to re-jet or not to re-jet

Joined
Jul 8, 2007
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#1
old cr 80 trys so hard to get to a smooth power band but always jumps in and out. it runs great until full throttle. ive heard rejet or new reeds. my needle is set on the lowest tick pulling it up as far as it will go and my main jet is ajusted as far down as it will go. is it still not getting enough gas? should i get a leaner or richer needle. please dont tell me that i have to mess with the reeds... but i guess you learn sometime

i havent done a plug test because i wasnt sure if i would get an accurate reading with it acting up
Thanks
 

RM_guy

Moderator /
Damn Yankees
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Nov 21, 2000
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#2
Full throttle is the main jet but can be affected somewhat by the needle. I suggest reading this guide http://justkdx.dirtrider.net/carbtuning.html and going from there.

Reeds will affect low end much more than the top end so unless they are really trashed I'd stick with the jetting. Just make sure everything is clean and up to spec before messing with the jets.
 
Joined
Jul 18, 2007
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#3
move your needle position up at least one, right now your running it too lean from 1/4-3/4 throttle. also what do you mean by your main jet is adjusted all the way down, on all the carbs i have seen, the main jet is screwed in all the way.
 
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#4
my main jet has this thin nut on it allowing you to back the main jet out and still tighten it down right now i have it barely screwed in and i snugged the nut up agains the carb so it stays in but i bought the bike used so it might not be right. is it not getting enough gas??? i could get a bigger main no prob and what else should i clean on the bike i clean the air filter, carb, and exhaust
 
Joined
Apr 18, 2006
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#5
Are you sure that it is the main jet? What you described sounds like the idle jet.

Most dirt bike carburetors have three adjustments. The easist to adjust is the idle mixture,which is usually a needle screw from the outside. Either a spring or jam nut is used to keep the idle jet in place.

The metering needle moves up an down with the throttle slide. You have to remove the top of the carburetor to get to it but that is usually a fairly easy operation. The metering needle can be adjusted up or down by moving a circlip.

The main jet is buried deep in the float bowl of the carburetor. To get to it you need to completely disassemble the carburetor and usually remove the float. The jet usually can not be "adjusted", you need to replace it with a different size.

If your problem is at high speed then you can pretty much rule out the idle jet.

If by chance you actually do have an externally adjusted main jet then it sounds to me like you have something backwards. You said: "main jet is ajusted as far down as it will go". When you have an adjustable jet turning the screw in will restrict the flow of gas, making it lean. Usually, "as far as it will go" is when the needle hits the seat, which if the seat hasn't been damaged will shut the flow of fuel off. To adjust the mixture rich you back the screw out. "As far as it will go" is when the needle falls out, although it would have quit having any effect long before that.

If you are adjusting a main jet lean, then adjusting your metering jet rich, the two are fighting each other. But, like I said above, I think you have been playing with the idle mixture, not the main.

Rod