new FMF jet kit installed


8 replies to this topic
  • rmx

Posted January 05, 2009 - 09:36 PM

#1

Just rejetted my carb and installed FMF spark arrestor along with a moose replacement air filter. As of right now:

Model: 2000 YZ426
Edit Revision Date:
Main Jet: 175
Pilot Jet: 42 (stock)
Starter Jet: 72 (stock)
Leak Jet (if any): (stock)
Fuel screw/Setting: stock @ 3 turns
Needle/Position: FMF progressive taper @ 2nd ring
Filter: Moose
Pipe/muffler: FMF Ti-4 w/header
Altitude Range: 0-100 Florida Panhandle (PCBeach)
Temp Range: 45-75
Humidity Range: 75-95

Comments: was 10 by the time I got finished so couldn't turn it up much or ride it. It was harder to start but not bad. idles very nice and throttle response was incredible. I'm hoping it fairs as well under load :thinking:

  • grayracer513

Posted January 05, 2009 - 09:51 PM

#2

Help out by posting this HERE

  • rmx

Posted January 06, 2009 - 09:45 PM

#3

I used the template so I do know about the link. Your post on that link said not to post discussions there, to use these forums.
To that end, I posted here since my jetting is a bit out from the norm from other riders. Also, the needle is FMF and set to the second notch...most of you posted using the fourth notch with another brand. Is this simply the lower the notch(higher number) used the more fuel can pass by the taper? Unfortunately I wont get to ride until the weekend but ran it at my work today and it is alot smoother and the popping has gone away during decel. So when I do a plug test and find out if I am lean or rich should I be adjusting the mix screw or clip position on the needle?

  • grayracer513

Posted January 07, 2009 - 07:39 AM

#4

I used the template so I do know about the link. Your post on that link said not to post discussions there, to use these forums.
To that end, I posted here since my jetting is a bit out from the norm from other riders. Also, the needle is FMF and set to the second notch...most of you posted using the fourth notch with another brand. Is this simply the lower the notch(higher number) used the more fuel can pass by the taper? Unfortunately I wont get to ride until the weekend but ran it at my work today and it is alot smoother and the popping has gone away during decel. So when I do a plug test and find out if I am lean or rich should I be adjusting the mix screw or clip position on the needle?

First, post your jetting there anyway. If it needs revision later, simply edit the post, note the revision date, and update your comments. You can still discuss any issues here.

The fact that your jetting is out of the norm, as you say, makes it more valuable (as long as it actually works). If everyone ended up with the exact same settings, what would be the point in posting that?

You should review the tuning section at the back of your manual, and if you don't have one, download one using the links down the page in the Common Threads Sticky. It goes into the effect of each component on various throttle opening ranges.

A plug check is virtually worthless on unleaded gas, to start with, since the lead is where the bulk of the color came from. The only part of the insulator of a plug run on UL that will really tell you anything is the narrow ring of color that develops at the base of the insulator, right against the shell of the plug, and you can't really see that unless you cut the plug open. Otherwise, the insulator will come out bone white on a clean main jet chop, and it will tell you nothing.

Then too, a plug check is done by making a full throttle run, chopping the throttle, and killing the engine, rolling to a stop. The plug is then removed and read without running the engine any longer. Such a check tells you only about the main jet, because the main jet does over 95% of all the fuel metering at full throttle. Pilot screw and needle adjustments have no effect on full throttle operation. If you do a plug check using leaded fuel, use a used plug in good condition, so there's some color on the plug to begin with. Clean new plugs won't show you enough in one run to be useful.

The pilot screw is effective from idle up to around 1/8 throttle, and can influence things up to 1/4 under some circumstances.

The needle is like two metering components in one. The straight part above the taper is effective from around 1/8 to 1/4 or so, and helps with the transition from the pilot circuit to the main metering circuit. Needles are available in varying diameters having the same taper cut and height, so this aspect can be changed either independently or in concert with other changes.

The tapered section of the needle starts to take over as it is drawn up past the top edge of the needle jet (discharge nozzle, emulsion tube, whatever) at around 1/4 throttle, and meters fuel from that point up to around 3/4 throttle, where its influence tapers off sharply, giving way to the main jet. And you're correct, the higher the needle clip, the lower the needle sits in the jet, and the leaner it makes the mixture. But, obviously, you can't compare your original needle position to your new one, since we have no way of knowing how the needles themselves differ.

It should be remembered that the needle doesn't operate independently. It modifies the amount of fuel that passes through the main jet. At 1/4 throttle, the size of the main doesn't make a great deal of difference. You could have, say, a 168 in it one time, and a 185 the next, and not tell much difference. But as the throttle opens farther, the amount of fuel becomes more and more dependent on the main jet, finally having almost nothing to do with any other component.

OEM FCR needles are almost all produced with a single straight taper. Keihin builds carbs, not motorcycles, and they make a wide enough range of metering components available for the motorcycle manufacturers to pick from to get things working more than reasonably well. Aftermarket jetting specialists like JD and FMF offer some specialty needles with dual or radiused tapers to address specific situations in specific engines, and if your bike needs one of these, it's the only thing that will produce the best results.

So basically, you need to get the pilot circuit ball parked first, then get the main jet in range, then settle the needle. Then, go back and fine tune the pilot and main.

Here's a guide by Eddie Sisneros to the basics of jetting by the seat of your pants, as opposed to a dyno and a gas analyzer:

http://www.thumperta...699#post2881699

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • rmx

Posted January 07, 2009 - 03:15 PM

#5

Thanks, rereading his(burned) post after reading your reply made more sense. I posted the template to the jetting link and will update it after a good weekend run. What I have rode so far now leaves me with other problems...bike runs like a contender but I can't keep traction. My buddies' 426/450 conversion is running a smaller rear sprocket and wider tire and shreds me of the line...might be going there when more funds are available. Time to go read the manual's tuning section, thanks again.

  • rmx

Posted January 10, 2009 - 07:43 PM

#6

Well the rejetting works out great with throttle opened up...but the idle is killing me. I read the back of the book on tuning and think I get it but don't now how to adjust the Pilot air jet. I know it is preset but I am idling at 844 degrees at the bend of the header. Before I change out the jet would like to try the air jet...any experience doing this?

  • grayracer513

Posted January 11, 2009 - 09:57 PM

#7

Read How to confirm your Pilot Settings

On the temp readings, how are you taking those?

Bear in mind that jetting is not to only influence on idle exhaust temperature. Ignition and cam timing both have a major influence on exhaust gas temp at idle, and a normal, healthy, properly jetted and tuned YZ450 will have the exterior surface of a titanium header glowing visibly red in the shade inside of 2-3 minutes, guaranteed.

  • rmx

Posted January 12, 2009 - 05:17 PM

#8

I am a heavy equipment mechanic, I used a thermal scanner I use reading block temp and radiator end caps. I know a well tuned bike is not set up to idle, the temp just seems high.

  • grayracer513

Posted January 13, 2009 - 08:28 PM

#9

885˚ F is the temp at which metal glows visibly red in twilight, and that is entirely normal for a YZ450.





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