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04 YZ 250f jetting question

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Bike is all stock except a FMF Q. The bike still has stock jetting and no o-ring mod yet. The jet sizes are 180 and 42 (stock). Since it has the FMF on it, should it have been jetted? If so what should I change it too for a starting point? The bike is used from 2000-maybe 6000 ft level.

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The bigger question is how does it run? What issues do you experience. You do not arbitrarily stick in jets because you changed something. You change something, test ride and determine if and at what throttle position a problem exists. Then you check to see what jetting you have that controls the fuel at that throttle position (not rpm or gear) and adjust accordingly.

I will venture the louder pipe will not require a change in jetting, just ear plugs. Stock pipe is about as powerful as you can get with a good, smooth spread.

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The bigger question is how does it run? What issues do you experience. You do not arbitrarily stick in jets because you changed something. You change something, test ride and determine if and at what throttle position a problem exists. Then you check to see what jetting you have that controls the fuel at that throttle position (not rpm or gear) and adjust accordingly.

I will venture the louder pipe will not require a change in jetting, just ear plugs. Stock pipe is about as powerful as you can get with a good, smooth spread.

Every bike I have ever owned has benefited from stock jetting being changed from easier starting to increase of power and better throttle response. Even FMF suggests to change jets when you add their silencer. Only problem they only show jetting down to the year of 05.

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Every bike I have ever owned has benefited from stock jetting being changed from easier starting to increase of power and better throttle response. Even FMF suggests to change jets when you add their silencer. Only problem they only show jetting down to the year of 05.

You make it sound like you have a bunch of experience jetting bikes, but your original question seems to be pretty basic. I think William hit the nail on the head with his reply. How is the bike running? Does it stumble at 1/2 throttle? Does it lack power at WOT? Not idling correctly? Answering those questions will tell you which circuit of the carburetor needs attention, and that will in turn indicate whether you need a different main jet, a different pilot jet, or simply a different needle clip position. Give some more specifics and I'm sure there is someone on here that can help.

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Every bike I have ever owned has benefited from stock jetting being changed from easier starting to increase of power and better throttle response. Even FMF suggests to change jets when you add their silencer. Only problem they only show jetting down to the year of 05.

Remember the old saying....."If it isn't broke, don't fix it".

Ride it and see if it needs fixed.

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The blanket statement that "every bike needs to be rejetted" is stating that manufacturers deliberately jet their bikes improperly for most of the buyers. Does that make any sense?

They actually jet for:

68 degrees

50% humidity

30.00 inches mercury

Sea level

As DG and zlathim points out (and you observed) aftermarket companies put out blanket comments and sometimes, none at all. FMF has not mentioned a changed needed for what, 7 years?

The reality is, ride, test, make notes. Adjust only if needed. One of the biggest mistakes people do is assume or worse "Joe Pro's bike runs this jetting, so I should too because I want to be like him". If Joe Pro cashes should you crash too? Ridiculous but that is why you need to be on your own. If... you have a buddy with an identical bike to yours and he methodically jets his bike and gets it perfect and you guys always ride together, then his jetting may (keyword) be correct for you too. Otherwise, you are blindly following a path to who knows where.

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