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post #1 of 8 (permalink) Old 02-05-2011, 04:50 PM Thread Starter
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YZ 250 Jetting Issue

Howdy all, I have searched the forums and havent found my problem and answer yet. So I figured I would go ahead and ask.

I have a 99 250 2 stroke, bought it from a guy that didnt know anything about it. It has a Pro Circuit pipe, boyeson reeds, and uni filter. The jets in it are stock, the problem Im having is the mid range, it runs great and starts fairly easy, just mid range is horrible, bogs down and then takes off.

What should I jet this thing at is my question. I looked on Pro Circuit website for jetting and they recommend dropping the main to a 168, but all others go up in size why would this one be different and go down?

Thank you.
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post #2 of 8 (permalink) Old 02-05-2011, 08:17 PM
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Mains are for full throttle, needles are mid range
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post #3 of 8 (permalink) Old 02-10-2011, 03:37 PM
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i have a 2000 yz250 running a 168 main but i have a boysen rad valve and a fmf fatty with the short silencer and some mild porting in mine , the reason you have to go down on the jetting is because the way the rad valve`s and the boysen reeds are more responsive and put the fuel in faster and more efficiently.
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post #4 of 8 (permalink) Old 02-11-2011, 05:21 AM
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Quote:
Originally Posted by 696z400 View Post
Howdy all, I have searched the forums and havent found my problem and answer yet. So I figured I would go ahead and ask.

I have a 99 250 2 stroke, bought it from a guy that didnt know anything about it. It has a Pro Circuit pipe, boyeson reeds, and uni filter. The jets in it are stock, the problem Im having is the mid range, it runs great and starts fairly easy, just mid range is horrible, bogs down and then takes off.

What should I jet this thing at is my question. I looked on Pro Circuit website for jetting and they recommend dropping the main to a 168, but all others go up in size why would this one be different and go down?

Thank you.
I understand dropping the main is counterintuitive. - the question is with your complete package what jet do you need?

IMO you will be lost without doing a plug check.
for example lets say the pipe requires a jet size up and the reed required a jet down . stock makes sense in that case , but actually going through the steps to get an actual plug reading is the right answer.

also for example you could have a power valve problem so getting all mixed up guessing on the jetting will make finding a broken power valve ( or some other mechanical problem) difficult .

good luck

David

Last edited by macattack; 02-11-2011 at 08:24 AM.
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post #5 of 8 (permalink) Old 02-11-2011, 04:54 PM
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you are absolutely correct macattack i assumed that maybe he already knew to read the plug , could even need a pilot jet change to help it.
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post #6 of 8 (permalink) Old 02-21-2011, 08:05 PM Thread Starter
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Thank you all for your help, I ended up taking the carb and reeds out and checking everything, come to find out the float was way off it was only allowing about 1/16" fuel in the bowl, and then I leaned out the needle 1 spot. Runs great now.
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post #7 of 8 (permalink) Old 02-21-2011, 10:56 PM
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Is it weird that I have never jetted a carb..

I just dropped my ATV to get worked on and had them do it for me for about $50.. I'm lazy


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post #8 of 8 (permalink) Old 02-25-2011, 11:36 PM
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Jetting carbs is something I don't recommend getting into unless you're planning on getting into it seriously. At one point in time I did a fair bit of it and kept a box with a couple of hundred jets in it, and even that was for one specific make of carb that only had to operate on 3 very similar engines. Proper fueling is important, but it's not something you should buy a "kit" for.

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