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YZ250 2004 YZ250 Idle knock with fresh top and bottom end rebuild

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 Recently rebuilt a 2004 YZ250 from the ground up, all new bearings and seals, new OEM cylinder (b size), new hotrods crank, and new vertex piston (b size). I am admittedly new to 2 strokes, and am not entirely familiar with the sounds they make. That being said, the knocking sound this bike makes at idle is disconcerting to say the least. The bike has 8 hours or runtime, I've torn it down twice and have yet to notice any abnormal wear. I've even taken the top-end into the shop I work at a few times and had other techs look at it, they all say it looks fine. I specced both the cylinder and the piston with a micrometer and they both measured out as they should. Piston was just a hair over 66.35mm and cylinder was just a hair over 66.40mm. I've checked timing with a dial indicator, and yet the noise persists.  I am at a bit of a loss as to what is causing the noise, and was hoping to get some direction as far as what to check into next. Video of the noise is below, thanks for taking the time to read.

 

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Here's my 04 after and brand new cylinder and pro x piston. I think these motors are just kinda noisy at idle.. The clutch itself causes a bit of noise pull the lever while idling and you can hear what I mean. I have 20+ hours on mine and nothing has changed. Except for a broken throttle cable, you might want to check that,imagine going up a ramp and it breaks midway up. 

 

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8 minutes ago, TwoFiddyYZ said:

Here's my 04 after and brand new cylinder and pro x piston. I think these motors are just kinda noisy at idle.. The clutch itself causes a bit of noise pull the lever while idling and you can hear what I mean. I have 20+ hours on mine and nothing has changed. Except for a broken throttle cable, you might want to check that,imagine going up a ramp and it breaks midway up. 

 

Couldn't hear it real well in the video but I'm assuming your bike makes a noise similar to mine then? If I pull in the clutch the sound from the clutch goes away, but the knocking remains, I pulled the clutch in midway through my video. Good tip on the throttle cable, breaking it sounds like a bad time!

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2 hours ago, Pooc40 said:

 Recently rebuilt a 2004 YZ250 from the ground up, all new bearings and seals, new OEM cylinder (b size), new hotrods crank, and new vertex piston (b size). I am admittedly new to 2 strokes, and am not entirely familiar with the sounds they make. That being said, the knocking sound this bike makes at idle is disconcerting to say the least. The bike has 8 hours or runtime, I've torn it down twice and have yet to notice any abnormal wear. I've even taken the top-end into the shop I work at a few times and had other techs look at it, they all say it looks fine. I specced both the cylinder and the piston with a micrometer and they both measured out as they should. Piston was just a hair over 66.35mm and cylinder was just a hair over 66.40mm. I've checked timing with a dial indicator, and yet the noise persists.  I am at a bit of a loss as to what is causing the noise, and was hoping to get some direction as far as what to check into next. Video of the noise is below, thanks for taking the time to read.

 

Gotta say that does sound excessive for once, maybe just a good mic and it's actually normal IDK

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11 minutes ago, Smoking 2's said:

Gotta say that does sound excessive for once, maybe just a good mic and it's actually normal IDK

If anything I think it's gnarlier in person. Two of my buddies have yzs, ones a 2020 250x and one is a 2004 250, neither sounds anywhere near as loud as mine. The sound is similar to my buddies 04, but much louder, which is why I was leaning towards piston slap, but that's why I tore it down to measure. Working at a yamaha dealership is brutal too, cause I hear new yzs start up every day and sound perfect.

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4 minutes ago, Pooc40 said:

If anything I think it's gnarlier in person. Two of my buddies have yzs, ones a 2020 250x and one is a 2004 250, neither sounds anywhere near as loud as mine. The sound is similar to my buddies 04, but much louder, which is why I was leaning towards piston slap, but that's why I tore it down to measure. Working at a yamaha dealership is brutal too, cause I hear new yzs start up every day and sound perfect.

Yeah the noise is normal just not that damn loud typically.

What head is that? How much compression

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2 minutes ago, Smoking 2's said:

Yeah the noise is normal just not that damn loud typically.

What head is that? How much compression

It's from Apex Technical Innovations. Kind of a small company, I had heard good things about them so I decided to give them a shot. Its their XC cut, supposed to smooth out power, so I'm guessing lower compression than stock. Just messaged them though so we'll see what they say.

