yz250 engine quirks

[Nick425]

I have an 03 Yz250 bored to 325cc, runs like a champ and is pretty fast.

But if I put my head close to the engine at low to mid rpm it doesn't sound that good. Almost like a knocking noise or even whining. Something doesn't sound 100%. Could an unbalanced crank cause this

I checked my clutch the other day, not too burnt but the clutch basket seemed like it had quite a bit of play from side to side and a little outward movement. Not sure if this is normal. Any tricks to removing the whole clutch assembly without an air compressor?

[Mikeyam187]

Cut a broomstick and put one end on the engine and your ear up to the other end and listen to the sound reverberating up the wooden dowel. Move the end on the engine around from location to location and listen for changes in the sound to pinpoint where you think it might be coming from. It will isolate the knocking sound from the exhaust tone...give it a try!

[socal18]

Cut a broomstick and put one end on the engine and your ear up to the other end and listen to the sound reverberating up the wooden dowel. Move the end on the engine around from location to location and listen for changes in the sound to pinpoint where you think it might be coming from. It will isolate the knocking sound from the exhaust tone...give it a try!

A screwdriver works just fine for that. Save the broomstick.

To hold the clutch basket/primary drive, take a shop rag and tie a knot in it near one corner. Feed the end into the gears while rotating the clutch basket. When the knot hits the gears, they will stop, then put a socket of appropriate size on the nut and turn it.

[flipside]

Ride up a hill in 2nd or 3rd. Let off then get back on it and if it knocks then it is detonating. Common with the YZ's. You'll need a head mod to run pump fuel. Ignition timing can help too.

[YZed250]

Could be detonation, as flipside indicated. Commonly cured with Av gas or head mod or leaded race gas or new crank seals.
If the piston is a forged piston, they are typically noisier than the stock cast piston, especially on startup and once the cylinder clearance starts to open up. Could be time for a top end and check the rod for play while you're in there.
You can always remove the flywheel cover and check the crank for play on the left side.
My YZs were all pretty noisy at idle with the clutch out/released. Clutch lash was basically non-existent as soon as I pulled in the clutch.
If you happen to have the clutch cover off, check that the primary drive gear bolt is still tight. If it backs out you'll hole the clutch cover, loose your oil and probably DNF. As the bolt is back out, you'll notice some more rattles and vibes.

[YZed250]

Whining or whirring is commonly associated with worn/pitted crank bearings.

[Shamus]

About how much time is on the thing?

[Nick425]

It does have a forged piston and about 20hrs on piston and 40 on bottom end. Just took out clutch plates out and the basket is pretty wrecked, so the bike is going into surgery

It could possibly have some worn crank bearings for sure


Cylinder and head are not OEM, and are meant for pump fuel

[Shamus]

A forged piston will make some noise until it's fully warmed up. But the clutch basket can make some racket when it's worn out too. Especially at idle. The clutch plates inside it rattle back and forth on the worn out fingers.

[Nick425]

A forged piston will make some noise until it's fully warmed up. But the clutch basket can make some racket when it's worn out too. Especially at idle. The clutch plates inside it rattle back and forth on the worn out fingers.

I bet it was both of these things. Worst basket I've seen, new one should help out.

[6IX]

Whining or whirring is commonly associated with worn/pitted crank bearings.

Could also be the transmission.  My '02 had a whirring noise in neutral near idle and while free revving it in the pits.  When we pulled it apart 3rd gear had been mauled bad and needed to be replaced. When we free spun the gears by hand with the cases sitting on the bench right in front of us we could recreate the whirring noise.