Yz250 rattle at 1/3 throttle under load

[Radmaverick69]

Hi guys. I have a 2007 yz250. The kid I got it from says it had 12 hours on top end and 50 total. I’ve got no reason to not believe him, but wouldn’t bet serious money one a craigslist seller either. Anyways, I have this rattle/ knocking sound at about 1/3 - 1/2 throttle under load only. Not sure what the deal is. I’ve got a ktm 300 as well so I’m not new to two strokes but this is my first yz. It does sound like it’s coming from the front of the engine, definitely  not the chain. Any tips? The gas was old, so I ran my ktm fuel 50:1 in it and no change. Ram that old gas 32:1 and same sound. 

[GSP2]

It’s the resonate frequency of the bike. No counterbalancer.  

[Radmaverick69]

And it sounds like a rattle to knocking sound? Doesn’t sound like a vibration to me. That’s what I would think of with a resonance sound. I originally thought it was my pipe gird and took that off and still same sound. 

[Piney Woods]

At low RPM and under a decent load my CR250 has a rattle that comes from the power valves. I have never even ridden a YZ but it sounds like that could be happening also.

[Radmaverick69]

That’s kinda what I was thinking also. The sound happens right before they feel like the power valve is opening. Right in that range. So that sound would be normal then? 

[Kinger317]

 I wouldn’t say it’s normal but 2007 could be somethings worn or broken in there?...

[ss-racing66]

1 hour ago, Radmaverick69 said:

Anyways, I have this rattle/ knocking sound at about 1/3 - 1/2 throttle under load only. Not sure what the deal is.

I’d prolly recommend you do some investigating. I have a 2005, and I don’t have any rattling/knocking at 1/3-1/2 throttle 

[mlatour]

As with any used 2-stroke, no matter what the seller said,

(keeping in mind 50 hours on a 2007 is an average of having only being ridden 3 hours or so per year....)

investing in a set of top end gaskets/seals and having a preventative look inside would be a wise choice.

 

A quick check of the plug, top of piston and combustion chamber could perhaps reveal signs of detonation (not uncommon for these bikes)

and, any excess play in the connecting rod/crank/bearings of a sooner than expected engine refurb.

 

Edited July 21, 2022 by mlatour

[Motomofo1]

Hard to diagnose over the internet… are you running pump gas? Put race gas in it and see if the noise goes away. These bikes often have detonation issues when running pump gas. 

[mog]

Detonation will kill an engine in a few rides ,trashes the entire top end 

 

[Radmaverick69]

So rode again tonight. It’s weird. It’s only at that certain rpm and clutch must be out and fuel on. I can be coasting down a hill and have the bike in a lower gear and no noise. But give it a little throttle just barely and the sound is there at that certain rpm. The bike absolutely screams and sounds great at the upper rev range. Wouldn’t detonation be at the upper range the most and under heavy load only? This rattle comes at all time in this certain Rpm range. If I take the top end off what would I be looking for as far as problems? 

[Piney Woods]

Are you riding in the woods. Woods is all I do and that creates the situation I described very often. I am also a gear high rider which magnifies the effect. My thought remains, good luck..

[mlatour]

Since it's a YZ250 with known detonation issues we quickly jump to that conclusion but...

if it's happening only in a very narrow range of rpm/engine load/throttle opening

(especially if not accelerating/decelerating but just coasting/cruising/lugging at part-throttle)

it could be as simple as 'chain slap' which is common on many bikes (but more so on 4-strokes I think)

 

Next time it happens, try either downshifting/upshifting one gear and/or, changing throttle position ever so slightly.

'chain slap' isn't caused by a loose chain but rather the 'harmonics' of the whole drivetrain under certain loads.

 

Though still confirm there are no signs of detonation.

 

Edited July 24, 2022 by mlatour

[Radmaverick69]

Okay great. Thank you. What is the best way to rule out detonation? Just use race fuel and see? 

[mlatour]

As I had mentioned earlier, for the cheap cost of a couple of head gasket o-rings,

I'd have a closer look at the piston's crown and cylinder head.

[Radmaverick69]

Just through the exhaust port or take off the top end? Will the exhaust port be enough? 

[mlatour]

8 hours ago, Radmaverick69 said:

Just through the exhaust port or take off the top end? Will the exhaust port be enough? 

Clues of detonation won't be seen that way.

You don't have to take the whole top end apart, just remove the cylinder head.

Drain the coolant, remove top engine mount bracket, spark plug and 6 cylinder head nuts.

The two cylinder head o-rings are like 6-7$ each.

 

Edited July 24, 2022 by mlatour

[Motomofo1]

A YZ250 that is pinging on pump gas will start rattling not long after you open the throttle, but SHOULD keep doing so beyond half throttle. That being said, situations like this are process of elimination. Cross it off the list so you don’t have to wonder or speculate. YZs (especially before the “global spec” head started being used) are infamous for detonating on pump gas with the stock head. 
 

I used to ride with a local A class offroad guy who didn’t know what detonation was… someone had told him that YZ250s were just “noisy.” He had close to 100 hours on his new bike before he learned the importance of race gas. Surprisingly, the bike never blew up on him. 

[Radmaverick69]

Got it. Yes I’m gonna look into it. I have also been told I may be a little lean on the bottom end. I’m gonna richen it up today and test it. I understand the detonation. But I’d imagine it’s the same as cars, when you increase elevation you can run lower octane fuel with no issues. I’m in Colorado at 5000 ft so I’m thinking pump gas may be just fine. I’ll see today. 

[Radmaverick69]

So bad news. The bike absolutely Grenaded. I richened the fuel up a little bit and it sounded better but went out on a ride yesterday. And after about 10 mins the bike had a loud snap and then the back end locked up and slid to a stop. Took off the top end and the whole rod was demolished. Engine cases seem to be fine so far but the cylinder is destroyed. Gonna have to replace that as well. How does my piston look? Thanks.