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jetting question 04 CRF

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my bike seems to run really well on acceleration, but at like mid throttle it has a "lurch" or something like if i'm cruising down a road with constant throttle. how do you cure that? there is also a tiny backfire that happens sometimes when i chop the throttle after some full throttle acceleration. it seems like it should be a simple fix, i just don't know what to change. thanks.

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Sounds like your fuel screw needs adjusting. You should start by turning the fuel screw out 1/4 of a turn and then ride it and see if it improves. The max you should go is 2 1/2 turns out, after that you should go up a size on the pilot jet. The fuel screw is on the bottom of your carb and is recessed and somewhat hard to see so consult your manual and it'll point it out. You can do this without having to remove anything and takes 20 seconds to do...

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i have a zip ty fuel screw, and i adjust it every ride to work the bugs out. but the fuel screw only really affects off idle stuff, and the lurch i'm talking about is not around idle. i think it has something to do with needle position. like i said before it pulls very strong, its just that when i'm not on it full throttle it doesn't run as cleanly as it should.

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Sounds like you need to mess with the needle to me. Given that its getting colder, I'd guess you need to richen it just a tad. Have you changed your Main Jet yet for the colder weather? Up the MJ, and maybe one step richer on the needle and see if that improves things.

Don't forget your PJ, you may need to richen that as well. But I'd start with the MJ and Needle first.

ben

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Sounds like the jetting is a little lean in the middle, usually a pop a.k.a backfire on decel is the pilot jet, just make sure there are no exhaust leaks which causes the popping too, if you go up a size then it will usually take care of the bakefire as well as the lurching at constant 1/2 throttle, the fuel screw will help a little, but doesn't affect anything past 1/8 throttle, and on decel the revs are still pretty high, it bypasses the fuel screw for the most part, a great product to get is a slide rule from mikuni or kehin, you can factor in temp, humidity, altitude, and it's usually pretty accurate and very easy to use, it's like $20.00, i've used in sx and mx and my rider doesn't complain much about jetting,

cheers

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This is the correct answer IMO. ONLY do one thing at a time though! Raise the needle ONE position, drop the clip one. Then ride the bike a loop. If still unhappy raise the main, which can be done in the field very easy. BTW - I am running a 175 in mine, and yes my needle needs to be moved up one in mine also due to the drop in temps. FWIW - 1 1/4 additional turns out (from stock setting) on the mixuture screw approximates bumping the pilot one.

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I think you are going to end up doing both anyway, so why not take the easy wasy out and pop in a new main. I would recommend a 175 if you have removed the backfire screen.

I ride from 4500' to 7500' in elevation. In CO you have to be close to that or higher.

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The 2002 CRF450 came stock with a 175 main jet, although, it had a leaner needle jet. I don't know why Honda changed the jetting so much... Maybe the new airbox?

You won't hurt anything by going to a richer main jet. The proper thing to do would be to go to stock jetting and make small incremental adjustments until you have it dialed in right. Even then you'll have to make small adjustments (fuel screw and maybe 1 size on the main) for the various seasons.

My suggestion... 1. stock setttings 2. Adjust fuel screw out until you no longer have popping on decel. If you have to go more than 3 turns out go to a richer pilot jet. 3. Try one size richer and one size leaner main jet than stock - stick with the one that runs best/strongest.

Don't rule out the JD kit... $79 bucks is cheap ($6,700 sticker price for a new CRF). It comes with an assortment of jets (which you should have anyways), instructions, and the best part is you'll have JD's help right here at TT. Can't beat that. 😢

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The question is, not always what RPM you are at, but what THROTTLE OPENING you are having jetting issues at. You say when cruising down the road, well, that could be like 1/8th opening...in which case fuel screw, pilot jet and needle position are in effect. Not the main...

Tell us more, more, more! 😢 :cry:

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