04 yz450f PJ air jet tuning:rich bottom end studder

Ok I got an 04 yz450f that I cannot get to run right on the bottom. No matter what I try, rich main lean pilot/rich pilot lean main it ALWAYS studders at like 1/8-1/3 throttle opening at a constant position. As soon as I stop opening(AP OFF) it starts to rich studder. I can manage to get passed it if I open the throttle quick enough but I would like to have it dialed. Specs:

First set up:



Needle position: clip 4


Elevation: 0-1,500'

Full white bros pipe

With this set up it studders very bad at any constant throttle position. Had to go up this far on the main to get smooth decel and it still has a little backfiring under decel.

Spec 2:



Clip #4 everything else the same.

This time it has a little less studder at 1/8-1/3 constant throttle with no backfiring on decel and the top end is much much better than a 175&172

If I go down to a 45 pilot it runs crisp everywhere but runs excessively hot and has excessive backfiring.. Can't richen air screw enough.

With the 48 pilot i cant turn the air screw in enough to kill it. The 45 will die when I turn it in. So my thought is the pilot air jet needs to go up a size then perhaps go up to a 175 main? Who here knows about tuning the pilot air jets?

You need to clean the passage above the pilot jet with compressed air and mono filiment fishing wire, fix the air leak in your hot start (probably a cracked top nut or stuck plunger), and go back to a 45 pilot.


 By the way, once you open the throttle past 1/16, the pilot has a very small effect. You are now on the needle.

Why you say the hot start cover is cracked? Wouldn't it want to run lean then? I tried going on clip 3 with a 175 main and a 45 pilot but it was absolutely terrible. Bad bad bad popping on the bottom end, horribly lean. Sputtered at constant throttle. Seemed like a rich sputter. No bog anywhere. I'm starting to think its the top diameter of the needle. What is the stock yfz needle code? Or anyone run leaner bottom end needles? Top end is crisp and perfect. I can't get that studder to go away though.. And not have it backfire as well

I think I'm in needle territory.. Unless I go up on the main to 180 and use clip 3

No, you are going in circles.



NCVS or stock needle

50 leak

75/80 power jet

stock air jet

Brass Fuel screw with FRESH oring and washer


You are popping with the 45 because you have an air leak, and the YZ/WR carbs have a shitty nut on the top holding the in the cable/plunger, and the plungers corrode in place, creating a lean condition.

The only place I would have an air leak is at the hot start. This really seems like a jetting issue. How can I tell if the hot start is all good?

The only place I would have an air leak is at the hot start. This really seems like a jetting issue. How can I tell if the hot start is all good?



Pull the lever in, and see if it goes back all the way on it's own.

If you have to push it back while holding the cable, it's starting to corrode (or the cable is pinched) holding the hot start partially open


Have you checked all your wire harness grounds?

Idk I'm gonna take a look again tonight. It does sound like an air leak now that I think about it. This is my first 4 stroke I've attempted to jet so I don't have the sounds down like a two stroke yet. As well as the fact that no jetting has fixed the problem. Does anyone ever mess with the pilot air jet? Or is that reserved for alc and big bores?

The hot start goes back pretty fluidly. I can feel at the very end some resistance but it feels like it seats no problem. Grounds? I have not. Originally when I got the bike it popped/bogged bad so I unplugged the tps and it was good. So I thought it was that and just thought timing was a little off and that was the studder. Eventually I got sick of the studder and did the jetting right(it was too rich on top/lean on bottom) but the studder remains. Tps plugged in or not. Tps seems to work just fine. Actually pops less on decel with the tps in

Just checked the hot start.. It's clean as can be. I can hear it click/seat when I release it no matter how far I release it from. And there is no visible cracks/rips or even discoloring. Looks brand new.. Idk guys I'm at a loss

It almost seems as though the needle is coming on too soon.. But literally besides that studder at low rpm constant throttle its amazing. Used to bog with the 175M 42P(rich bog) when the needle would start to hit under very high load. Now it's like no matter how much traction I'm getting or if I'm down way low in 3rd I can open it up with no hesitation. That's what leads me to believe the jetting is spot on. If I go up on the PJ air jet do I have to jet up on the main some too cause its already running pretty hot and runs extremely hot with 45/170 jets

Also it gets better if I tilt the carb sideways. You think my float level is super off? If anything it's super low. Maybe I need to raise it a little and that will help out with the slightly lean 45PJ? I'm gonna try going 172M/45PJ. I can't remember if I tried that. I think I did and it sputtered up top and had a different popping on decel. More inconsistent and the tone of each backfire was different. Some seemed pretty long too. Sorry for all the posts. This is really bugging the hell out of me. I thought I had it but nope. I'm trying to hit the track tomorrow night too and I hate the track with that studder although its easy to work around on a track I still want it crisp as can be

You should not have the change the pilot air jet

All it does is set the range for the air side of the pilot circuit.

The Pilot jet sets the fuel side.


The main has nothing to do with it.


You have an air leak, that is what is making it hot.

Check your hot start nut.

Check what about it? I've already checked it. It's clean. Nothing wrong with it. It's not cracked. The hot start functions 100% as it should. I'm gonna take it off and see anyways I guess

Well the hot start nut wasn't loose but it wasn't too tight. I tightened it up a little. Should I put a zip Ty around the base of that boot that sits on the nut? Seems like there is a perfect spot for one.

I put the 45pj back in as well as a 172 main. No matter how fast I rode with the 45/170 the header would glow about halfway through the bend or just at the bend. Maybe it always does and I never see it cause I ride in the day. I was worried about it so I figured try the 172 and see if I can get it to run well. Tightened the hot start nut and zip tied the boot. Set the air screw to 1.5 out so we'll see how it does in the morning

Header glow has nothing to do with the main

At 10:44 pm, you would normally see a glow.....

get it running and spray brake cleaner around the carb to the head area on the boot etc ,that would find your air leak

Idk why I didn't think about carb cleaner. What do you mean the header temps have nothing to do with the main jet? I'm not sitting idle, this is moving 4th mid throttle on the street at around 70° my yfz only glows when it's at idle. The yz would only glow at idle with the 175 main. I can't remember if I tried the 172/45 main yet. I'm gonna give it a go here soon. See if I can trace the source of this issue

Just rode around with a 45PJ/172MJ and there is no studder in the ranger there was before but as soon as the needle taper really starts to hit it sputters. No backfiring though. This is also after I've zip tied the boot on the hot start nut and tightened it a little. 170/45 is where it seems to want to be. Otherwise maybe a 46.5 would do the trick but its good enough I guess. I hear other people's bikes pop like hell and they think nothing of it.. I can't stand that. Anyways I'm gonna put an inline thermostat on this thing to keep an eye on it. What is the Max temp these things should be at?

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