RM250 low comp. HELP!

[bucfan11]

Very low hour 06 RM250. Was running great, son said it started wanting to
die when he let off of throttle. No power loss, nothing seemed wrong other
than not idling. shut it off and when he tried to restart... very little
compression. I checked the plug to make sure it was tight. Before I tear
down and replace rings/piston what else could cause low compression on a 2
stroke?

[blowndodge]

do you know what "low compression" is on a 2 stroke? You SHOULD have at least 115lbs cranking pressure. You need to test that first.

If low then you've done one of many things:

seized a ring, blown a head gasket, blown a base gasket, holed a pistion, broke the skirt on the piston, loose spark plug, or crank seals are shot. All these cause low compression. Get a compression tester then remove the spark plug and screw in the compression guage and hold the throttle wide open and crank about 10 times on the kick starter as fast and hard as you can and let me know what the guage says.

compression guages are fairly cheap to buy.

[ringadingh]

You can also take a look inside the cylinder through the spark plug hole, by using a small flashlight with a flexible wand. If it ran fine prior to acting up, Im suspecting a detonated piston, from running to lean.
And I just replaced my compression gage last week, it was only $15.00 here in Canada.

[rflnomad]

Yes, I would drain the coolant and pull the head and then the cylinder. If the piston has a hole in it, the question is "why?"
Is premix ratio, gas octane and recomended oil all correct?
Don't laugh, check the air filter and the boot. See if there are tears or failures that may allow dirt in the motor.
Good luck and keep us fellow two stroke dirt riders posted!!!!!

[MikeInFla]

I don't know how many 2 stroke pistons I've gone through and they did the same thing. I think Ringadingh is right.

[bucfan11]

Pulled the piston out today. Both rings are stuck. Cylinder looks great, no obvious signs of wear or scrapeing. Piston...well, along 2/3rds of the edge it is pitted, scrape marks down the front. It looks to me like it got hot although the bike showed no signs of overheating. Spark plug looks good, not ash colored. I'm stumped as to what it could be, maybe just a freak thing.

Rob

[macmac]

Down the front means down the exhaust side right? If so this is typical of not enough oil in the mix. If it kixed gas, was it? If it is injected from a bottle was it full?

If the clyinder walls are covered with stuck on alloy from the pistion use acid to etch it off, and hone the cly lightly after.

Another thing I believe but am told am wrong about is ethenol, and IMO it cuts the 2 stroke oil. I ate a 120 cc piston on a saw and Sachs Dolmar sent me a service rep and that was his opinion at the time.

With saws I now run some SEAFOAM, but by the tank as the saws are filled. This will not harm the saws in my case, and tends to clean carbons and burned oils that get in behind the rings.

So the piston is getting cleaned of carbon and gook while it runs and soaks when it isn't.

Carbon is hard enough to skuff alloy, and then the alloy eats itself. With all 2 strokes I tend to run them a tad richer than they want, in hopes of more cooling.

Maybe you can come by Marine gasoline with No ethenol.

[bucfan11]

I kinda think maybe there was not enough oil mixed, plus the ethenol gas. My son swears he mixed 4oz to the gallon and he is the one who always mixes it. I'll add an extra once from now on. I'm going to pull the carb. and check the jets, raise the needle one notch so I know it's running a little rich.

Rob

[bucfan11]

Well, My like new, barley rode RM250, only ridden to church on Sunday, has been re-jetted... A LOT! I now know what caused the piston to chip. Needle-two notches lean, Power jet- from a 55 to a 45, Main jet from a 172 to a 168, Slow jet from a 50 to a 48. I've ordered all the stock ones.

[macmac]

lean is mean as they say.......

[rflnomad]

Wow, that is pretty lean. I put our KX 250 at the stock setting after we bought it. The PO had leaned it on the pilot,needle and main jet. How much more power do you really need?
I told my son to put it back to stock, and leave it alone.
Perfomance comes from riding smarter, not just motor.

[bucfan11]

Originally Posted by rflnomad

Wow, that is pretty lean. I put our KX 250 at the stock setting after we bought it. The PO had leaned it on the pilot,needle and main jet. How much more power do you really need?
I told my son to put it back to stock, and leave it alone.
Perfomance comes from riding smarter, not just motor.

Exactly, these things are so strong I don't see anyone needing more power unless you're proffesional. We had a KX250F and it was strong, nothing like the 250 2 stroke but still strong. These machines amaze me.

Rob

[ringadingh]

You can get away with them running lean on the warmer days, but when it cools down and the barometer drops, you can blow a piston in a hurry. Its best to leave it with stock jets, you lose some power, but it should stay together longer.