CR250R question

Bought a new (06) CR250R about a month ago, after breakin it fouled
plugs bad and was making a strange knock when its barely revved right
when the rpms start dropping, sometimes 2 or 3 knocks. We tried
changing jets and it might have helped a little bit but not much and it
fouled a plug again.
Any help would be great.
Thanks

 

Two-strokes run on both oil and gasoline. If you have too much oil in
the mixture,
the effective octane of the fuel is lowered and the engine knocks.

I am NOT telling you to use less oil in your pre-mix, if you are
running pre-mix.

But, it is very possibly that your crankcase has a lot of fuel and oil
mix collected in the crankcase, or even in a bend of your expansion
chamber. If the fuel/oil gets blown back into the cylinder and manages
to ignite, whether by spark ignition or dieseling before the spark, a
two-stroke engine will knock.

 

Thanks for your input FB, I dont understand how the fuel/oil could get
in the crankcase, but it was blowing alot of oil out the exhaust so we
figured it must be running rich. We changed the jetting and it doesnt
blow oil but it still knocks although less. Do you think maybe there is
still quite a bit in the exhaust that could still be causing the knock?
Thanks again

 

FB,
I really like your posts, and have gotten a lot of useful
information reading you and John Johnson, but I don't completely
agree with you on this one.

The extra oil should effectively raise octane, by raising the flash
point of the fuel.
However, extra oil will mean less fuel per volume of measure, which
would lean the air/fuel mixture, and could cause the engine to "ping"
or "knock".

Jim Cook
Team LAGNAF
'06 GasGas DE300

 

The CR250R is a 2-stroke isn't it? My CR250M was a 2-stroke.

Two-stroke engines use the upward motion of the piston to suck fuel/air
mixture into the crankcase. Then the piston goes down and pushes the
mixture up through the transfer ports into the engine.

If you leave the fuel petcock turned on overnight, it is possible for
pre-mix to leak past the float valve in the carburetor and it drains
down into the bottom of the crankcase and sit there.

When you go to start the engine, all that pre-mix gets pushed up into
the combustion chamber, fouling the spark plug. Some of the pre-mix can
get blown right out of the engine and go into the bend of the exhaust
pipe and stay there.

I remember the first time that happened to one of my Yamahas. It drove
me nuts, fouling the spark plug over and over and over.

One Suzuki even had drain plugs in the bottom of the crankcase to drain
out any oil that accumulated down there.

It is possible that there is oil in the bend of the exhaust pipe and
the pressure waves blowing back and forth through the exhaust system
could make the oil go back to the combustion chamber.

Another possibility is that your gasoline is very low octane, or very
old and has lost its
octane rating. Check your owners manual to make sure you are using the
right octane
gasoline.

 

Thanks for all of your inputs.
We rejetted again and the knocks slowed way down but then began again.
Still lookin. Rick I dont know of any dealer that gives a warranty on a
dirt bike after its out the door.
Thanks again
Bill

 

I'll answer this part for him. There is NO warranty on race bikes, which
the CR250R is considered. Once you sign the papers, it is yours and yours
alone. Dealer service is expensive compared to indys and they are more
expensive than the advice that he will get here.

Eric