stalling with rapid throttle twist

42 replies to this topic
  • Fastest1

Posted May 09, 2002 - 07:55 PM


First of all a Thumper isnt quite as sensitive to jetting as a 2 smoke! You would have to get your jetting off pretty bad to sieze it. Anyway quit whacking the throttle in neutral! Put the bike in gear and ride it does it still do it? These bikes have plenty of power stock.The BK mod did help my starting problems and maybe crispened response a bit. Read the manual and do what it says about jetting. It is good information. Good Luck

  • Mark_Cantrell

Posted May 10, 2002 - 04:44 AM



Yeah, it is a little hard to get started in the carb. It can also turn into a multi day ordeal with lots of orders to Sudco. If you're interested, I posted 'Jetting 101' on the 250F forum that gives the basics of getting into the carb and very basic jetting.

You didn't say how bad it is. When you ride it, does it have an impact on your style or ability to take obstacles, speed, etc? If not, and its new, just go out and ride it and worry about this later.

As Fastest says, they aren't as sensitive to lean problems as 2 strokes. That doesn't mean lean them way lean and run them hard. You could hurt one of these too, it would just take more.

The manual mxrider quoted either directly contradicts what I just said (and would mean if you need to do anything, go to a larger leak jet) or if whack and let off and the problem is after letting off, does not impact what I said. Hopefully someone smarter can clarify this.

Finally, the two workarounds that might mask the symptoms, high idle and fuel screw, are just workarounds but why not? What is happening now won't hurt the bike. If you mask the symptoms enough it's fun and you won't get hurt (stalling on the face of a double, etc.), just do it.

Good luck,

  • Hick

Posted May 10, 2002 - 07:52 AM


Hey Mark,

I don't think you can change the leak jet on the 39 Flat CR. I don't know why you can on the 250's carb and not the 426...

It was suggested to me once that perhaps the float bowl could be retrofitted...

  • Hick

Posted May 10, 2002 - 07:54 AM



Who cares what the damn thing runs like when its on a stand? How does it go with you on board?

That's how I jet my bike, anyway. With me on the bike, and the bike on two wheels.

Hope this helps.

  • yznvegas

Posted May 10, 2002 - 11:05 AM


OK, update, just got the bike back from the dealer and they said all they had to do was richen some screw on the carberuater. You can access it with a special screwdriver without even taking apart the carb. I will let you guys know how it runs later tonight. Thank you.

  • yznvegas

Posted May 10, 2002 - 11:28 AM


Oh yah, they said the oil was overflowing because the lot boy put way too much oil in it. They said don't worry about it, the mechanic took a siphon and pulled the oil out.

  • Hick

Posted May 10, 2002 - 01:04 PM


"Danger Will Robinson, Idiot Dealer Alert!"

If it were me I'd change the oil anyway and put 1.7 liters in to be sure.

BTW a few drops, even as much as a teaspoon full, out of the breather hose after a ride is normal.

BTWII the screw they reference is the idle mixture screw, and you don't need any kind of "special" screwdriver, just a really short, small slot. Or you can just use a slotted bit from a bit driver and turn it with a pair of pliers, this way it is easy to measure 1/4 turn, for example.

Hope this helps.

  • yznvegas

Posted May 10, 2002 - 01:23 PM


Hick, would you change the oil and filter or just the oil. I only have about 1 hour of ride time right now.

What kind of oil would you use?

  • skthom2320

Posted May 10, 2002 - 05:39 PM


Change the oil often on your bike. I would probably change after each ride the first few times. I change mine after every other ride and clean/replace the filter every other oil change.

Inspect the filter for tears each time. If it is fine, just clean with carb cleaner and use it again.

Use a high quality non-synthetic for break-in then change to a synthetic. Beware though, oil brand is like religion. :) You are safe with Yamalube 4 (non-synthetic) then switch to 4R (synthetic). Use the searh function and enter "oil" as your search term. Plenty of reading. The bottom line is that you can use anything that is the appropriate grade (see the manual, I think it is SJ or higher) and NOT "energy conserving." It will say in the bottom part of the API label on the bottle if it is "energy conserving." For example, I and others on the board use Mobil 1 20-50 (in the red cap).

  • Hick

Posted May 11, 2002 - 09:17 PM


What he said.

You can clean the filter by backflushing with contact cleaner or diesel or....

You may find some metallic debris, don't be alarmed, that is normal for a new motor, and is also why I think the first few oil changes are critical.

For my own piece of mind I just round-filed the filter and replaced with a new one (about twelve bucks) after the first few changes. By then you should not be seeing very much debris at all.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • yznvegas

Posted May 12, 2002 - 10:18 AM


Okay, I went riding yesterday and the bike still stalls with a rapid throttle but now upon engine braking the bike pops and backfires???? The dealer said all they did was richen up the idle screw.

Also, upon your guys advice I went to change the oil yesterday but couldn't break the strainer bolt. I posted the question about it under "oil change"....thanks guys, your tips are very much appreciated.

  • Mark_Cantrell

Posted May 12, 2002 - 11:55 AM



They may have gone the wrong way on the fuel screw. It is backwards from the air screws found on almost every other carb. You needed richer and now your symptom is even more lean than it was.

