426 low-end hesitation- again

i've got an idea but because it's not the usual kind of answer & it costs a lot you might not want to try it.

we used to increase the snap on the road racers by making that little archway at the bottom of your slide smaller. the engine i s trying to suck "x" cub. ft. of air & it has only a small cutaway. you make it smaller & the air has to go through a lot quicker to keep up.

this means it is able to suck on the fuel harder & to respond quicker!

the slide cutaway on the wr is "15" if yours is the same try down a size to a 10. this will suck so hard that your pilot jet may have to go down a size as well, so buy the next size down at the same time.

i believe you can get this from someone motogreg knows. carbwarehouse or carbwholesale or somebody.

at the end of the day it's not my money i'm spending, so...


I know this has come up a jillion times before, but I have looked and can't seem to find the posts so here we go again. My 426 has had the low end hesitation I've heard so many talk about but it hasn't been bad enough on mine until after a stall in a moto last friday night. I think I'm finally getting to the point in my riding where this will be a factor for me. Anyway, I read the MXA article and brought the pilot screw out 2 turns, this helped but it's still there. They also suggest dropping the needle jet clip 1 position but I thought I'd ask you guys if this would help before I take the carb off. (help me Hick)... Any other suggestions would be appreciated, carbs are not my strong point... Oh yes, I am riding mostly between 600' and 1200' altitude. thanks...



Yes I would try richening the needle a clip or two to get rid of the stumble. If one clip position has no effect then try two (clip down, needle up to richen, of course). If things still don’t improve I would put the clip back where it was and go one richer on the pilot jet and start over. If you change the pilot you should probably also go back to 1-3/4 turns out on idle mix and reevaluate.

If the stumble you are describing happens only when the throttle is severely whacked open you may be able to cure it by lengthening the stroke of the accelerator pump. If the stumble happens during quick throttle openings to less than half throttle then this will probably have no effect. A benefit of this adjustment is it is very easy and feedback is very noticeable. Just make sure you remember the stock position so you can return to the base setting if things get out of hand.

True, in the manual it very clearly says not to adjust this screw (it also says there is no need to adjust idle mix :) ) but I have had some success with this on my 426 with no ill affects. I had cured myself and my bike of the “YZ stumble” (by rejetting) but I noticed while trying some low speed wheelies that the stumble was still there when I quickly rolled the throttle beyond a half turn. It wasn’t noticeable during “ordinary” riding, but I never do moto.

All you need to remove to get at the adjuster is the small allen bolt on the black plastic piece that covers the slide pulley and throttle cables on the right side of the carb. Remove this fastener and pull the plastic away and you will see a small screw under the pulley. The end of the screw butts against a plastic cam. By tightening or turning the screw clockwise the cam is pushed further back. This increases the starting point and thus increases the stroke of the accelerator pump.

Try turning this screw a half turn counterclockwise. You should be able to easily stall the motor now with a quick twist. Go back to the stock setting and then try a quarter turn clockwise. The affects of a quarter turn either way should be very noticeable. I ended up one quarter turn in over stock.

So at that point where my YZ would cough is where, I assume, the pump had completed its stroke. By lengthening this my YZ is now still getting extra fuel where it wasn’t before. So now I can be below idle and really whack it open and it doesn’t stumble. I almost looped it trying to make it cough. Okay I did loop it, but I caught it before I trashed my fender. I can now no longer create any stumble on my YZ no matter how hard I try.

Make sure your bike is fully warmed up and your idle is set properly before trying this. I imagine this screw exists to realign the pump stroke after slide height is set (set screw on top of slide). Also, keep in mind that all this works in concert with jetting. I may indeed be simply covering up a crappy jetting job by playing with the pump, but I don’t think so. If you think you are jetted lean then fix that first.

A side issue is that the pump on the FCR tends to collect grunge. Frequent washings or water crossings may also corrode the pump push rod. I have experienced the grunge effect, YZ stumble was so bad the bike would die because pump wasn’t working properly. I now have a fuel filter on my bike. The corrosion thing I don’t know about because nothing corrodes in New Mexico, I think I read that in Dirt Bike…

There are also a few companies that make replacement pump housings. I think the lastest Dirt Rider has a small test on one made by Factory R&D. In the article it was noted that the ’01 YZ pump delivered less fuel but over a longer stroke, so maybe we should all try to swap the ’01 PN (when they get here).

Someone on WR side also tried another brand a while back called the P-38 I think. I’d love to hear about it if anybody else has tried one of these things (they were both about $80). Another curious thing is that there are like four different pump diaphragms offered from Yamaha. I have asked everyone I know to ask and nobody knew the difference between the different PNs. I bet some displace more or less fuel than standard. So I’d love to hear if anyone has tried changing that also. Because with two aftermarket pump housings and a change to the pump for ’01 I am convinced that the cure for infamous “YZ Stumble” lies in part with the acceleraor pump.

Sorry for the long post…

Hope this helps!

Thanks Hick- I finally found on dirtrider.net where this had come up before but that really helped. I'll probably live with it until after a moto on saturday, but then I plan to do some tinkering and I'll post back what I found. Thanks again...



Interesting results on the pump adjuster screw. I've been wondering if there would be some benefit from altering it.

The Factory R&D accelerator pump body is named the P-38 Lighting. Just the one and the same.

Not even Sudco has been able to tell me what those optional diaphrams are for. Still curious about what they do too. :)


[This message has been edited by James Dean (edited 09-20-2000).]