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1 hour ago, Smoking 2's said:

Gotta say that does sound excessive for once, maybe just a good mic and it's actually normal IDK

That’s what I was thinking. Wish you revved it in the video to see what happens

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Vertex is cast like ProX and Oem no?  Your piston to cylinder gap is about spot on too for a cast piston.   I run Wiseco forged pistons with a slightly larger clearance and get some slap on a cold motor at idle, but its louder and different that yours .  Mine goes away when warm.  Yours sounds like a lighter softer sound,  a tap tap tap tap consistent with rpm.... almost like a fourstroke rocker arm and not slap as I have experienced.   Does yours go away after warming up?   

Sorry, I'm not much help.  Following to see what transpires.  

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2 minutes ago, Captain.Olives said:

Vertex is cast like ProX and Oem no?  Your piston to cylinder gap is about spot on too for a cast piston.   I run Wiseco forged pistons with a slightly larger clearance and get some slap on a cold motor at idle, but its louder and different that yours .  Mine goes away when warm.  Yours sounds like a lighter softer sound,  a tap tap tap tap consistent with rpm.... almost like a fourstroke rocker arm and not slap as I have experienced.   Does yours go away after warming up?   

Sorry, I'm not much help.  Following to see what transpires.  

Vertex is cast with a Teflon coating.

did you measure the runout in the crank?

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Disclaimer: you have to understand I have never opened a bottom end.  I don't know how they go bad, and I don't know personally the sound of them failing.

I saved this video when I stumbled across it one time, in case I needed it in the future.

It *may* be your crank bearings, but I hope not...

 

 

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32 minutes ago, Sullysully said:

That’s what I was thinking. Wish you revved it in the video to see what happens

I can shoot another video with a rev in a bit. It basically makes the same sound but faster until a certain rev, then goes away, then comes back as the bike decels. Also have not measured runout. Might have to break out the dial indicator. 

Edited by Pooc40

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33 minutes ago, Captain.Olives said:

Vertex is cast like ProX and Oem no?  Your piston to cylinder gap is about spot on too for a cast piston.   I run Wiseco forged pistons with a slightly larger clearance and get some slap on a cold motor at idle, but its louder and different that yours .  Mine goes away when warm.  Yours sounds like a lighter softer sound,  a tap tap tap tap consistent with rpm.... almost like a fourstroke rocker arm and not slap as I have experienced.   Does yours go away after warming up?   

Sorry, I'm not much help.  Following to see what transpires.  

Unfortunately my sound does not diminish after warming up.

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12 minutes ago, egab said:

Disclaimer: you have to understand I have never opened a bottom end.  I don't know how they go bad, and I don't know personally the sound of them failing.

I saved this video when I stumbled across it one time, in case I needed it in the future.

It *may* be your crank bearings, but I hope not...

 

 

Its definitely a good thought. Engine turns over smoothly by hand when broken down. No strange noises or hiccups while turning over. Checked play on stator side and clutch side and couldn't get any movement. Only noise I can get from the bottom end is if I turn the flywheel back and forth I can get a clack clack noise. I imagine it's just from the gears on the clutch side.

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1 hour ago, Pooc40 said:

Its definitely a good thought. Engine turns over smoothly by hand when broken down. No strange noises or hiccups while turning over. Checked play on stator side and clutch side and couldn't get any movement. Only noise I can get from the bottom end is if I turn the flywheel back and forth I can get a clack clack noise. I imagine it's just from the gears on the clutch side.

Crank bearings will often be bad before you can even feel it. I always measure runout before installing a crank. My HR crank was a perfect .01, kudos to them. I wish they welded their pins tbh

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Just now, Sullysully said:

Crank bearings will often be bad before you can even feel it. I always measure runout before installing a crank. My HR crank was a perfect .01, kudos to them. I wish they welded their pins tbh

Right, I hope the crank bearings wouldn't be bad already as they are OEM and have only been in 8 hours..... I'm assuming I could check runout with the crank installed. Just pull the clutch and flywheel? 

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2 minutes ago, Pooc40 said:

Right, I hope the crank bearings wouldn't be bad already as they are OEM and have only been in 8 hours..... I'm assuming I could check runout with the crank installed. Just pull the clutch and flywheel? 

If you do pull the side cover take some pics of primary drive meshing with clutch basket and other parts in that side

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5 minutes ago, Smoking 2's said:

If you do pull the side cover take some pics of primary drive meshing with clutch basket and other parts in that side

Will do, I'll probably break into it to measure runout tomorrow. Guessing its .01mm? Cause .01in would not be particularly impressive. 

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58 minutes ago, Pooc40 said:

Will do, I'll probably break into it to measure runout tomorrow. Guessing its .01mm? Cause .01in would not be particularly impressive. 

I think so, I’d have to check my manual. Even if it’s .01in it’s not meant to be “impressive” just whatever the manual specifies. There are some skilled rebuilders that can get zero runout.

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