Jump to content
Sign in to follow this  

How/why does the fuel screw work (NOOB!)

Recommended Posts

How do I adjust the fuel screw for riding conditions (elevation) My rides can start at 5,500 feet and end at 8-9K feet.

can someone explain how I would learn how and WHY to adjust it?

I know on my E I would use the Flex-Jet adjustable one so i can make changes on the fly. but I have no idea what effect it would have and how it would benefit me, I know it's important, but I dont know what to do with it.

Help?

-Zach

Share this post


Link to post
Share on other sites

What you do is, (short hand version) first, let your bike get to full operating temperature, ride it around for a few minutes or so, then, adjust the idle in as low as you can get it to run without the engine cutting off, then, you adjust your fuel screw in until it starts to sputter wanting to cut off, then, start adjusting the fuel screw back out before the motor stalls until it idles itself up to its highest rpm before leveling out and not idleing any higher, the motor will actually idle up from fuel screw adjustment, that is the sweet spot right where you want to be, then you can adjust your actual idle knob up or down which ever need be. Must be at operating temp to be correctly adjusted, you can do this at any altitude and if you have the right pilot jet installed you should have a fairly good bit of adjustment to work with.

Share this post


Link to post
Share on other sites

So this will help with the performance of the bike at altitude?

I has having a bog when I would whack the throttle open, Eddie says it was to rich so I got a leaner jet but I was worried it would limit my performance at my home altitude (5,500ft)

Please excuse my ignorance, but I would be able to have some adjustment in the lean/rich setting with the fuel screw?

Share this post


Link to post
Share on other sites

fuel screw adjustment is really all about idle, and the transition from idle to mid range RPMs, the reason it needs adjusting is Air pressure is greatest at sea level (or below) so your bike is getting Air and fuel at a certain mix (ratio) as you change elevation air gets thinner (ever breathe hard hiking way up in the mountains?) As you go up into the mountains there is less air but your jets are providing the same amount of fuel, so you then need to reduce slightly the amount of fuel the bike gets at idle and you can do that by slightly turning the fuel screw in slightly closing a small passage that allows fuel into the cumbustion chaimber, the result? a snappier throttle, and a more crisp throttle response at least at the crack of the throttle, It has a very small effect on the rest of the throttle range as far as I know... but helpful in the mountains if your bike is already jetted fat.. Hope this helps...

Share this post


Link to post
Share on other sites

The fuel screw and pilot jet control the fuel flow in the idling/pilot circuit...

Needle and Main Jet are the controlling items to be concerned with in respect to performance at altitude..

General rule of thumb is as altitude increases ,jetting size decreases ...i.e.less fuel is required for correct combustion because less oxygen is available ..

The pilot circuit also requires less fuel and can be adjusted clockwise to reduce fuel flow...if altitude increase is significant enough ,you may have to go to a smaller PJ..

Share this post


Link to post
Share on other sites

I fully realize air pressure changes at altitude, I understand rich/lean conditions, but I have little understanding of carbs and their details.

Good to know about the off idle response being effected by the fuel screw, from what it looks like its mostly just a way to tell if the bike has the correct jet at whatever altitude you are at, which does me no good as I probably wont be changing jets in the middle of my ride.

Share this post


Link to post
Share on other sites

currently I have an EMP needle and a 150 MJ, (CE removed)

after speaking to Eddie about my bog he sold me a 140MJ so that should cure my issue, but I have not been able to touch my bike in two weeks so I have not been able to install it. I guess that jump from the 150 to 140 seemed like a big one, but I trust his advice, thus my question about the fuel screw.

-Zach

Share this post


Link to post
Share on other sites

The mixture screw is a parallel path for fuel to take in addition to the idle jet. Say your bike has a 25 idle jet but really needs a 26.39 pilot jet (which is not made). The mixture screw lets the little extra fuel to tweak the idle mixture. If the idle mix is correct the transition to the needle will be smoothest.

The small amount of fuel getting through the idle circuit will have nearly no effect on higher engine speeds.

The reason there is an idle mixture screw but not for higher revs is the idle mixture needs to be much more precise due to the low air flow.

Idle circuit approx idle to 1/4 throttle (above that not enough fuel flow to affect mixture)

Needle approx 1/4 to 3/4 throttle

Main Jet approx over 3/4 throttle

some overlap at changeover points.

Share this post


Link to post
Share on other sites
I fully realize air pressure changes at altitude, I understand rich/lean conditions, but I have little understanding of carbs and their details.

Good to know about the off idle response being effected by the fuel screw, from what it looks like its mostly just a way to tell if the bike has the correct [highlight] PILOT [/highlight]jet at whatever altitude you are at, which does me no good as I probably wont be changing jets in the middle of my ride.

It only affects the pilot circuit

Share this post


Link to post
Share on other sites
which does me no good as I probably wont be changing jets in the middle of my ride.

And now you know why people like fuel injection.

Share this post


Link to post
Share on other sites

if you get to your extreme ride altitude and find it too soft(rich),just remove the airbox door to let in more air to lean out the main jet some.then when you get back down reinstall the door to richen it back up.

  • Like 1

Share this post


Link to post
Share on other sites
The mixture screw is a parallel path for fuel to take in addition to the idle jet. Say your bike has a 25 idle jet but really needs a 26.39 pilot jet (which is not made). The mixture screw lets the little extra fuel to tweak the idle mixture. If the idle mix is correct the transition to the needle will be smoothest.

The small amount of fuel getting through the idle circuit will have nearly no effect on higher engine speeds.

The reason there is an idle mixture screw but not for higher revs is the idle mixture needs to be much more precise due to the low air flow.

Idle circuit approx idle to 1/4 throttle (above that not enough fuel flow to affect mixture)

Needle approx 1/4 to 3/4 throttle

Main Jet approx over 3/4 throttle

some overlap at changeover points.

Nice, simple explanation, thanks.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

  • Similar Content

    • By smokey9lives
      Hi,
      I have a 2003 DRZ (actually a KLX400) and the head tube bearings are shot.  I ordered a new set to install but I'm wondering if anyone has any recommendations on other parts I should replace while I have the front disassembled.  I was thinking about new rubber fork protectors, but are there other things that wear out on the DRZ front ends that you can only get to when disassambled?
      I also broke off a replacement key that I got for the steering lock.  I must have been made of cheap pot-metal.  Should I just remove the whole lock mechanism?
      Any tips or tricks for getting the old bearings out and the new ones in would be welcome!
      Thanks!
    • By antonyp
      Hey Guys,
      I recently bought a 2003 DRZ400S and thought it would be a good idea to check the valves clearance etc . I timed the engine on TDC with the magneto mark aligned as per the manual but I find the cam timing slightly odd. The arrows pointing upward are on the 1st and 15th pin as they should but with the magneto timing mark aligned they are not perfectly straight. Same goes for the arrows that should be parallel to the cylinder head casing. Is this normal or should they be absolutely perfect ?
       

    • By 4strokeOnly
      My bike was stolen, so I'm liquidating my spare parts. 
      I took this off my S and replaced it with an SM unit so my speed would be more accurate with the street wheel setup.  I no longer need this spare.  I greased it regularly and it was in good working order when I removed it.  Included is only what is pictured.  Cable not included.
      Suzuki part number:  54600-29F00
      buyer pays shipping.
×