If you haven't already done it, the fuel screw is in a recessed hole in the front middle of the carb just in front of the bowl. You get to it with a small screwdriver (you may have to shorten one) going up from the trans case. It is easier to twist the bottom to the shifter side and use a regular small screwdriver, although if you've not twisted it many times, it is stiff. Anyway, turn the fuel screw in for less fuel and out for more. According to your description, you need more fuel (out or CCW). First count the turns (in 1/8th or 1/4 turn increments) to turn it all the way in. Then back it out to where it was plus 1 turn. Then try it. If the screw is out more than 2 1/2 turns, you may need a bigger pilot jet. Don't back it out more than 2 1/2 turns because it may fall out while riding (although it will still run above idle even with it out). The bike comes with a spare but I think it was smaller.

Post your results,

  • yubs

Posted May 12, 2002 - 02:10 PM


I've got an 02 426 and it occasionally dies with a rapid throttle hot or cold. What is the BKmod I keep hearing about? I am a new rider and don't know a lot.

  • RSA210

Posted May 12, 2002 - 03:07 PM


YZNVEGAS Do a search on Kouba T-Handle,Fits over fuel screw.Makes it real easy to play with idle mixture.

  • Mark_Cantrell

Posted May 12, 2002 - 04:08 PM



Four stroke carbs usually have accelerator pumps. When the throttle opens rapidly, especially at low RPMs, the vacuum goes away reducing the 'signal' (or indication or method) to the jets to pull gas up from the bowl. As the revs build, the vacuum comes back. The accelerator pump, unlike any of the other fuel circuits on the bike, doesn't rely on vacuum to pull fuel. Instead, an actuator connected to the throttle pulley goes into a plunger (diaphram) on the bottom of the carb, squirting fuel as the throttle is twisted. This produces a squirt only while the throttle is being added, not at a constant throttle or trailing throttle.

Like any other circuit (e.g., pilot circuit, needle circuit, or main jet circuit), there may be more fuel added or less fuel added than is needed by the AP. In addition, unlike any other circuit, the AP squirt may be the right amount but not last long enough (to build revs) or too long (being rich after revs build).

Newer bikes (either 2001 and up or just 250Fs) have a leak jet that 'leaks' some of the squirt back into the bowl. The AP is purposely built too strong so a smaller leak jet would send most of the fuel into the throat and a larger leak jet would send less into the throat (that is, more would leak back into the bowl). This allows adjustment from too much to too little.

Similar to the way leak jets reduce the volume of the AP squirt, the BK mod reduces the duration of the squirt. The BK mod consists of drilling and tapping a blind flange on the carb, putting in a small screw with a spring to keep it in place, that limits the travel of the cam connected to the throttle pulley that actuates the AP. It will not make the squirt longer, but by limiting the travel, you can adjust the squirt shorter than whatever it naturally is.

Most of these bikes come with enough or too much and too long AP squirt. Larger leak jets and the BK mod control each. DOC has found bikes that have too little duration and/or volume. He plugs the leak jet entirely and then uses the BK mod to adjust the duration back down.

yznvegas did not have enough AP squirt (in my opinion) and the shop he took it to intended to mask this by adding fuel all the time (larger pilot and/or backing out fuel screw). Instead, it sounds like the reduced the amount of fuel (now it pops more on the pilot circuit, a sure sign of lean) making it worse.

Oh well, hope that made sense,

  • Mark_Cantrell

Posted May 12, 2002 - 05:36 PM



As hard as this is to believe, I expanded on that reply on the 250F forum as Accelerator Pump 101.


  • Hick

Posted May 12, 2002 - 08:54 PM


Originally posted by Mark Cantrell:
Newer bikes (either 2001 and up or just 250Fs) have a leak jet that 'leaks' some of the squirt back into the bowl. The AP is purposely built too strong so a smaller leak jet would send most of the fuel into the throat and a larger leak jet would send less into the throat (that is, more would leak back into the bowl). This allows adjustment from too much to too little.


Please stop it with the leak jet talk. That doesn’t apply to the 426.

Either that or I’m an idiot. So call it 50/50.

  • Thumpin_426

Posted May 22, 2002 - 04:48 AM


Dude, You NEED the BK Mod!! Every 426 has that problem off the show room. Do the mod and fine tune the jetting according to your elevation and your bike WILL rip!!! Forget what the dealers tell you. This web site is better than any dealer or any manual. Here you are talking to people who has experienced it first hand and the hard way. Simply conduct a serch on this page for "BK Mod" for the last year or what ever, print them out and do it. It takes about 30 minutes.

This WILL help!!

  • yznvegas

Posted May 22, 2002 - 05:33 AM


Thanks Thumpin, I think I will do the mod after the bike is out of warranty in another 2 1/2 months.

  • gonzo

Posted May 22, 2002 - 12:58 PM


I had the same problem and I took it to a mechanic. He told me there is a set screw on the accelorator pump in the carb. That has to be adjusted.

Related Content


YZ450F 03 Sparks driving me crazy by SirAttard

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 5 replies

Help with 2000 yz426 by weekend woods racer

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  36 replies

Is it worth it? (Cam and High compression piston) by macgi77

Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies

Thinking about Yamaha 250... by Arctic Pride

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  491 replies

Is the 2016 GYTR Yamaha YZ450F too fast? by Fryboy

Dirt Bike   Special Interest Forums   Pro Racing
  • * - - - -
  • Hot  241 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.