04 YZ250 Engine Problem

So I was riding my YZ yesterday and all was well.. Went for a couple of
rides in the desert with my kid.. Came back after a 1 hour ride and
let the bike site for a while while we ate lunch.... Started her up
again and was just doing some light riding on this little hill when al
of a sudden the motor started bogging down on me then died... tried
kicking her over and it was so hard to kick.. Like the bike was
flooded of something.. So I pulled the plug and all looked fine..
Kicked over no problem with the plug out...

I put the plug back in and still the same issue again.. Was really hard
to kick over with not a hint of engine spark at all. Like its totally
dead..

Anyone know what to do or try?

I just rejetted everything back to stock settings last Saturday.. PC
Pipe.. Other than that its all stock..

Thanks in advance...

 

So, is it the 2-stroke or the 4-stroke YZ?

 

Hey B-12 It is the 2 stroker.. Sorry I should have made that clearer.

 

www.partsfish.com has diagrams for the 2003 model. There probably
aren't that many differences between a 2003 and a 2004 unless Yamaha
completely redesigned the YZ250.

The obvious thing to check first is whether the spark plug is any good.
If it is carboned up inside, the high voltage can leak away too fast
for a spark to fire.

The hard kicking is probably because the exhaust power valve is closed.
That
causes the engine to have higher compression at lower cranking speeds.

If the engine is really hot and the power valve is closed, it will be
hard to kick even if the spark plug doesn't fire, because some gasoline
is still in the crankcase of a two stroke and it will diesel off when
it is compressed.

I have seen two stroke engine run as diesels twice. It's amazing that a
low compression engine like that will diesel on gasoline, but they do.

Looking at the parts diagrams on www.partsfish.com, I don't see what
controls the power valve.

There's a strange mechanical gizmo called a "governor" in one of the
diagrams. I don't know what it does.

Regarding the lack of spark, I would check all the connections at the
kill switch on the handlebar, the power to the ignition coil and the
connections to the CDI box.

The generator probably makes a high voltage of about 100 volts and that
goes to the CDI box and on to the ignition coil. A trigger coil in the
generator unit will signal the
CDI unit to fire the coil. The trigger coil probably only puts out
about 2 or 3 volts.

You would need an analog volt ohmmeter to see the pulsing of the
trigger coil as the engine turns. A digital voltmeter wouldn't react
fast enough to the short pulse from the trigger coil.

There is a throttle position sensor on the carburetor and a electric
fuel shutoff valve. They probably work together to prevent a runaway
motorcycle if the throttle slide doesn't follow the movement of the
twist grip.

I didn't see any control unit for the electric fuel shutoff valve, but
the control circuit might be built into the CDI control box.

 

<snip>


*********************************************************

The governor, through centrifugal force, activates the powervalve
control/activator linkage.


Jim

 

Your suggestion is noted and ignored. You misspelled "advice".

 

Hey guys...

Well I pulled the carb apart last night and all is good. I pulled the
plug on the bike and what i noticed is that while i kick the bike i am
getting no spark at all..(should i sbe seeing any spark)? Could my CDI
be bad...?

 

Motorcycle ignition systems don't put out fat, crackling sparks like a
car ignition.

Somethimes they are incredibly weak and anemic looking. You have to try
the spark test in the dark to even see a really weak spark.

And, if your spark plug is really carbon fouled, the low voltage might
leak away before it gets high enough to jump the plug gap.

It's possible that the CDI box might be defective, but I recommend that
you take the generator side cover off and determine whether the
generator is turning or not.

I have seen the nut holding the generator onto the crankshaft loosen up
before, so the generator rotor stopped turning and the engine suddenly
quit running.

Your machine doesn't have a battery, so it has to get the high voltage
for the CDI from the generator.

And, the CDI won't work if the trigger coil doesn't get the signal from
the generator rotor either.

I don't know what the exact ohmmeter reading through the stator coil
and the trigger coil would be. Maybe you can google that info up off
the web, perhaps Electrosport USA has the info.

 

The OP doesn't have a problem with his power valve, I was pointing out
that the hard kicking was symptomatic of a closed power valve.

The OP doesn't see a spark, and he wants to know if he should see one,
so why don't you concentrate on useful information?

Otherwise, just bugger off.

 

Oh. I see. You're mentally retarded. Into the killfile with you.

 

Usually when they go like that if you don't have any funny sounds, knocks
or whatever, it's electrical.

You should check your generator and battery. If the brushes on the
generator are worn out, the gen won't be charging the battery and the
bike will start running off battery. Once battery voltage drops below
a certain threshold, the CDI units will stop firing.

Of course, it is always possible that a switch snapped, a wire broke,
or a CDI unit died. But check the simple stuff first.

Ted

 

It probably is an electrical problem, but one with a strange twist.
This machine has a generator that operates the CDI unit and it also has
a fuel shutoff solenoid on the carburetor. If the solenoid doesn't get
any power, the carburetor doesn't get any gas.

There are no brushes and the motorcycle has no battery. It's a
motocross bike.