2006 CR250 Jetting Help

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3/1/2018 6:46 AM
Edited Date/Time: 3/1/2018 9:31 AM

I picked up a new-to-me bike last night. 2006 CR250 with 11 hours on the frame rebuild.

Its got stock internals, bore, stroke, carb, everything is said to be rebuilt with OEM parts.

It also has a FMF fatty works pipe with shorty silencer

He mentioned that he was only able to get it to run correctly with straight VP C12 40:1. He said when he tried to mix 50/50 pump gas "it sounded like a coke can rattling in the cylinder"

Isn't there something I can do to make this run better on either straight pump or 50/50 mix? I don't have any C12 and I'm wanting to go riding this weekend, also I would be wanting to run VP T2 over C12 if I were to buy race fuel.

I'm clueless when it comes to jetting a 2 stroke. I had a YZ250 last year and it ended up running perfect with the mixture I went with so I never had to adjust.

Any help here?

Thanks in advance!

Jonah

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3/1/2018 7:10 AM

I'm in the process of fine tuning an 06 model. Maybe I can help steer you in the right direction.

What is the current jetting in the bike ? Is it the stock oem TMX ? What are your temps, humidity & elevation ? Will you be riding mx or woods type of riding ?

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Speak softly and carry a big stick.

3/1/2018 7:15 AM

H4L wrote:

I'm in the process of fine tuning an 06 model. Maybe I can help steer you in the right direction.

What is the current jetting in the bike ? Is it the stock oem TMX ? What are your temps, humidity & elevation ? Will you be riding mx or woods type of riding ?

I will confirm tonight but he says it was probably jetted for the fuel and pipe.

I live in Texas. Elevation 500' max, warm temps, humid as hell. and I'll be riding MX.

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3/1/2018 10:27 AM

H4L wrote:

I'm in the process of fine tuning an 06 model. Maybe I can help steer you in the right direction.

What is the current jetting in the bike ? Is it the stock oem TMX ? What are your temps, humidity & elevation ? Will you be riding mx or woods type of riding ?

jonahdru222 wrote:

I will confirm tonight but he says it was probably jetted for the fuel and pipe.

I live in Texas. Elevation 500' max, warm temps, humid as hell. and I'll be riding MX.

I'm a little south of Houston so maybe I can help. In all my CR250s, I've ran the JD jet kits in the stock Mikuni and the bikes ran great. Even have one in my 07 CR125. Never could get the jetting right moving the needle clip and switching out jets on the stock setup. I think the difference is in JDs tapered needle they give you to use. I just through in the jets and needle recommended with the JD instruction and the bikes ran crisp. With that being said, I never tried the Keihin carb that seems to be so popular.

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3/1/2018 10:53 AM

Classic detonation. The "Coke can" sound the dude was referring to was the fuel exploding before TDC because the engine is running too hot. The mixture will detonate before the spark once compression reaches a tipping point on the upward stroke. As you might guess, this is hard on pistons. Running straight race fuel helped because the extra octane in the fuel cooled down the total mixture and burned a little slower. The Fatty pipe is adding to the problem by flowing more air through the engine, and I'm guessing the previous owner did not change the jetting to compensate.

The first step is to richen your main jet. This will likely fix all the problems, or at least get you 90% of the way there. Run the carb out of fuel if you have a drain screw. Loosen your clamps and turn the carb sideways (you may need to remove the cap of the carb since the throttle cable won't always give you enough play.) Then, remove the plug on the float bowl (Have a rag handy if you didn't drain the fuel,) and find the main jet. It is a hexagonal brass fitting that has a small hole bored in it. It should be located just about in the center of the hole left by the missing float bowl plug. There is another brass fitting just forward of that one called the pilot jet; we'll get to that later if necessary. Remove the main jet with a 6mm socket and look for the size etched on the outside. Go get a few jets that are the next few sizes larger at your local moto shop and replace with one which is 2 sizes larger.

That will get you in the ballpark; then you can see how it's running and let us know if there are still any issues.

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Braaapin' aint easy.

3/1/2018 11:19 AM

Falcon wrote:

Classic detonation. The "Coke can" sound the dude was referring to was the fuel exploding before TDC because the engine is running too hot. The mixture will detonate before the spark once compression reaches a tipping point on the upward stroke. As you might guess, this is hard on pistons. Running straight race fuel helped because the extra octane in the fuel cooled down the total mixture and burned a little slower. The Fatty pipe is adding to the problem by flowing more air through the engine, and I'm guessing the previous owner did not change the jetting to compensate.

The first step is to richen your main jet. This will likely fix all the problems, or at least get you 90% of the way there. Run the carb out of fuel if you have a drain screw. Loosen your clamps and turn the carb sideways (you may need to remove the cap of the carb since the throttle cable won't always give you enough play.) Then, remove the plug on the float bowl (Have a rag handy if you didn't drain the fuel,) and find the main jet. It is a hexagonal brass fitting that has a small hole bored in it. It should be located just about in the center of the hole left by the missing float bowl plug. There is another brass fitting just forward of that one called the pilot jet; we'll get to that later if necessary. Remove the main jet with a 6mm socket and look for the size etched on the outside. Go get a few jets that are the next few sizes larger at your local moto shop and replace with one which is 2 sizes larger.

That will get you in the ballpark; then you can see how it's running and let us know if there are still any issues.

Wow thanks for the elaborate response! I'll let you guys know once i try it out.

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3/1/2018 12:06 PM

Fix your squish first, then fiddle with jetting. The stock squish is likely to be greater than 0.080". After having it set on my previous 2005 model, the stock-ish jetting worked fine.

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3/1/2018 6:09 PM

jonahdru222 wrote:

I will confirm tonight but he says it was probably jetted for the fuel and pipe.

I live in Texas. Elevation 500' max, warm temps, humid as hell. and I'll be riding MX.

Sounds good.
FYI - The MJ works in the 3/4 - full throttle range & is only part of the equation when jetting the carb. When is the rattle happening at ? Idle - 1/4, midrange or WOT ? One of the 1st things I'd check is make sure the carb is clean inside with no clogged jets & the float level set to oem specs.

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Speak softly and carry a big stick.

3/1/2018 6:17 PM

Assuming the engine was rebuilt to oem specs. It doesn't have the cyl base or head shaved as that will raise the compression. Have you taken a comp. reading of the engine ? That can be a good indicator if the cyl. base or head have been modified. Just some things to check before getting into jetting the carb is know what has been done to the engine. Unfortunately I've had bad experiences when buying used dirt bikes & take the previous owners info given at face value.

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Speak softly and carry a big stick.

3/1/2018 8:16 PM
Edited Date/Time: 3/1/2018 8:16 PM

A little tip. You have a municipal airport close by called Taylor Municipal Airport that sells 100LL AVGAS for $3.80/gallon. Just go there with some cans and fill up. Cheap enough to run it 100%, its a lot more consistent than pump gas, and has a low lead content for some added protection. I run it straight in my KX 125 and it loves it.

https://www.airnav.com/airport/T74/A

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