mid-upper range jetting?

I've read 100s of posts on jetting advice, but not stumbled across this issue. Low-end is great, and acceleration up through top end is awesome. But, when holding the throttle steady at say 3/4 open it starts to miss (or pop?) after it's fully accelerated. Happens in any gear.

Bike is an 01 WR426 with FMF IV Q pipe. Jetting is stock except an extra 1/2 turn out on the pilot screw. 42 pilot, 165 main, DRR needle on #4. I'm assuming it's a main jet issue, but don't know if the missing means it's too lean or too rich.

Advice appreciated. thanks

it doesn't seem too rich-on paper! try dropping the needle one clip. this should sort it.



When I hear pop, I think lean. Using this logic the accelerator pump keeps it rich enough while accelerating but he has a lean spot in his jetting that is uncovered after the sqirt, top of needle, bottom of main.

Is there a rich pop also? Did the missing bring up the rich?

Forget it, I just reread the post and you dropped the needle to enrich it at top of needle. Sorry.


when they're too rich they have a flat spot at the bottom. when they're too rich in the mid/top they sound like the rev limiter is cutting in.

i totally ignored the back popping right up to my final tests in "jetting Qs". that's because the pipe will re-fire any unburnt gases and you've removed the air bleed valve. to me thismeans that even with the correct jetting there is an element of back-popping.

to get rid of it is to run rich. the other way is to put the airvalve back on again when it's truly set up right.

dropping the needle LEANS it BTW.



I love reading your posts, I really want to understand this carb better. I read the post and even copied it right but was thinking dropped the clip instead of dropped the needle. I'm mildly dyslexic aslo.

1. My bike ('01 WR250F, airbox out, YZ cam, grey wire cut, Vortex, stock pipe w/Vortip, air cut valve still in place, 170/42/ELP#2,1 5/8). Runs fairly well, slightly rich now but fine in winter, without a baffle. With a baffle, either Vortip or stock, it has a wicked bog in the bottom RPM range, maybe up to 4500 or 5000RPM. I put a 38 instead of 42 in it last night to try to clean it up, could only ride on road so not 100% sure but there is still some (bbooooowwwwaaahhhh) type bog there. My AP already has a short <1s squirt. Still lower or it is a cam timing or ignition timing problem. I was getting pops on decel with the 38.

2. Is the air bleed what I call air cut valve, the decelleration enrichening circuit under plate on left upper side of carb?

3. Do you have a preference between plugging and removing spring/diaphram on air cut valve.

4. Is there a good explanation of why a partially plugged pilot jet, in inside of jet above the cross drilled holes, would cause a rich mixture?

5. Emulsifying air comes in through pilot air jet into the cross drilled holes of the pilot jet? This is why excess filter oil plugs pilot and not other jets? Where does the main air jet air go? What could plug fuel screw circuit, my fuel screw seems unresponsive.

I got JDs jetting spreadsheet and love playing with it but need/want to understand carb better.



p.s., is there a logic diagram for the FCR, there is a simple one in the Tuning section of manual but it is very basic.

there's a lot here.

the first thing is that i hope you mean airbox lid removed!!

1) PAJ must get smaller to suit the smaller PJ. that's why your PS is unresponsive! the ratio is out.

2) yes

3) don't know it. i had a '99. block it to do your work. put it back to normal when your done to stop the back popping.

4) it wasn't blocked, it was partially blocked. therefore it's the same as too small a PAJ or large PJ; you get a rich mixture.

5) the air through the PAJ goes over the PJ tube and sucks it upo just like those old blow sprayers who did custom cars. or a car spray gun come to that!

JD's jetting chart isn't of any use to me. i've looked at it once. i was given it "on the house" BTW. i did all my work before i got it anyway.

now if i can't find a use for it....

hope this helps.


Taffy, are saying to go one notch richer on the needle or one notch leaner for the symptom. (clip up = needle down = leaner, clip down = needle up = richer)

I think you are saying try one richer, but it's such a bear to get in there and change the clip position I wanted to make sure.

Anything with the main you'd suggest?

btw, most my riding is < 1000ft above sea level.



Again, thank you.


Not to speak for Taffy, but he reiterated after I misread it that the needle needs dropped which would lean it out. To do this, you raise the clip.

The other thing is it doesn't have to be that hard. I have a WR250F so it may be slightly different, but I don't have to take off or even twist the carb to get the needle out. Take off the seat and tank, then the two allen bolts, lift off top of carb, use throttle (once only if possible) to bring throttle valve to top, 4mm allen to take off needle holder, needle out with tweezers. 5-10 minutes start to finish.

Hope this works for WR426 also.

BTW, in the WR250F Tuning section, under "Examples of carburetor setting depending on symptom" table, row 'Poor response in the low to intermediate speeds' it has two Settings: (1) Rase jet needle clip position (2) If this has no effect lower the jet needle clip position. Let us know.


i always talk about what i do with the needle. so lean it by "dropping the needle one clip position".

now if you want to know about the MJ, yes i think a 160MJ would be better. this has the net effect of leaning the needle as well. it's as though you've dropped it 1/2 a clip.

what i would therefore suggest is that you first go from 165 to 160 MJ. then try the needle down one clip. IN THAT ORDER. if the needle doesn't want dropping you'll soon find out!

when i did my testing i had no bolts in the tank and only one in the seat. no fanny (&%$#@!) flaps on the rads. also both engine plates were on the left so i could just twist the carb. i got most jobs done in 10 mins or less.


