2004 Yamaha YZ 450f Minor Jetting Issues

hey guys,

I am about to blow my brains out trying to get this carb dialed in, and I am not sure what I am doing wrong, but I have read all the jetting articles as well as looked all of the stuff up, and every time it seems that I still cannot get it running right.  Maybe someone can shed some light on the problem because I think I am it.

The bike is a 2004 YZ 450f, I have a FMF power bomb header/FMF Powercore 4 Slip on

-I am located in Raleigh, NC and the temp here is around 90 degrees, and we are 3500ft above sea level

-I have a R&D Powerbowl 1 installed on the bottom of the carb with adjustable leak jet

-New top end, other than that everything is stock as far as jetting would be concerned.


Started @

Main Jet=170


Needle Clip-Using Red Needle out of JD Jet Kit @ 4th notch from top

Air/Fuel Mixture Screw on carb=2 turns out

R&D Powerbowl Leak Jet=2.5 turns out


With this, the bike was doing pretty good, but still had a popping, and the header pipe would get pretty nice and orange by the engine, if I am riding too slow in technical portions of trails, guaranteed overheating issues.


Changed Carb to 

Main Jet=168


Needle Clip=Using Red Needle still, went from 4th notch from top to 2nd notch from top

Air/Fuel=2 turns out

R&D Powerbowl Leak Jet=2 turns out


After riding at these settings, the bike felt like it was holding back on power, didn't feel to have near as much power as normal (when I say power I guess I mean the bike isn't constantly standing straight up in the air, almost feeling like it was taking a few split seconds to really hit hard power)


Does anyone have any suggestions PLEASE on what they may think the problem is?  Also, if anyone can give me a good tip on the proper way to adjust the leak jet on the Powerbowl it would be greatly appreciated because right now I am trying to use the instructions to set it, but I will be honest in saying I don't know exactly how to set it properly or what to look for in case it isn't set properly (how would I know other than the bike feeling like it's not running right)?



Thanks again guys for all the help you all give.


Assuming what you started with was stock jetting, the upgraded exhaust will usually cause it to run leaner as your engine breathes better. The conditions you mentioned you had with that would indicate a lean condition if not normal. The only changes you made would only cause leaner conditions except for the leak jet adjustment so that was kinda goin the wrong way imo. With what you started with was is only popping a little on decel? And would the header glow only when not moving or barely moving at all? Also with the leak jet i just adjust it until i can crack the throttle quickly without bogging. It basicly just adjusts the amount of fuel that makes it from the ap bowl to the intake with each pump

It is normal for a bit of popping on decel and the headers all glow when there is no air moving past them


I am not completely sure what this particular part is/does but it's the plug that comes out of the carb(has 3 wires total in the plug: 1 black w/ blue stripe, 1 blue wire, and one yellow wire) - anyways, I was just about to take the carb back off when I realized that I overlooked that connection apparently when I cranked it up and tested the bike with the other jets in it.  This plug that goes in the carb was unplugged, which is what is causing me to really believe that that is the reason the bike felt so weird as far as throttle response and everything on that test ride.  Can someone tell me if that would have impacted how the bike ran at the given time?  I mean I am 99.999% sure that it is the cause because I plugged it up and rode again and the bike seemed to do night and day better and seem to be on track, but I just didn't know if the bike could possibly run without that wire plugged in, or if it can in fact crank and run without that wire plugged in that goes in the carb?

That plug is your tps (throttle position sensor). Being unplugged will disable the 3d timing map thus timing map will be as if you were always wot. You will lose throttle response and "crispness" but bike will run completely fine

On carb settings you started with can u explain a couple things with more detail? Did the header only glow while no air moving past it or even while moving fairly good? How bad was the popping? Just on decel? I ask because like i said before a litte bit of popping on decel is normal and header glow is normal when barely moving or not moving.

Ok, the TPS sensor being unplugged was definitely that problem of feeling WAAAY off and to answer your question an the details-

Header glows while sitting as per normal, but also will glow a little bit when riding (not get red hot like a stove top, but definitely hot enough that sometimes on technical trails if we have ppl riding with us and they're taking their sweet time I'll have to sit and cut my bike off so it won't overheat. Popping was only on Decel and I know it's suppose to pop some, just seemed like it was popping a little excessively? And I read while researching jetting that you could have the bike too rich or too lean and it help to cause the bike to overheat-it never overheats if I'm riding pretty good on it, whether it's drag racing or going through wide straight trails, but gets extremely hot in the woods when doing right runs. I don't have a flywheel weight but I ordered a 13oz weight to hopefully help with the woods riding but I have never used one so I don't know what to expect.

With that adjustable leak jet, is it actually helping the bike to run richer by screwing the needle clockwise (in) or is screwing it clockwise running more fat and counterclockwise is more lean?

By the way thank you guys so much for all the responses it's really helping me to learn this stuff

I believe turning the adj leak jet screw out (counter clockwise) lets more fuel "leak by" thus less fuel making it to the intake

Do u have a manual for your bike? I would get one they are free online someone here can send you a link. Tgere is a tuning section that will help you properly jet your carb. Follow those instructions

I should add that you should have your carb jetted right before setting the leak jet

O and i dont think changing main jet or needle clip position is going to change anything your trying to fix as your problem is low speed

IMO, I'd get rid of the JD needle and use the OEM part.  You might also inspect the top of the main discharge nozzle to be sure the top hole, which is the "needle jet" is round.  If it's ovaled, it's worn. 


The link to the manuals is in the Common Threads topic, top of the index.

Gray Racer,

I still have the OEM part for the needle, and also I have inspected the top of the main discharge nozzle in the carb and it hasn't become an oval shape yet, it is in fact round still.  What are your thoughts on the normal OEM needle as opposed to the JD jet kit needle? I will admit, the two needles they give you are confusing as hell because one is blue and says for high altitude/cold temp, and the red it says is for low altitude and high temp.  This makes no sense to me, but I am assuming both of these needles have a different diameter making them different.  So your saying you would stick to the OEM needle?

In general, yes. 

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