2002 426 missing at 1/8 to 1/4 throttle

Hey guys.  So I am new to the 426.  I got the bike about 2 months ago and have been working out some kinks in it.  One that I am having a hard time with is a miss/surge/cutout at 1/8 to 1/4 throttle.  Bike makes great power.  I can twist the throttle through the miss but if I hold it steady or play with it at 1/8-1/4 it will miss.  I have had the carb apart and cleaned and inspected all mains and jets.  I have it set up to factory specs(according to Yamaha manual).  I have been checking out other post that may be related but have come up with nothing.  I do not have the BK mod but I am not sure this is my problem as it is a miss not a bog.  Its almost like I have weak spark at that point, or I am over fueling.  I ride with a 2011 YZ450F and a 2013 CRF450 and I have not issue keeping up as long as I twist past the miss.  Bike has a new plug.  I have not had a chance to pull it out and check it since I put it in(5 rides ago).  I have also just brought home a timing light from work to check my timing but im not sure im headed in the right direction.  Im a little baffled as is my YZ450F buddy.  We are both car mechanics so this is making us look a little bad that we have not figured this one out yet lol.  Any ideas/questions would be great Thanks

To start with, your instincts that this is not going to be cured by the BK mod are correct.  That's an accelerator pump tweak, and it does nothing to resolve a problem at a steady throttle.


Second, just for shits & giggles, try unplugging the TPS and running it without it.  If it cures things, test, adjust, and/or replace it per the manual.  Or just run without it.  Only less-than-full-throttle ignition timing will be affected.


Next, you need to figure out whether it's rich or lean at that throttle opening.  If the miss is regular, and acts like the choke was left on (and/or the choke makes it worse), it's rich.  If it's lean to the point of missing, it should be irregular and erratic, and may "surge" and/or hesitate if accelerated slightly.


You will probably find it rich.  The range from off-idle to a quarter or so is primarily controlled by the upper, non-tapered portion of the needle, with some early overlap from the pilot. Inspect the needle for wear, and also look at the hole in the top of the main discharge nozzle, or "needle jet" to see if it has an "ovaled" appearance.  If you find either, that's likely your problem. 


Another place to look is at the vacuum release plate on the slide, and the little seal that fits it to the slide.  Look for cracks, missing corners, and improper installation.  It should be installed with the "square" side that has the hole near the edge down, rounded end up.  The seal will typically come out of place swollen and wavy and useless looking.  Let it dry overnight and it should be in better condition.


The carb could also have a leak or blockage between the upper and lower sections of the main body, which joint is closed by 4 screws (often 'security' Torx).  Only JD Jetting offers the formed gasket that goes there.  Get one BEFORE you separate these two parts.


Failing all of that, replace the carb with one from an '05 - '09 YZ450 and wonder why you didn't do that first.

Wow. Thanks for the info. I have read your posts on my other pages and sounds like u have been around the block once or twice. So it sounds like I should just get a newer carb. I read other post and I believe that u said that the throttle cables and hot start lever/cable with have to be changed. I assume I get ones of the same year as the carb?

Oh and jetting. Do I run the stock jetting as a base and go from there or should I be running the stock jetting for the year of carb I can get?

You don't need a new carb.

Replace the needle jet. It will be worn.

You probably should start by checking the current carb to see if you can easily fix it, but the 450 carb swap is an absolutely excellent mod for the 400/426.  Yes to both the questions you asked above, and the throttle cables will fit your throttle tube.

I ordered a carb kit off of eBay this morning so I will update once I get it and install all the new seals and jets. Thanks for the advice guys. I really enjoy the bike so I will look into a newer 450 carb as well.

Hey guys. So I had an Exhaust on order as the one on the bike had a broken mount and was bent. When I was installing it just for shits and giggles I pulled the plug out of the bike. Black black black. So I pulled the carb off and raise the clip one slot on the needle(so the needle is now lower) to lean it out. I have been riding all afternoon now and the bike has not stumbled once. Guess that's a simple lesson. Look at your plug dummy

Edited by 02-426F-124

Good info as always Rich...I was coming on here for the very same problem on my '08.  It is stumbling at low, steady throttle but runs like a champ everywhere else...very annoying.


I pulled the carb and gave it a thorough cleaning and even replaced the pilot and main with new to be sure there is no varnish buildup but it still does it.  I inspected the needle and it appears to be fine. I have fiddled with the fuel screw and it made only a very slight difference.


All that said, all my recent riding has been at higher elevations (3,000 - 6,000') in fairly warm weather and I am running stock jetting and needle/needle position so I am definitely in an environment that will cause a rich fuel/air ratio, however in the past I have never had to rejet for this area/temp...adjusting the fuel screw has always taken care of it. According to the jetting spreadsheet I should drop to a 42 pilot but I have never had to before.


I will ride it in my neighborhood (500' elev) and see how it does. Also going to check the seal on the vacuum release plate as you mentioned.


Are there any adverse effects to running with the TPS disconnected?

Unplugging the TPS turns ignition mapping from 3d to 2d, which means instead of the ignition reacting instantly to a twist of the throttle, it will rely on the RPM.

On my 400f I found that unplugging it dulled down throttle response a lot up to 1/3 opening. You might have different results though depending on your jetting.

In the end my needle jet was worn, and a simple $10 replacement completely fixed my st-st-stutters.

Results on an '08 450 are apt to be a great deal different.  A lot don't notice anything.


The map does go from 3D to 2D, but it's analogous to disconnecting the vacuum advance on a car old enough to have one.  The ignition is advanced under light loads to improve part throttle performance.  When the TPS is disconnected, the system assumes full throttle (since that's the safe thing), and times the spark based only on RPM.  No danger to the engine or electricals.

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