Hey titanium,

yes it does sound like it is running a little too lean.
more air often times makes the engine act like it has the throttle held open a little.

normally the carb gets more air when the throttle is opened. so, high idle often occurs when the fuel mixture is a little lean. and normally after warm up.

i would start by turning the fuel/air mixture screw in a little. but use small increments and mark where you started from so you don't get it out of whack and can't get it back where ya had it.

start maybe with a 1/4 turn to begin with, ride it for a few minutes to let it work on this new setting before adding anymore. and add 1/4 each time, but like i said, keep up with how many 1/4 turns you make so you can get it back if needed.  

Hello Wolf1,
Thanks for the insight you have given.  I am having some struggles with my new 2008 YFZ450.

Essentially in order to have it run well at idle, I have to have the choke at 1/2 always.  I have been reading a lot, and most places say that the pilot jet is too small (42) and I should at least move it up to a (45).

Everything is stock on mine right now (my carb is currently disassembled to find what I have)
pilot=42
main=155
needle=NGN R (with only one slot for the "e-clip")
mixture screw=unsure, I have not messed with it
intake=stock
exhaust=stock  (but I ordered one: Procircuit 496)

I was planning on going to get a pilot (45) and some mains (165 and 170) and maybe the GYT-R needle.

Then starting with the 45-pilot and the 165 I was going to see how the idle changed.

Any recommendations?

Thanks,
Todd (San Diego...too bad I am not in the Chat. area, sounds like you know what you are doing and you love to figure out things...)  

if you have to run the atv with the choke on at all after it warms up, then it is not getting enough fuel.

use a larger pilot, you don't have to spend a fortune on exhaust but your get a few more horses if you do, just yank the airbox cover off and through it in the garage somewhere, and take the baffle out of the muffler.
the yamaha yzf450 was designed to do this.  be careful not drown the airbox in the nearest creek.
but then most of the fun of riding is those wicked jumps and awesome air.

every yzf450 is built with the idea that the rider is going to race it. so they don't plan on the airbox cover or baffle staying on the bike or the carb jets remaining stock.

the yzf450 is an awesome atv!!!  killer performance if you rejet a little larger, remove the airbox cover, and yank the baffle or buy a good high volume exhaust.
you'd be surprized at the performance once you do these things.  

Wolf1

I am new to this site and after reading several of your posts you really seem to know what you are talking about. With that being said I have a similar question regarding jetting. I purchased a used 2005 YFZ 450 for my daughter several months ago. The bike is pretty much stock with the exception of wheels, suspension, and the baffle was removed. The problem I am having is that the bike skips and breaks up at high rpms (not the rev limiter). I bought a new NGK plug and it ran great for a few hours and then the same thing. I cleaned the carb and jets and again it ran great for a few hours and then the same. I put a fuel filter on and again cleaned the carb. Same thing. The air filter is old and has signs of comming apart, the air box is cleaned every time and it does have the lid. I cannot recall the jets but believe they are stock. The bike starts, idles and runs strong (in my opinion) until you really get on it. Even if you ride it down the road it spits and spudders when the rpms get up. I am contemplating buying a jet kit with a K&N filter which is around $125 and hope that this solves my problem. I need to buy a new filter anyway so I figured to do both at the same time. What gets me is that it runs great after I clean the carb but only for a few hours which leads me away from jetting being the issue. Could it be just my luck that it keeps getting a piece of trash in one of the jets? Thats why I put the filter in line and try to keep the filter box area as clean as I can. It doesnt seem to matter if it is dusty, muddy, or normal riding conditions.

Any help or suggestions are greatly appreciated. BTW, I live in central florida so altitude and temp are not an issue.

Thanks,  

it sounds like your filter is coming apart and clogging up the carburetor.

when the filter media starts to break loose and fragment it gets into the carb and the fuel breaks it down even further.it can really clog up a carb.

however lets cover all the bases to be sure.

have you thoroughly inspected the floats when you have it apart?
check to be sure that the floats have no cracks or holes in them that would allow fuel to fill them up and weigh them down.
check the seams.
inspect the needle under floats and be sure that they clean and clear.

yank the jets and look through them.
if they are clogged use a wire bread tie (burn the paper or plastic from the tie)to clear them
or use a .10 guitar string.

the cutting out and spitting can be a sign of two possible different problems.

1 it's running too lean (getting too much air) should be easy to fix just turn the fuel/air mixture screw to the left in 1/4 turn increments and check it after each 1/4 turn (let it run for a few minutes with each 1/4 turn)
and don't forget to mark where it was originally and keep up with how many 1/4 turns you make.

The other possible problem is that it has some trash in it.

check your gas tank, drain all of the fuel out into an open container so you can see the fuel and any trash or water that may be in it.
just don't get a spark near it, and do so in a well ventialted area.

once you have drained the tank look inside the tank using a flash light, remove the fuel shut off valve, and look inside it for signs of trash or dirt.


once you have cleaned the tank reclean the carburetor.

right now with a bad filter in it that is coming apart it would be better to run it without one, but the best bet is to go buy a new one.
REMOVE THAT BAD FILTER.

if you have cleaned everything out and tried the fuel/air mixture adjustment and still no luck, check the stator to see if it has gotten wet and the contacts have rust on them. this can cause this as well.

once you get this thing set right and running well you'll be surprized at the amount of power it puts out.

the yzf450 was designed to be race ready without having to spend a fortune to prepare it.

very nice machines!!!
am thinking about picking one of these up myself.

there are other things that cause this but it doesn't sound like what you are dealing with.
they wouldn't get better by cleaning the carb.
things like head or cylinder gaskets that are leaking, or a dried out and cracked intake boot.

try the cleaning and inspection and then the fuel/air mixture screw, if no change there let me know,
we'll look into other possible causes.  