I suspected the R&D unit was the same one talked about on the WR side. Plus it was Dirt Bike that has the “test”, not Dirt Rider. Oops. :)

I wonder which part will be changed on the ’01 pump? The rod, housing or little plastic arm? Or nothing at all? The same DB issue that said the pump was revised said that another change for the ’01 will be a magnesium clutch cover. Funny, my ’00 says “Magnesium” right there in raised letters…


All I have to say is "great description" on how to adjust the accelerator pump. I have had this problem since I got my bike and no one has been able to put a finger on it. I've also heard that all the problems lie in the accel. pump, but we'll see what they actually change for the 01. Thanks again and I'll let you know what differences I find in the adjustment. BTW, what is your current jetting? M- P- AirScrew? Thanks.


Hello group- I thought I would report back what I found. I adjusted my pilot screw out to 2 turns and this helped noticably. The thing that helped the most though was changing the spark plug. This was the first time I've changed mine (probably 25 hours or so) and I couldn't believe the difference, had to even lower my idle a bit. It's alot tougher to change on this bike than say a 2 stroke but was definately worth the trouble...



Every bike I’ve ever owned ran rich here (NM, 4,500 ft. elev.) so when I got my 426 I started playing with the jetting and leaning things up. That is when I discovered the existence of the accelerator pump and the dreaded stumble. I liked the leaner pilot and main but went back to stock clip position and the stumble was gone.

I didn’t notice it before I started rejetting, that is why I think it is usually on the needle. Anyway this is where I ended up, but I also added a PC T4 when I jetted the bike:

160-40-1.75 clip in stock position (or all stock w/ one size leaner M & P)

But I ride from 4,500 to 7,500 ft., 90 to 110 degrees, 20 to 40 % humidity, so this may not help too many people. It is beginning to cool off again so I may need to revise this.

BTW, don’t call it an air screw. I’m not one of them, but some folks around will be quick to correct you and say, “it’s a FUEL screw.” :)


Yeah I was not happy the first time I replaced my plug but I'm used to it now. But its easier than an R&R of the needle...

Also, Im going to Glamis next month. I was planning on just throwing a 45/165 in there and maybe raising the needle and hoping it ran well, I don't want to have to rejet once I'm there. Maybe I'll start a new topic to ask what will work, but I'll check here if anybody has some suggestions.


[This message has been edited by Hick (edited 09-22-2000).]


There needs to be a minor correction above. After looking closely at the accelerator pump screw, it appears that backing out the adjuster screw (counterclockwise) makes it pump earlier. This lowers the rod down closer to the diaphram and reduces the gap.

Turning it clockwise delays the pump action. The stock setting on my '00WR was 1 turn clockwise from zero clearance. Zero clearance would cause an instant pump action from idle and would squirt fuel against the slide below 1/8 throttle. The freeplay can be felt by rotating the cam arm with your finger. With 1 turn on the screw the pump starts to actuate at 1/8 to 1/4 throttle. What do you have yours set at in light of this reference position??


[This message has been edited by James Dean (edited 09-27-2000).]

For anyone interested this topic has sort of migrated over to the WR side under “James Dean – Accelerator Pump.” James has some good input and information on this issue, and I think anyone experiencing the “YZ stumble” (MXAs words, not mine) should have a look.

Thanks for the info Holeshot.

A few comments:

I believe the test you read is the same product, I have seen two small tests, most recently in Dirt Rider.

The manual also states, among other things, that the idle mixture does not need to be changed from the factory setting. :)

SoCalErik has the Factory R&D pump housing on his ’00 KTM 400 SX and has said it works well and did improve throttle response.

Turning the screw further delays the pump action relative to the slide position. The pump does not deliver fuel immediately upon opening the throttle, turning the screw in increases the delay but does not change the amount or velocity.

More than one part may be changed but I agree with your statement about using part(s) off the ’01 (assuming it is different) instead of the $75 (or so) for the P-38, it is bound to be much cheaper that way even if all the pump parts are changed.

All this stuff is pretty well hashed out on the WR side under the “Accelerator Pump” heading.

I'd forgotten about the WR forum.

Whew! There's a lot of info about the accel. pump there.

Thanks for the insight on how pump adjustment works.

Apparently, the '00 426 squirts a bit too much fuel at initial throttle openings, causing a temporarily rich condition and a stumble. I haven't really noticed the problem at elevations I normally ride at (500 to 2500ft), but the problem did surface at Big Bear, CA (6000ft).

The slight "burble" when rolling the throttle back on doesn't bother me much, because I rarely ride at that elevation, and I adjust my riding style to avoid it.

A company called Factory R&D sells a replacement bottom plate that is claimed to better atomize the fuel and improve throttle response on the 426. A couple of guys tried it on their 400's and didn't notice any difference, but the 426 guys who tried it said there was an definite improvement. I saw an article in one of the recent dirt mags about a plate or aftermarket part that allegedly solves the problem (I'll check and and post again if it's not the Factory R&D part).

For '01, Yamaha has changed the accelerator pump to deliver less fuel when the throttle is initially cracked and (possibly) more fuel as the pump nears the end of it's cycle. If the parts are easily interchangeable and cheap enough, I think I'd put the '01 part(s) on instead of aftermarket.

I've looked at the pump adjustment on the carb, and noticed that that the pump plays "catch up" when the throttle is opened quickly, in other words the pump is not linked directly to the throttle like the trigger of a squirt gun. I feel that adjustment of the linkage is not covered in the manual for a reason – Yamaha doesn't want owners to mess with it.

If I felt there could be a benefit from turning that screw, I'd twist away, but I can't see it.

Have any of you guys ever had your bikes hooked up to an exhaust gas analyzer? The shop that I worked at in CA would check the bikes as a part of the setup. It's a good place to start. Anyone in the socal area with these woes should email me, I know a guy who's pretty good with the thumpin' beasts.


The faster I go the harder I fall....

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