Pilot Jet Screw ? & stuff 03WR450F 160MJ, 48PJ

I Just checked my pilot Jet Screw. It was 2 turns out stock. I changed it to 1-1/2 turns out. Did anyone else check this? I also changed the stock Jet Needle to 5th position (richer).

I'm still waiting on my YZ throttle stop screw. The manual refers this as the screw that sets the Idle "Turn the throttle shaft assembly left while holding down the lever 1 5 and fit the throttle stop screw tip d to the stopper e of the throttle shaft assembly pulley." page 4-16. Is that correct? It don't sound right to me. Where is the throttle stop located? :)

When my PAJ #100 gets in this week I will also put it in. I know I'm changing too many things at the same time, but don't have enough time before first race? Does this look like I'm going in the right direction? Hopefully I will be able to ride Friday! and see what I've done! I'm basically trying to get rid of the off idle bog.


Motorod, the throttle stop is an allen head screw on the carb. From the right side of the bike look at the area where the two throttle cables go into the carb. Go down a little and to the left. It's kind of hidden on the back side of the carb. Pull it out and cut 11mm from the total length. I bought the YZ stop and measured both of them.

Good Luck, Jeff

Thanks for the info. I hadn't looked for it yet since my stop screw hasn't arrived. The shop I bought from cut the screw too short, so I had to order a YZ screw.

motorod, me ol china there's one born every day!

what do ya mean yer in a hurry? you could be running to shoot your own foot off! slow down! test the changes, get a feel for it. who on earth told you to put a 100PAJ in it?

crazy! stick with the 75PAJ it's better and get a 40PJ. oh! and don't take even my word for it. test it first and back to back.



You guys talk about changing/testing jets like its a 2-minute procedure. How are you doing it?

I'm starting to wonder about all you WR450 guys. Are you new to the forum or what? It's pretty well established that smaller diameter(richer) needles in combination with a smaller pilot jet(AKA Taffy Jetting) produce dramatically better bottem end response. I ran the 48PJ/100PAJ/EKP-4 jetting in my YZ timed WR426 and thought it was fine until I went to Taffy jetting and found out I had been riding a blubbering POS around. There is a test for blubbering thumper syndrome. It goes like this: approach some nasty whoops about 25 MPH then close the throttle and coast in. Try to blip the throttle on/off to keep your front wheel from slamming into a whoop and pitching you. If you can't do it and compensate by always accelerating continuously through the whoops you are riding a blubbering POS. Fortunately the cure is only a few jets and some testing.

Maybe you guys should just save yourself some time and skip right to what works?

Or maybe your test for blubbering POS's will land us all in the ER with broken collar bones. NEXT>

"who on earth told you to put a 100PAJ in it?"

I got that from your post in the " Jetting Qs " (http://www.thumpertalk.com/bike/ubbthreads/favlinker.php?Cat=&Entry=1459&F_Board=UBB3&Thread=17653&partnumber=&postmarker=).

Your last chart read:

" my chart therefore reads;








Is this not a good place to start on this bike?

How do you time the squirt? It looks to me like it just squirts as you turn the throttle and the adjustment just controls the amount of gas coming out.

Thakes me about an 1-1/2 hours to change out. Have to take the seat, tank, and pipe off. Have you changed a plug yet, that looks fun! :)

i can change the jets in about 20 mins tops.

The pilot jet and main jet can be easily accessed by removing the float bowl drain bolt. I only remove the seat and tank when I have to work with the needle and other jets.

what i gave you were ratios sir.

waht pairing i feel you should use is another. anyway let's chillout coz you certainly don't deserve our wrath individually.

just to cut things short and real simple.

the PJ/PAJ ratio is only about 20% of idle. it's there for 20 mph in 4th gear and snapping the throttle.

a needle that reads well over 'M' for sea level and 'P' for altitude to keep the 'fat' in the hole and run it lean will need lots of help from the pilot circuit to 'juice it up'. thus the high numbers you inevitably employ.

i'm guestimating here but a typical scenario at sea level is 'P' needle (as in EKP) and 42/45 PJ.

on mine it would be a thinner 'M' needle and 35PJ and a nice middle one would be EMN and 38/40 PJ.

now the difference at tickover is diddly-squeek but when you roll that throttle in fourth at 20mph we are now on the pilot circuit.

now we can have a rich mixture or a leaner but correct mixture. it's up to you. so always pick your PC jets in the ratio i suggest but go lean and take the richer needle straight.

having ridden no higher than 1,000ft in my life i can only take hicks word for it but some of the 10,000ft boys are on a **Q needle and nice 35 or 38 PJ's.

the squirt should be as short as is humanly possible, BTW 1 second is the minimum before the system refuses to function properly.