Thank you,

I will pick up an new air filter & fuel filter, drain and clean the tank, clean the carb & jets (checking all seals and seams) and then try the air/fuel screw. It will be this weekend before I have at it once again but at least now im a lot quicker than the first time.

I will let you know how it works out.

Again, thank you for your knowledge and assistance.  

welcome, do let me know how it works out for you, if needed we'll look at other possibilites  

Wolf1

I ordered a kit today which included a K&N filer, prefilter and jet kit. Hopefully I will have it by next weekend. If not I will have to wait until the following weekend to install as I simply do not have the time to work on the bike during weekdays. I will also be replacing the fuel filter, spark plug, and clean the air box, gas tank, and carb real good at the same time.

I do have 2 more questions for you though. Which screw is the air/fuel adjustment? Also, I have read that some remove the lid to the air box in order to get more air to the filter. If you do this do you have to make other adjustments at the same time (air/fuel screw or jetting)?

Thanks again and I will definitely let you know how it turns out.  

the fuel/air mixture screw is the one on the bottom.

if you remove the airbox lid you'll have to adjust the air/fuel mixture screw a little.

but Yamaha designed it to be run without the airbox lid.

just don't drown it in a pond or creek deep enough to get water in the airbox.

if you need help adjusting the carb let me know and i'll try to talk you through it.  

Hi I have 07 Yfz 450 Full yoshi Exhaust Stage 1 cams 490 big bore kit aftermarket rev limiter. Im having problems jetting the quad when i start it up it idles fine and everything when i push in the throttle about 3/4 of the way and let of it Backfires LOUD Once Maybe twice Im Just wondering if anybody can help my on what they think the problem might be thank you and i appreciate it  

do you have the airbox lid still on it?

if so remove it and see if it gets better or worse.

if it gets better then you aren't getting enough air in the fuel/air mixture.
turn the fuel air mixture screw in.

if it gets worse, then aren't getting enough fuel.

mark where the fuel/air mixture screw is setting before you start so you have a reference point, and make turns in 1/4 turn increments.
keep up with how many 1/4 turns you make.

sounds like you have put some serious money in that thing.

rule out the fuel mixture first, if you can't get it to clean up then we'll look at other possibilities.  

what jets do u think i should run being under 3000 feet And what slot on the needle Just so i can compare to what i already have put in the carb thank you  

i would probably run anywhere from a 165-170 main and a 45 pilot.

and you'd probably need to run on the second or third groove on the needle depending on which needle you have. i have seen some with a 2 slot needle and some with a 3 slot.

yuou are running a 490 big bore kit so you are turning some rpms' and need a higher cfm with that compression.

just make sure you have removed the airbox lid.
you already have an exhaust system as long as that yoshi is big enough to breathe.

sounds like that thing should scream when you get it tuned in.  

Wolf1,

I got my kit in the mail Friday and decided to put it together that nite. First I turned the air/fuel screw in and noticed that it was about 2 1/4 - 2 1/2 turns out. once slightly snug I marked the outside with a black sharpie. The kit included a DynoJet kit and a K&N air filter. I cleaned everything and went strictly by their stage 2 instructions on installation. Put it bak together and it ran a little rough on idle (kit says 1 1/2 or 1 1/4 turns out. Tried turning the screw 1/4 more out a few times and really didnt get any better. Then decided to change jets (the kit came with about 10 or so jets from 122? - 165). tried the same routine over and over and over. Then decided to try stage1 which is basically lid on versus lid off and clip#4 versus #5. still the same thing and I gave up for the nite. Saturday I went back to it and after several attempts I decided to just give up on the jetting and go back to stock. Now the thing will not even start. Nothing. checked and rechecked everything and killed the battery 2x over these 2 days. Gave up on saturday and called it a nite. Sunday I went back and decided to take my time and figure out why it wasnt starting at all. I pulled the plug and it was nasty (new NGK). Ok, I fouled the plug so I put the one that I just removed on Fri nite and still nothing. Took it out and pluged it into the cap, laid the bottom of the plug against metal and tried to start it. No fire coming out of the plug.... AHHHHH. I know on an auto you can place the bottom of the plug near of touching metal and it should spark so I figure this should work the same. Then I figured that the 50 of so times I turned the carb (only removed the initial time) that perhaps I have knocked a connection loose?? I have checked everything that I can think of with no luck. I will admit that when it comes to electrical systems, other than a house, I am pretty ignorant. There are 2 fuses on top of the batter and I checked those, even though I have no idea what they are for. All connections etc have been checked.

Where do I start and what do I do to narrow down my issue? I do not want to just start replacing electrical parts as they are expensive. My daughter advised me to give up and take it to the dealer ($75/hr). I am very mechanically inclined but just not in the electrical/fuel related areas.

HELP.... If I were not in central florida I would just drop it of with you for a week or so but your about 10-12 hrs away and the wife will not let me trailer it that far.  

Not to be a butt head or anything, but these quads are not for anyone with no experience so I hope your daughter it more than a beginner level rider.  Even if she has had a quad before.  Just for the sake of safety.