If Fuel was Flooding,,You would absolutely have gotten some POOR RUNNING(as if on Choke) Before it died.
--Plus,,,it would be overflowing from the carb while just sitting there.

Water,,,even just a Droplet,,,can Plug a Carb Passage and cause the Symptoms of Either Flooding or Starving.
And it can do so without producing the Other accompanying symptoms of Actual FLOOD/STARVE.

It sort of masquerades as a Different fault than "just water".

But if Carb is NOT leaking/overflowing while SITTING,,,it hardly could have caused flooding to kill engine without producing at least a moment of really poor Running.

And,,starving is starving,,,if it had fuel blockage to the point it just simply Died while running,,,
It "couldnt" wet the plug with Fuel

So,,,Fuel Fault is a Low Probabilty.

A bead or 2 of water wandering thru Carb is a SLIM possibility

........................................................
SPARK:
Electronic Ignition has some peculiar properties.

Older ignition with Breaker Points either work,or not.
They Can get Intermittent Spark from
a)Dirty Contacts
b)"Sticky" points arm
It's Possible,,but Extremely Rare for either of those to occur on a "Sudden Fault" basis.Majority of times it will act up first and give some warning.

Reason for that typical failure mode is because Both conditions are basically a matter of "deterioration",,,not a Hard Failure.
So the symptoms begin intermittently,then Grow worse.
It may take MINUTES,,or a MONTH.
But usually Some kind of "warning"

ELECTRONIC CDI is different.
When it's part begin to fail it has a way of Skipping every other spark,,,then every 5 sparks,,,every 7~10~20 sparks and so on.

So you can SEE a spark,,,
but it's hard to visually perceive that it's only Intermittent and Occasional.
And impossible to tell by eyeball if it's Firing in correct spot.

Meanwhile,,,the electrical values,,,Voltage/Amperage of the Spark is degraded.
So it may Spark in Open Air,,,but NOT under compression in the engine.

A 3rd effect also comes into play,,Ignition Timing is altered by the faulty values in the failing CDI components.
They can Spark strongly,,,but be actually Firing way out of place.

All sounds like VooDoo,,,but it's the way Resistors and Capacitors interact.
Leaky Capacitors Lower Voltage.
Increased Resistance in any of a number of places in the circuit DELAY timing signals.
A negative feedback resistor which is meant to Delay the Spark Trigger point,,if it fails the Spark will occur waaay early.

A bad Cap can also Not hold enough charge for Long enough to fire at a Slow Speed.
Such as trying to kick start.
But kicking it Faster while Plug is OUT and checking for spark,,,that raises Charging Frequency HIGH enough to produce a visible spark.
Flywheel is Charging the Caps ,,Faster than they're Leaking.

All of which is why a Visible Spark from a Battery/Coil ignition,,or a breaker point magneto will USUALLY start and run ,,,however poorly.

But a Electronic Ignition/CDI can LOOK like it's sparking Great,,,
but actually be "So Dead" it'll never start or run Anything.

There's little a Home Mechanic can do to check them.
1) get an old Plug that DOES fire,,,bend the Ground strap far away(or clip it Off).
This makes for a VERY LONG arc gap.
If that "Test Plug" Fires,,,and the spark still looks "Blue"
Then it Probably has correct /adequate voltage/amperage to the plug.

2)You Need a Ignition Timing Strobe Light to determine if it's firing in correct spot.

The Voltage Output,,,,and the Trigger point are 2 entirely different things.
BOTH conditions must be met in order for it to run.
A CDI ign system can fail Totally Dead,
or in either function--Voltgage or Timing

..............................
A New Plug NOT Starting the bike,,,
But Getting WET w/ fuel,,,
THEN actually firing way out of time,,,"Miss Firing"....

That sorta Verifies Fuel,,,
but Condemns Ignition .

There is Obviously SOME form ignition,,,
You SEE Spark,,,and hear the Effect--but at "wrong time"

That tentatively Verifies Voltage Output,,
but Condemns Timing

Timing is controlled by:
*Flywheel Key
*Trigger Coil/Pickup on engine,in Magneto Assy
*CDI Box

***Dirty,,Corroded,,or LOOSE wiring connections are ALWAYS a Suspect in any ignition probs.
CDI ignitions are particularly sensitive to it.
The effect of the increased resistance drives CDIs crazy because it influences Voltages which control the CDI.
Check connections FROM> Engine/Magneto TO> CDI Box

.....................................................................
One other possibility,,,I hate to even mention.

Piston Seizure can produce the symptoms you describe.
Especially if Ring gets stuck to Piston during the process....either from physical damage--smearing metal over rings,or "pinching them in the ring grooves",,,,or Carbon sticking them in their grooves.

You can run a compression test,but thats only a partial indication.

Best bet is a Visual Inspection.
*Remove Exhaust and LOOK into the exhaust Port at the Piston for any scarring,burning,etc

*And /Or Remove Cylinder Head and look at Cylinder Walls for any "black Streaks",,,seizure/galling/aluminum deposits smeared onto cyl walls

ODDS are SLIM,,,those bikes are VERY tough engines and rarely seize.

But however slim,,it is a possibility.
I only mention it because you DO have some degree of Fuel and Spark.

If I had to GUESS,,,Id most suspect a CDI Fault/Failure

Avoid/Minimize using Choke,,by the way.
Especially when troubleshooting a No Start.
That can ruin even a Brand New Plug.
Too much fuel can saturate it and short it out internally.
Which can Really make ya pull your hair out,,,"I know it Cant be a bad plug,,cuz it's Brand New!"

Plugs are like Light Bulbs,,,when they're bad,they're bad.
And it can happen at Any point,,for a number of reasons

...............................
Dont be too concerned about engine seizure.
Odds are terribly low even though it IS a possibility.

Wouldnt surprise me if the Culprit is a Bad Ignition PickUp coil/Trigger Coil in the magneto.
Yamaha calls them "Pulser Coil"

2nd Guess would be Source Coil in the magneto or CDI Box itself
....................................................................................................
Good Luck,,probly nothing very serious

Answer 4

I would say by my past experience check the points b/c i have a 78 yamaha dt125 and the points were set wrong and just b/c you get spark doesent mean its a good spark it could be sparking every other time b/c mine was hope this helps.

Logan

Answer 5

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Answer 6

well this could probaby be that you need new piston rings or a complete rebore. this is common in Yammaha DT 125's. ok if you try to crank it over by your hand and you can crank it over by your hand that means you have no comprssion. or another way is that if you take your spark plug out press your finger in the hole and kick it over and it doesnt blow your finger off the plug hole this is another way of testing if you have no compression. if you have no compression in the clyinder you will either need new piston rings or you will need a rebore. i think you will need a rebore. if it is the case that u have no compression you will need to take it to a motorbike shop they will bore the barrel to a bigger size to take the scores out and then fit it with a new piston and rings and yoyr problem will be solved. but remember when u have a rebore you must run it it for about 500 miles. if you rev the granny of it then you will be back to square 1.

Answer 7

A quick test -
Drain the carburetor (into a clean cup if possible).
Might have water in the fuel.
It will look like a jelly fish floating around in the cup of fuel.
If you see any rust or other crap, clean the carb and look in the fuel tank to see if any is any in there.
If so, drain the tank and install an in-line fuel filter.

Answer 8

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Answer 9

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Answer 10

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Answer 11

The same happened to mine and it turned out that the fuel pump wasnt putting the fuel through to the engine. unless your a dab hand with bike mechanics i would recommend taking it to a dealership, you dont want to inccur further problems