I tried a 160MJ, it pops and crackles holding at mid throttle quite a bit instead of cutting out.

When I pulled back into the garage, the header pipe was glowing bright red quite a ways back. It's done that when idling before, but not been red so far back on the header before. That something to worry about? I didn't try dropping the needle since the extra red did worry me.

Now I'm thinking I need to go richer, not leaner.

Never mind this last post, I think I figured out at least part of the problem. Feel stupid admitting it, but looks like the air box lid was not really removed, just the snorkel thing was taken out. Removing the whole lid helped a lot already.

I've got the jet settings back to defaults, and will take it from there after some hard riding and taking notes on what it's doing.

Thanks for the advice Taffy and Mark. Keep up the good work. Thumper talk is a great place to get help.

I'm having a simililar problem. It sounds like the rev limiter is kicking in on top. I don't think I'm any where near the to limit. I still have the air box lid on and the snorkle is still in there. Looks like I have something to try this weekend.


Taffy just taught me this. From above on this post:

when they're too rich they have a flat spot at the bottom. when they're too rich in the mid/top they sound like the rev limiter is cutting in.

I was running jetting tests to prove this theory Sunday when I lost compression (unrelated, don't worry). He is exactly right. I made mine way too rich by raising my needle way up (from clip #2 to clip #6). It was slow on the bottom and cut out, mimicking the rev limiter exactly, as RPMs increased if I was still on the needle (roughly between 1/4 and 2/3 throttle).

You didn't say where. In my case, when needle was way rich but pilot and main were still good, I would run fine at low throttle, the lack of acceleration would creep in as I got on the needle at around 1/4 throttle, then quiet short misses or skips as RPMs built. By the time I rolled off needle onto main, it ran great and would rip on up to the real rev limiter. (was in mid gears).

Use a mid gear test, roll on throttle slowly and smoothly enough that AP isn't much of a factor and try to remember what ranges of throttle position gave this effect. Then lean out those areas. If some needle areas are rich and others aren't, then it's time to go to school, Jetting Q's.

If this isn't enough or you need more detail about the test or basic jetting questions, reply back. I also posted a ' If you have never jetted but think you need to... ' on the 250F forum that might help if you've never opened your carb, skip it if you have.


well done mark. hopefully next time you get a question that's a rich needle clip you'll keep answering them. phew!! mouse-wheel-mouse-wheel!

i would add that i did my needle tests as follows;

flat out in top sat down. close throttle while you say right here (right here, right now is exactly two seconds) remember the fat block who did that track) puff daddy or someone (i'm way up on the music scene!!!!!) oh i just remembered "fat boy slim".

anyway, flat out close throttle for rendition of "right here" and open again. THAT'S THE NEEDLE.


Thanks for the help Mark. My bike is still completely stock. The furthest I have tried was removing the baffle from the pipe for a couple runs. I thought the bike had more of a problem on the high end with the baffle removed but this may be me noticing it because it was so much louder. How much a difference should removing the air box lid make?


Sorry, I'm about to lecture again. The power an engine makes (skipping lots of other limiting items like heat dissipation here) varies with the amout of air it gets. To reduce power, close the throttle, which blocks the air intake, which reduces the amount of air. This assumes the carb is doing a reasonable job of adding fuel in the appropriate proportion to the air. To increase power further, increase the amount of air forced into the carb (supercharger or turbocharger). At high altitude, there is less air pressure, so less air goes in, the engine can't make as much power (only 1/2 sea level power at 16,000 feet). On a hot day the air is thinner, less power. On humid days more of the air is water and therefore less is oxygen, less power (in planes we call this the HHH effect for High, Hot, and Humid).

The various things between your bike's intake manifold and atmospheric pressure are the pressure loss in the manifold, carb, boot (slight), the throttle slide (under your control), the air filter (ok a substantial drop but if clean and not overoiled it's the best you can do) and the airbox lid and snorkle.

Going to a more open pipe requires a richer mixture (don't know why but almost universal reports of it). Going to a more open airbox might require a leaner mix due to more vacuum upstream of the throttle slide. But there are so many effects (less air through the air jets and emulsifier tube, etc.) that I can't predict it.

Netting it out, the airbox lid will restrict flow, primarily at high RPM, and your bike will have more power with it off. I can't imagine it would be more than a few percent (that said, if you closed it off to the point it was as small as the throttle opening, it would reduce the power as much as that throttle position does). It may have an impact on jetting, but I doubt you could tell.

Your original post described a rich condition. Ride it like it is for a few rides. It won't have much power and until you get used to it that may be a good thing. Then if you choose to do the various free mods, do them before climbing into the jetting because they may change the jetting requirements.

Where is Benwood? I ride around Pgh sometimes.


Benwood is about 5 minutes outside of Wheeling, WV. I'm about an hour south from Pittsburgh.

Again, thanks for the help.


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