Need Re-Jetting Help? ~YFZ450 Jetting Guide Inside~

James_Dean · 10-07-2003, 07:31 PM

There is some truth to the reference. A little popping is not a major concern and is a lean deceleration condition. From 0 - 1/4 throttle is where the condition occurs.

The popping isn't necessary for good performance though, and can be reduced by adjusting the main circuit (as mentioned) for deceleration. The needle straight diameter is what meters fuel from the main circuit on closed throttle deceleration. The GYTR needle has a leaner diameter and produces the popping unless you increase the pilot jet to richen for it, but this also richens the idle mixture. The JDJetting kit for the YFZ has 2 needles, one is richer(blue marked) and the other is close to the GYTR in diameter(red) so you can adjust with either diameter if you want. The blue marked needle rarely pops on deceleration.

I run an air/fuel gauge on my handlebars for testing and make test runs from lean to rich settings on each component in the FCR carburetors. The gauge always shows a lean condition in unison with the popping on deceleration.

James

www.JDJetting.com

FAST1 · 10-07-2003, 08:32 PM

James
Thanks for responding on this jetting subject.

Your synopsis appears to differ somewhat from the Mikuni site in terms of what causes the popping and occasional backfire. Based on your observation it can be cured with a different taper needle.

Any idea why Mikuni states this?
When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.

Is the main system delivering any fuel when your not applying any throttle and downshifting through the gears while braking? Essentially isn't this what the Mikuni site states? Is fuel really flowing through the main and needle during idle throttle settings?

Or is it a result of the special carb on the YFZ?

DirtDragon · 10-08-2003, 09:17 PM

I currently have lid and sparky removed, cam mod and uni filter. Based on feedback in this forum I jetted at 45pilot, stock position on needle and 165 main. 2.5 turns on screw. It pulled real good but seemed to lack midrange and there was popping on decel. So, I put in a 170 main, moved clip to 6th position and went to 48pilot. screw 2.75 turns out.

The idle seemed to be much higher and it seems like it has more lowend now. Pulls extremly smooth all the way to the top with no popping or backfires on letoff. I was pretty happy until I did some drag racing and found I was loosing almost 2 lenghts in 300'. Could I be too rich even though it appears to running well? I'll be checking the plug tomorrow but I'm running the CR8IEX plug so not really sure how to check it since the electrode is so small. Any suggestions?

Pahrump · 10-08-2003, 11:00 PM

Depending upon your elevation you may want to drop to a 168 main and put the screw at 2 turns

James_Dean · 10-09-2003, 01:30 AM

Quote:

Is the main system delivering any fuel when your not applying any throttle and downshifting through the gears while braking? Essentially isn't this what the Mikuni site states? Is fuel really flowing through the main and needle during idle throttle settings?

Or is it a result of the special carb on the YFZ?

Fast1,

Yes, this is the case. The Mikuni reference states the main circuit doesn't deliver fuel at closed throttle, but under deceleration it does provide fuel and becomes more important (think engine speed in rpm at closed throttle). At idle, the main circuit is not nearly as much of a factor. Fuel will flow thru the main circuit on deceleration and changing nothing but the needle straight diameter will impact the fuel/air mixture and the popping out the exhaust. I've seen it and heard it enough to know and invested more in brass needles to test than most could imagine.

The reference is avoiding the lengthy discussion on slide cutaway, needle diameters, and nozzle screen effects that all enter into what happens thru the main circuit at varied engine speeds and closed throttle. Keeping the discussion simple avoids having to explain these aspects. The older Mikuni carbs may have had slides with more cutaway and less influence from the main circuit, but as an example, the '78 YZ250 was a typical plug fouling bike on downhills and it had a Mikuni with small(rich) needle diameter causing the closed throttle jetting problems.

James

FAST1 · 10-09-2003, 12:28 PM

Hey James
Thanks for the details on this subject...

I am hoping you might offer some more advise. Although I don't have your jetting needles, I am wondering if you might offer some of your jetting advise based on the following mods to my YFZ? Also are your needles available for purchase without actually purchasing your entire jetting kit?

1000 foot elevation
40 to 70 degrees F
Full GYTR carbon exhaust and GYTR header
removed a/b lid with GYTR dual stage foam filter
no cam mod at this time
GYTR needle

Also what would you recommend with your JD jetting kit?

Thanks again for takig the time to contribute on this subject and sharing your knowledge.

edgerat · 10-09-2003, 02:42 PM

Fast1,
JD helped me jet my YFZ in person so I feel confident in answering your jetting question. Clip #4 (GYTR NCVQ) needle
168 main
45 pilot
I highly recommend the JD jet kit because when you use the blue needle that comes in his kit the midrange top-end pull in unreal. You dont get that with the GYTr needle. In my opinion they are far too lean.
Also if you have jetting questions about the JD jet kit you could read the first post in this thread that I wrote to get a general idea on where to jet your quad.

FAST1 · 10-09-2003, 09:21 PM

Edge

got the jetting settings in tonight ,, that you suggested.

Although I didn't get to run it much.. just close by the house, this is what I discovered right off the bat...

1st.. I now have the "fall flat on your face" symptom when I stab the throttle from a near idle position while in 1st gear.

