CR 250 Knock

[Dragmeistr]

Can someone tell me what the most likely cause of a nearly brand new 2003 CR 250 knocking at 1/4 throttle would be? It gets more pronounced as it gets hotter. Stock jetting, plug looks good. I pulled down my exhaust and looked at the front side of the piston and there isnt any major scoring and the backside of the cylinder looks good. Is this a lean knock or is there something else wrong?

 

[Joepro9]

Doe the clutch affect it a all, try pulin in the clutch and such. Also see if you can locate the noise, might be a loose bolt or something. Maybe the gas you are running? How many hours you have on it?

 

[marcusgunby]

If its very rich in that area of the throttle, you can get a hydo lock that sounds a bit like that.

 

[RGalesi]

Could be also old gas (older than two weeks) or even a loose chain making noise... Maybe something wrong int the tranny? It might worth checking it...

 

[jjcobra99]

I posted a thread about the same thing in the 2-stroke forum. Might want to check it out.

 

[Dragmeistr]

Nope, clutch doesnt affect it at all. No loose bolts, I have been totally anal with this bike so far. I have been running 50/50 pump gas and c12 so I doubt it was the gas, and it was mixed and pumped two days before I used it. I went ahead and tore it down just because I would rahter spend 50 bucks now than 300 later. Nothing was wrong with the piston or cylinder, everything looked perfect. I am assuming it was a lean knock at this point. When I get it back together, I will try some jetting changes.

 

[2000SE]

keep us informed on the progress please

 

[Studboy]

I doubt that it is a lean knock Dragmeistr. What you are describing just sounds like your bike is rich on the pilot jet? The uneveness in sound does not mean that it is knocking. Anybody else?

 

[Dragmeistr]

Put the bike back together with a Wiseco piston and broke it in with the jetting that I used at the sand dunes(rich). Went back to stock jetting after break in, bike ran ok, but still had that detonation point. Tried moving the needle, pilot, A/S. Could not get it dialed out. Went and bought A PWK Airstriker, put it on, no knock, throttle repsonse crisp, bike pulls harder now than it did before. The only thing I had to change on that carb was the idle screw.

 

[Studboy]

The stock carb is a Mikuni TMX?

 

[Dragmeistr]

Yep, TMX 38mm, replaced it with the 38mm Keihin PWK quadvent airstriker.

 

[Studboy]

I did find that on my friends RM it was hard to tune the pilot circuit/low speed jetting on his flatslide TMX. Thats interesting, so did the carb provide more power or just more response/crispness?

 

[Dragmeistr]

To be honest, its tough to say, but the bike definately feels more powerful and I know it has a slight bit more top end.

 

[Studboy]

Good Deal.