OK here's what I did. (I'm only riding down a 600' gravel twisting driveway to test, 2nd gear tops, and tonight pitch black :) )

Yesterday I changed the stock needle (OBDUT-4) from 4 to 5th from top. I changed the squirt 1/2 turn shorter from stock (Seems to be right at 1 second), and set the Jet screw 1-1/2 turns out (stock was 2). It ran good but had the off idle bog (2nd gear about 10mph) and backfired while in neutral while goosing it.

Today, Replaced the stock needle with 5TA-14916-JN-00 (#NDJN) (YZ-450F 2003). Left all else alone. Backfire in neutral gone, OFF IDLE BOG GONE :D I have always ran the 160 MJ, 48 PJ, 70 PAJ and am at 800 ft. very humid.

If it quits raining I'm track ready now. Need to test at greater speeds and conditions. While writing this I remembered I had to turn that ignition off. :D Ha, I didn't mean to offend anyone, I guess I didn't understand those were ratios, sorry! :D :D Can you explain the ratios?


48/110-125 ?48PJ =110-125 is this a % of something? :D

Taffy, I read all your posts in the jetting Q's with open eyes, you took great pains, I just did't understand everything.



OK, I timed myself tonight. It took me 1 hour to change the jet needle. I droped the spacer from the air box boot clamp and had to take off the skid plate to get it in the process, so probably about 45 min. tops for me.

a lad here in the UK is ecstaatic about the DMM needle i gave him. but it's on clip 1 which is a bit extreme. to have the needle on a middle clip but be identicle would be a DFM or even a DFN if we take you're needle straight code. so you see DFN to DJN is only 1.5 clips apart and this can be accounted for in the different MJ's you both run etc.

so you see you're closer than you think!

the 48PJ/100-125 PAJ implies that the amount of air and fuel is so high as to barely be worth recording the difference.

now you've got a thinner needle in you can afford to go to lower PC ratios so start work now!

you'll be good for a PAS and a ratio of 40PJ and 75 (or 70 PAJ!)PAS.


Taffy- tell me if I am correct or not but instead of running the needle at #1 is it better to go down on the main and reset the needle at #3 or #4?


Hi: Could you define these abreviations for me please

48PJ/100-125. I am pretty sure the 48PJ stands for 48 pilot jet but I dont know what the rest of it refers to.

What is PC and PC ratios

What is a PAS

Does 75(or70 PAJ!)PAS refer to what the manual refers to as a starter Jet

Can the blip of throttle low end bog be cured with a accelerator pump screw adjustment and pilot jet adjustment or is the BK mod and all that just about required to cure the bog.

Few clarifications for ya...

The "PC" is the pilot circuit. Folks usually refer to the carb having 3 different "circuits"...a low speed pilot circuit, a midrange (controlled predominantly by the needle taper and clip position), and the top end (controlled predominantly by the main jet).

What he was referring to by the 100-125 was the pilot AIR jet. There are a few combinations of pilot jets/pilot air jets that seem to work well...and he's stating that the 48PJ works well with a pilot air jet in the range of 100 - 125.

The PAS is an adjustable pilot air jet (known as the pilot air screw). It can be purchased at Sudco for about $15 and can be adjusted to provide you with a pilot air jet range of somewhere around 50 up through 125 based on the number of turns out from fully seated that the pilot air screw is set to. Good purchase for you to make, since you'll never need to buy another pilot air jet.

The starter jet is a completely separate jet used to squirt additional fuel into the carb throat during startup...not used for anything else. Some folks have changed the starter jet from the stock one and have noticed some improvements in "start-ability".

To cure your low end bog, you should both jet your bike correctly AND do the accelerator pump mod...or atleast verify that it's currently squirting fuel for about 0.5 secs. Most bikes "off the shelf" have been known to have upwards of 3 secs squirt duration...way too much fuel than what is required...and thus, folks do the mod to eliminate the hesitation from being way too rich for those few seconds.

Finally, some camps go with the larger PJ/PAJ ratios for tractor like torque. Others go with the smaller PJ's/PAJ ratios for a "snappier" throttle response. As long as your PC ratio and needle diameter are close to being matched, you're bike will run well off the line.

Hope this helps...


couldn't have put it better larry! a very good descriptionand very thorough too.

jim i don't follow you're individual needs therefore you're talking dutch to me. run the FIGURES by me.


Thanks very much LarryCo and Taffy. I was up on almost all the jargon but sometimes the acronyms start to run together and this is my first pumper carb on a dirtbike. Got it runnin like a stripped but ape except for the bog which seems to be common. Tim

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