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

daddogrider · 10-02-2003, 01:27 AM

as the weather cools we are up to 180 jet main and gytr race needle at #4 and pilot 48 and we hit 77 mph without a stumble, pulls real good also. Starts on the first or second turn with no extra throttle, and idles smooth, no lag on accelaration.

try it for yourself if you are at sea level, this was done at Florence at 70 degree ocean breeze temp.

we had gytr foam filter with airbox lid off and gytr carbon pipe

175 was too lean and popped all the time on deceleration. now that is gone.

that is with 21 extreme haulers 12 inches wide and 8 inch rims aired up to 2.5 pounds 8 paddles on mild to rough sand.

daddogrider · 10-02-2003, 01:33 AM

Quote: Originally Posted by Swale

yeh i really does help, I've also found that these carbs are fairley easy to work on as each day goes by i'm finding more and more good things about the yfz 450

I'll get the 45 pilot can i ride until they come in i don't think my dealer will have them for a few days friday is when i should get them

dude try a 48 ppilot and a 175 main and the needle on #4

you will haul.

Nice thing is you can try all kinds of jets and they all run pretty good.
except the stock ones, they pop and backfire all day.

Pahrump · 10-02-2003, 11:38 PM

Daddog, with the 48 pilot where should the mixture screw be?

1 turn maybe?

I am at 1500-2000ft, 85-95 degrees and am currently running lid off, baffle out, and cam mod; with 168 main, 45 pilot, GTYR (YZ) Needle on 4th clip, and 2 turns. Seems to run good and I spanked the crap out of everything at Dumont on Tuesday but I am going to be putting the K&N on soon and am not sure what to do. Also the temps will be dropping to 70-80 degrees here soon.

Swale · 10-03-2003, 03:58 PM

i've just received my pilot jet (thank god) but its a 48 however i still put it in, the popping has dramaticley decreased to next to nothing and i can go from idle to WOT a lot better but it still stumbles a little but just not as bad, the only one problem is if i go up to through the rev range in idle slowly it backfiree a bit in the higher rev range, could this indicate anything?

Pahrump · 10-04-2003, 09:21 AM

Might be a little lean up top?

What is your main jet size, and what altitude are you at and temp there?

Swale · 10-04-2003, 10:55 AM

temp is about 18*C at sea level i think my main is 168 but am not to sure what should it be

Pahrump · 10-04-2003, 02:26 PM

Try a 170 and see what happens.

Heck they are only $4 bucks

Pahrump · 10-04-2003, 02:27 PM

Especially with the mods you have you might even moving the clip down on the needle one spot with a 170 main.

FAST1 · 10-05-2003, 09:21 AM

Need some jetting help...

Sparky out and lid off using GYTR foam filter

165 main
45 pilot
GYTR needle on 3rd clip

Seems to run fine and accelerate great however upon decceleration I occasionly get some loud backfires.. thought someone shot a gun at me the first time it happened..

My qeustion is what do I need to change on my jetting to eliminate this??

Pahrump · 10-05-2003, 10:24 AM

go to a 168-170 main and make sure the mixture screw is at 2 turns out.

Might drop the clip to the 4th spot on the needle.

FAST1 · 10-05-2003, 05:17 PM

Quote: Originally Posted by Pahrump

go to a 168-170 main and make sure the mixture screw is at 2 turns out.

Might drop the clip to the 4th spot on the needle.

So do you think I am running lean? I thougth I might be running rich.

I haven't checked the mixture screw yet.. just left it in the stock position.

Pahrump · 10-05-2003, 05:35 PM

Yes you are a little lean.
Backfiring especially upon deceleration is a sign of being lean.

The stock setting on the mixture screw is SUPPOSED to be 1 1/2 turns

FAST1 · 10-05-2003, 05:47 PM

Is it just a sign of being lean on the low end?

When I finished re-jetting to what I mentioned and tried it with the air box on or with just the baffle out, it would sputter and not reach max rpms.. It wasn't until I pulled the baffle and removed the air box that I felt the engine start to sing.

Could it be just my air mixture screw or needle clip position? Would it mean that I am lean on my main if the backfire occurs when just down shifting and after 5 to 10 seconds from running at rpm while the engine is idling and I am coming to a stop?

Pahrump · 10-05-2003, 05:59 PM

The pilot is for idle to about 1/4 throttle the needle for 1/4 to about 3/4, and the main for 3/4 to WOT.

So use that as to where you are getting the backfiring at and go up in size in that area.

You may just need to adjust the mixture out 1/2 turn or so.

What is the altitude where you are at?

FAST1 · 10-05-2003, 06:06 PM

1000 feet

I can't believe the main is lean since if I either stick the baffle in or stick the lid on,, it will hardly run on the top end.. I have to have both removed for any type of smooth rpm.

It backfires after coasting for a 5 to 10 seconds with the throttle fully closed (idle position) and not all the time..just some times.. I would assume that I am lean in the low end???

Help ,, I don't want to cook this engine..

What exactly does the air mixture screw affect? What part of the Fuel delivery?

Pahrump · 10-05-2003, 06:58 PM

Mixture screw is from idle to about 1/8-1/16 throttle so when you let off it comes into play.

Try a larger main, and dropping to the 4th clip it will not hurt anything, and might just fix the problem, but going lean could melt her.

FAST1 · 10-05-2003, 07:43 PM

I've been reviewing threads on thumpertalk.com and have concluded that it might be lean on the fuel mixture screw...

I also found this posted by one memeber.. any truth to it??

From the General Tuning manual found at Mikuni.com

4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) 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.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
5) Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.

Pahrump · 10-05-2003, 08:36 PM

That quote has alot of truth in it.

Like I said you may want to check your mixture screw and set it to two turns rom seat.

Have you done a plug check yet?

DirtDragon · 10-06-2003, 02:19 PM

I went to a 48 pilot and the popping went away. If the post above is correct then maybe I'm too rich on the pilot? Anyone else pop when letting off the throttle? I thought this was a bad thing and was happy it was gone.

FAST1 · 10-06-2003, 05:52 PM

Edgerat and Dogdad,

What are your thoughts about the mikuni info?

Do you guys agree or feel this is false?