DT125R melting pistons?

My 89 dt125r has melted 2 pistons so far, after the 1st piston melted
i've removed the thermostat, set the carb needle as rich as possible
(jets are stock), checked the oil pump, replaced crank seals, reeds are
fine. The exhaust system is totally standard, infact the bike is
totally standard according to the haynes manual apart from the servo
motor which has been fitted to operate the ypvs system.
After the 2nd piston melted I had to have it rebored and now im scared
to run it incase it happens again. The guy who bored it said it looked
like detonation or pre ignition. Anyone any ideas as to why this is
happening? Could it be a fault in my igniton system?
When the bike is running it sounds rough (perhaps knocking, i'm not
sure) around 4-5000 revs.

Greg

 

If your ignition system's timing is firing too early, it can cause this.

If your compression ratio is too high for the fuel you are using, it can
cause this.

If your fuel/air mixture is too lean, it can cause this.

An air leak somewhere in the crankcase can cause this.

If nothing else was changed, I would check the ignition first, then do a
rigorous check for air leaks. How is the seal around the crankshaft?

Ed Cregger

 

Is the power valve opening all the way soon enough? You may be trapping
too much heat in the cylinder for too long, and the spark plug might be
heating up to where it's glowing red hot and it then acts like a glow
plug. Then the ignition timing doesn't matter, the fire is getting lit
*before* the spark jumps the gap. (1)

When engines are running too lean and preigniting they make a sour
sound that I can't describe, then they start "shooting ducks" out the
exhaust pipe. The distinctive sound is a BOOM, like a shot gun.

You might try a larger main jet, going up in sizes until the engine
starts
4-stroking, i.e., the cylinder fires every other time and the exhaust
sound is blubbery. But, if the problem seems to happen during part
throttle, swapping main jets won't help, you need a bigger pilot jet...

You can tell that an engine is preigniting by the little specks of
carbon burned onto the spark plug insulator nose. They look like
pepper.

Other possibilities are that your silencer is plugged up or that your
spark plug heat range is wrong. But, don't try to cover up a basically
lean fuel air mixture with a cold spark plug, the mixture can still
detonate without warning...

(1) Honda actually built a 2-stroke 400cc motorbike for the Dakar Rally
which had a power valve that almost completely closed at low RPM and
small throttle openings. The ignition system was shut off and the
engine ran as a diesel to get good gas mileage in the desert...

 

Check for exhaust leaks particulary where it bolts to the head .
Other than that ignition timing caused by faulty cdi .If it only breaks down
at high revs it could be advancing to much .

 

Thanks for the replies, I'll try some of that as soon as I've got money
again lol. Has anyone on this forum got a dt by any chance?

Cheers
Greg.

 

Picked up a thoroguly abused one with a melted piston (and a variety of
other probs) for $100, it was a 90 or 91 I believe. I rebored it, sleaved
it, dealt with its other problems (locked forks, wiring was f'd etc) ran it
for fun for a bit, but then sold it for profit. Maybe a design flaw
considering I picked it up in the same condition that your dealing with.

 

The float bowl may require adjustment. The bike will run too lean if the
bowl cannot hold the maximum it is designed to hold when you're using a lot
of throttle. Double check that setting and make sure that your spark plug
is the correct one for your bike.
-jeff

 

Something else just came to mind. Also check the intake manifold betwwen
the carb and the cylinder for any leaks. That added air can screw up you
mixture something awful.

 

Yeah. Right. A design flaw. One of the design flaws inherent in any
mechanical device is that the engineers are unable to make it
*foolproof*.

Problem with a little 2-stroke engine is that it has to be run at
*really high RPM* to get the desired power out of it. And, even when
the engine is liquid-cooled, the fuel/air mixture has to be richer than
what is needed just to burn the part of the fuel that actually gets
used to produce *power*.

A lot of fuel gets partly burned and turns into carbon monoxide and
unburned hydrocarbons fly out the exhaust pipe just to keep the engine
cool instead of burning the fuel air mixture completely, turning it
into carbon dioxide and water vapor. This keeps the aluminum piston
from melting.

If you look at the parts fiche for the DT-125R's carburetor, you'll
probably see half a dozen or more main jets, and the intelligent rider
who owns such a machine realizes that a 2-stroke engine is very
sensitive to altitude and atmosphere *temperature* and pressure
changes.

The jet which works correctly on a standard day at sea level when the
air temperature is 59 degrees Fahrenheit is going to be TOO LARGE when
the air temperature is 80 degrees and the rider is a mile above sea
level in Denver...

But, that's not as bad as what would happen if the rider lived in
Denver and re-jetted his 2-stroke for operation at 5280 feet above sea
level on an 80 degree day and then he went down to ride at sea level on
a 59 degree day.