2nd When in neutral, and free reving the engine slowly from idle.. I find that the engine runs fairly smoothly for the first 1/8 or so throttle (I've got a 70 degree twist).. however when I hit a 1/4 to 1/3 thorttle.. the engine starts flubbering,, popping and occasionaly small backfires..this doesn't change up to 1/2 throttle while in neutral.. didn't want to run it anymore r's than that in neutral.

I had neither of these two problems while jetted at 165 main, 3rd clip position on GYTR needle with a 45 pilot.. with GYTR dual stage filter, top off and sparky out.

I think I may be rich in the mid range and may have to drop back to the third clip position and maybe back to the 165 main.

I did notice that the GYTR header has a smaller diameter header for the first 8 or so inches of pipe from the cylinder than the stock header.. the rest of the header appeared identical in diameter to the stock header.

Your thoughts on correcting this?

Daddog, I respect your opinion too.. if you care to voice it.

Thanks

Pahrump · 10-09-2003, 10:45 PM

I am not either one of the ones you asked for but I would try putting the needle at the #3 clip position. But for sure keep the 168 main in there.

edgerat · 10-10-2003, 12:12 PM

Fast,
The biggest thing that I learned from James Dean when I met him was that with this big of a PUMPER carb you will not be able to snap the throttle open from idle to WOT. The carb just isnt efficient enough to do this. You can give it a little bit of throttle to get it off idle then snap it WOT and it will be fine. As far as your 1/4-1/2 stumbling, my YFZ exhibited this same condition when I tried to use the JD Red needle. So I think if anything it is lean. Also, for the most part my jetting FAQ was just so I could give guys a solid guideline to help them jet their bikes. And as a side note to get rid of 99% of the redundant posts that we see. It is in no way a dead-bang once and for all thing for all setups.
Isaac

FAST1 · 10-10-2003, 12:43 PM

Ok

I totally understand your reason for this thread..

Should I drop the clip on the needle or just install a 170 main?

I apologize about the multiple posts on this subject but I'm just trying to figure this out and it has been a bit challenging..

The reason I mentioned the "fall on your face" throttle reponse is because with the sparky out and lid off (before installing the pipe and re-jetting again to compensate for the pipe) I did not have this at all. Shouldn't it still perform like it had?

edgerat · 10-10-2003, 01:22 PM

well if you think about it you are giving the engine alot more air while also letting it dump alot more air. So you have exceeded the settings that the airbox/engine combo was designed to work with.
Always keep these in mind when looking at jetting these are the following adjustments that affect the throttle settings:

mixture screw 1/8 throttle
pilot screw 1/8-1/4 throttle
needle 1/4-3/4
main jet 3/4-full throttle

Now the needle does affect all of these as well because the needle is the only component that is physically moving. But, those are the main throttle settings that will affect your jetting. So, if you are having a popping weird condition when you try and roll the throttle on and it occurs at 1/4 throttle or so I would say try a different pilot jet. You really arent going to hurt anything if you are just testing these settings in your driveway. I suggest messing with all of these settings on your own so you can see what each of them do.

Pahrump · 10-11-2003, 09:15 AM

Great advice Edgerat

RiPPiNiTuP7 · 10-14-2003, 10:28 PM

for the people that are ALL STOCK..have u tried rejetting to a 168 main, 45 pilot, and 4th notch on needle? were the results good?

Pahrump · 10-14-2003, 11:01 PM

Unless you are at sea level or below the 168 may be a little rich, but hey taking the lid off and pulling the baffle only takes like 2 minutes and will get you about 5HP along with your jetting mentioned above.

YFZdream450 · 10-16-2003, 06:08 PM

Ok, heres what i have...Baffle out, Cam mod, K&N filter, lid off. Whats the specs for that?
Someone said
-48 pilot
-180 main
-and YZ needle, but i dont know what clip to put it on.
And if thats right, where should the specs be when i put the baffle back in?

And what about that airscrrew, i have no idea haow to deal with that, and what to do

YFZdream450 · 10-16-2003, 06:36 PM

Is there a step by step guide to how to jet the carb....thats what i need

YFZ450Inferno · 10-17-2003, 12:49 PM

I was just wondering about the clip. like more info. on it, i just picked up my GYTR kit and and confused when you guys talk about the clip set to ___
but other than that i think i got it ;) Thanks, any usable input would help, thanks.

DirtDragon · 10-17-2003, 10:15 PM

I currently have lid off and sparky removed with the cam mod. I have a 165main, 48pilot, & 5th clip on stock needle and Uni filter. I'm at 900' elevation and the temp is high of 50.

My brother runs same exact setup but 168main, 48 pilot & 3rd clipt on GYTR needle.

We drag raced tonight and I was faster if we put the lid on but If we pulled the lids he would pull me.

We have drag races tomorrow and I need help on what to do. I have a 170main and was wondering if I should swap out the 165? I do not have a 168.

Any suggestions are appreciated!
Thanks!

wired · 10-18-2003, 07:01 AM

I had Kevin from Lost Creek set mine up, And here is the deal. I have no boxlid, cam mod 170 main jet, stock pilot and needle on the fourth clip from the bottom. It is PERFECT. I live in lower NY state by NJ and PA. Like all race quads, you just can't jab the throttle from idle and expect it totake off like a rocket. But if you race you are at 1/2 to 3/4 throttle when the gate drops so that shouldn't even be a concern.