fuel screw problems


7 replies to this topic
  • rasta

Posted September 24, 2005 - 09:49 PM

#1

put msr air fuel screws on my 02 426 and 03 450 nothing but problems, i live and ride at sea level the bikes are doing alright not the same as before, when i go to about 3000-4000 feet to ride , bikes run like crap, i keep playing with them but no luck. they say to turn screws til you get the highest idle but no luck, bikes keep running like crap. anybody else have this problem? any advice? thanks

  • bg10459

Posted September 25, 2005 - 04:21 AM

#2

The pilot screw is the fine adjust for your pilot circuit (pilot jet). When you go from sea level to 3 or 4000', your pilot circuit (as well as all other circuits) is starting to lean out. If your pilot jet is already on the lean side (maybe you're 2 or more turns out on the screw at sea level) it will be worse at altitude. If you regularly ride in this range of altitudes, you probably will want a pilot jet (and again, maybe all other circuits) that is slightly richer at sea level, so you have the adjustability on the screw when you get up in elevation. Try one size bigger on the pilot at fewer turns out when at sea level, and turn it out as you ascend.

  • SureBlue

Posted September 25, 2005 - 05:41 AM

#3

Try one size bigger on the pilot at fewer turns out when at sea level, and turn it out as you ascend.


Incorrect. Turn it IN when you ascend. Turning out the fuel screw increases gas and the mixture becomes richer. When you ascend (go riding at higher altitude) you need to turn the fuel screw in to make it leaner. It is the opposite to 2-strokes' airscrew where you turn it out to give it more air.

Rasta, replacing the stock fuel screw with adjustable screws don't change anything if you adjust the screws to the same spot as they were adjusted with stock screws. It only gives easier access to the screw so you can adjust it to your riding altitude, temperature and humidity.
If your bike runs fine at sea level, just screw the fuel screw in (clockwise) a half to full turn from the stock screw starting point to lean it out as oxygen levels drop at higher altitude.
Humidity replaces oxygen, so the higher humidity, the less gas the bike needs since there is less oxygen.
Cooler air temps contain more oxygen, so you also have to add gas, that is turn out the fuel screw.

It's not really that difficult, but takes some testing to find the correct settings. Start with 2 turns out from seated position and try. If it's lean, open it up. In case you need to open it over 2,5 turns, you should go for a richer pilot, possibly also a richer main. But first find out the pilot circuit and when it is fine try to find out the correct main. In case you need to go richer both on pilot and main, you may also need to raise the needle one clip (put the clip one notch down in needle), that also increases fuel at 1/2 throttle.
And naturally vice versa in case you need to go leaner.

  • bg10459

Posted September 25, 2005 - 05:01 PM

#4

Strike that, reverse it.

SureBlue is right, my mistake. :banghead:

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  • bg10459

Posted September 25, 2005 - 05:06 PM

#5

The pilot screw is the fine adjust for your pilot circuit (pilot jet). When you go from sea level to 3 or 4000', your pilot circuit (as well as all other circuits) is starting to richen. If your pilot jet is already on the rich side (maybe you're 1 turn or less on the screw at sea level) it will be worse at altitude. If you regularly ride in this range of altitudes, you probably will want a pilot jet (and again, maybe all other circuits) that is slightly leaner at sea level, so you have the adjustability on the screw when you get up in elevation. Try one size smaller on the pilot at more turns out when at sea level, and turn it in as you ascend. Remember though, richer is better than leaner, so do't get it too lean at seal level.

Hope my brain fart didn't cuase you any trouble.

  • yzf450jack

Posted September 25, 2005 - 06:08 PM

#6

There is a O-ring -washer- spring- fuel screw if you loose the O ring it may be lean and getting a air leak it's so easy to loose were did it go :applause: :banghead:

  • lee_nuttall

Posted September 27, 2005 - 05:09 AM

#7

I'm trying to install an MSR screw on my 05 CRF250X. It appears that I'm going to have to pull the carb out to get the stupid thing to turn far enough so I can back out the stock screw and install the MSR thumb screw . . . can anyone verify this?

Also I want to be sure I'm putting it in the right place. My manual doesn't have a picture of where the stock air-fuel screw is. I'm assuming it is the slotted screw that sticks out the bottom . . . any help here?

  • MotoGoalie

Posted September 27, 2005 - 09:47 AM

#8

I'm trying to install an MSR screw on my 05 CRF250X. It appears that I'm going to have to pull the carb out to get the stupid thing to turn far enough so I can back out the stock screw and install the MSR thumb screw . . . can anyone verify this?

Also I want to be sure I'm putting it in the right place. My manual doesn't have a picture of where the stock air-fuel screw is. I'm assuming it is the slotted screw that sticks out the bottom . . . any help here?



The bottom front of the carb. Slotted fuel screw. Yes.





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