Then the engine would be jetted far too lean, it would run hot, the
piston might melt.

And, it's all a "design flaw". The engineers cannot design the engine
to operate at maximum performance under all conditions without
re-jetting, and nobody can get the riders to read the owner's manual
and re-jet the carburetors for the altitude and temperature that they
ride in.

 

Also, check that the spark plugs are tight. I had a little Yamaha YAS1
125 cc TS twin many years ago; one of the plugs wasn't quite tight
enough causing an air leak which resulted in a seized piston. I wasn't
even going very fast when it happened.

Leon

 

Did you hear a strange buzzing sound, like the "jake brake" on a diesel
truck? That's what the compression release on my 2-stroke Yamaha DT-1
sounded like...

 

Today i tried 2 tzr125 cdi boxes, but made no difference, a new br9es
plug, stripped and cleaned the carb and set it back to what it says in
the haynes manual, tried it without the silencer and it fouled my spark
plug, tried another plug and it fouled it too, fitted the silencer
again and another plug and it worked but still not running properly.
The engine is still making a knocking noise and sometimes the knocking
gets soo bad it cuts out, after it cuts out it starts again ok, the
engine sounds smoothe and crisp at idle and idles perfectly, its not
untill the revs come up to over 4k that the problem starts.

 

What, did the plug wet foul right away, or did it wait until the engine
warmed up good?

It sounds like you have quite an accumulation of OIL in the bottom of
the crankcase, and it DETONATES when the engine gets hot and the oil
starts passing through the transfer ports. Detonation makes a big
CLANK! while pinging makes little clink-clink-clink sounds...

It sounds like more of the oil started coming out when you removed the
silencer, because there was more airflow through the system. Some
motorbikes used to have crankcase drain plugs to get excess oil out the
bottom end. But, when that oil turns into goop in the bottom of the
crankcase, it's hard to get out.

I still suspect that the YPVS valve isn't opening all the way, or that
the silencer baffle is plugged up. Maybe you even have a lot of carbon
accumulation in the exhaust header.

 

What kind of provision is there for advancing the spark as the engine revs
up? Is it a vacuum/centrifugal mechanical system? Or is it a chip that
controls spark advance?

Ed Cregger

 

The engine was warm before i removed the silencer, it fouled only
seconds after i started it with it off, the plug was oily when i
removed it so it could be excess oil in the bottom end (the bike is
pretty smokey).

 

I doubt its to do with the ypvs because when the bike is restricted the
powervalve is pegged in the closed position but i will look into it, as
for the timing, its the cdi box that controls the spark advance.

 

Tried a timing strobe on the bike today and found that when i started
it from cold and revved it, i could see the timing advanceing, then as
i revved it more it seemed to retard again (perhaps rev limiter, but it
was no where near the red line). Then as the bike got warmer i notice
the mark didnt seem to move as i revved it, and also the mark on the
flywheel doesnt seem to line up properly with the mark on the stator
plate under the strobe light. Will hopefully be trying another cdi from
a dt tommorow.

Greg

 

The timing specified in the manual is not necessarily the timing that
conditions in the engine will allow the spark to fire at.

Did you ever see one of those car distributor machines in a shop that
had a compressed air hose going into the chamber where there was a
spark plug being fired by the ignition system under test? Cylinder
pressure *matters*...

The manual specification is like a "ball park figure". An ignition
system will not fire the spark plug until there is enough voltage to
jump the gap against the pressure inside the cylinder. If there is too
much pressure in the cylinder, the spark will be retarded, or if there
isn't enough voltage, a spark won't jump the gap at all.

OTOH, a true CDI system makes the voltage rise incredibly fast, like in
microseconds, so very little voltage is lost to leakage across fouled
spark plug insulator.

The temperature of the spark plug electrodes also influences spark
timing.
The spark prefers to jump from the coldest point on the electrode, but
sparking heats up that point, so the spark moves to another cooler
place to fire from. A tiny bit of metal erodes from each point where a
spark has taken place, rounding off the electrodes, changing the timing
slightly...

That aspect sounds like the static timing is off. Is it advanced or
retarded to start with? The electronic advance curve does NOT know
whether the ignition static timing is correct or not, it just advances
according to its internal circuitry if there is no knock sensor built
into the engine.

It would definitely be nice to use a cdi that was specifically designed
to work with your particular model motorbike. Experimentation is
educational, if you can guess what the results of your tests really
mean...

 

Couldnt get a hold of a dt cdi, tuned out that the one the guy had was
another tzr one (damn).

 

You said in your first post that you'd removed the thermostat, so I
guess this is a water-cooled machine. Does it have a water pump? Is the
water pump actually pumping? Does water circulate freely through the
radiator?