2002 YZ426 Questions


14 replies to this topic
  • Spartan426

Posted December 14, 2011 - 04:08 PM

#1

Hi all,

I have a 2002 Yamaha YZ426f and have been rebuilding it from boxes, its been a great project and I've put about 15 miles on the engine, Innnncredible bike. (also my first)

Anyhow, I have been reading all over the 400/426/450 forums and there are a lot of mods, I've been seeing for mainly 98 - 01 years, from air box mods to carburetor to clutch, even 03 exhaust cam mod,

My question is, sense mine is an 02 YZ426, what mods are recommended for my year/model if any? and how much does the mod benefit/cost? (my bike is a completely stock engine but has been rebuilt from the ground up
(look at my garage page for info on my bike, i keep it up to date)

Thank You!
-Spartan :bonk:

  • DeeCee

Posted December 14, 2011 - 09:01 PM

#2

I guess the mods depend upon where and how you intend to ride the bike.

I have been riding a 400 for a number of years now in the desert and have found the four most beneficial mods were (not is order of benefit):
Fitting a side stand (Trailtech stand)

The decompression cam mod (to ease kick starting, especially in awkward situations).

Reworking my suspension with Race Tech gold valves (makes the rider much smoother)

Fitting a steering damper (for the sandy ruts)

  • RasmusDK

Posted December 15, 2011 - 02:47 PM

#3

Get some miles on it first, if anything works for now like you think it should, i wouldnt do anything. It depends on what you use it for too... The best thing i did to my 400 was to put a 08 450 carb on it, the power seems higher and its sooo much more smooth.. nice rebuild:thumbsup:

  • cowboyona426

Posted December 15, 2011 - 03:46 PM

#4

Auto decompression cam is a great mod for your bike. You don't need the 01 clutch mod, you already have it.

  • Spartan426

Posted December 15, 2011 - 06:53 PM

#5

I guess the mods depend upon where and how you intend to ride the bike.

I have been riding a 400 for a number of years now in the desert and have found the four most beneficial mods were (not is order of benefit):
Fitting a side stand (Trailtech stand)

The decompression cam mod (to ease kick starting, especially in awkward situations).

Reworking my suspension with Race Tech gold valves (makes the rider much smoother)

Fitting a steering damper (for the sandy ruts)



Thanks for the suggestions I do plan on getting a kick stand of some sort and a steering damper/suspension work when I have the $

Get some miles on it first, if anything works for now like you think it should, i wouldnt do anything. It depends on what you use it for too... The best thing i did to my 400 was to put a 08 450 carb on it, the power seems higher and its sooo much more smooth.. nice rebuild:thumbsup:


Thank You! :bonk: that's the kind of mod I was thinking of, as a 400 has a 08 450 carb you've updated the bike and it runs better

Auto decompression cam is a great mod for your bike. You don't need the 01 clutch mod, you already have it.


could you explain the benefit of the auto decompression? my bike starts first kick from cold if you do the "trick" right, (though the wired thing is once its ran, its a pain to get started again) does it just save you from having to use the compression release? does it add HP?


I'm sorry I should have clarified when I posted (sorry It's my first post) I'm looking for info for my bike on specific year upgrade mods, like if you had the 98 - 2000 models you can use a 01 clutch mod, or the newer carburetor from a 08, as RasmusDK was saying, things like this, taking a older year bike and putting newer parts on it from a newer model to have the new-er part/engineering in your engine. These are the kind of mods I'm looking for, the bike is incredible, but I'm just wondering if there are parts that can be put on it to make it even better from newer year models.

(also this bike is mainly for trails, though I plan on using it for everything when I have the $ and ability to go to the track and do Enduros, I'm 17 Right now and wont have my license till febuary lol)

Thanks For Your Responses!

  • RasmusDK

Posted December 16, 2011 - 01:46 PM

#6

The auto decompression is basic just damn handy if you want/need to kick start in awkward situations like DeeCee said :bonk: You will find out when you get there, or maybe go up a 45 degree slipy hill, stop in the middle of and kill the engine and start it again =) a 20 degree hill will make you understand too hehe. IDK about HP, but expect it to have more the enough already :smirk:
If you want a new carb go for one that has a leak jet, i think the 450 got them in 06, search the forum or just buy a 08 one.

  • Spartan426

Posted December 16, 2011 - 03:59 PM

#7

The auto decompression is basic just damn handy if you want/need to kick start in awkward situations like DeeCee said :smirk: You will find out when you get there, or maybe go up a 45 degree slipy hill, stop in the middle of and kill the engine and start it again =) a 20 degree hill will make you understand too hehe. IDK about HP, but expect it to have more the enough already :banana:
If you want a new carb go for one that has a leak jet, i think the 450 got them in 06, search the forum or just buy a 08 one.



So it just makes it easier to kick over? how so? you don't need to do "the trick" anymore just kick it trough? or still do the trick but with better odds of it starting? And that's true the beast has enough HP as it is, haha :bonk:

Whats a leak jet?

so I'm guessing sense mine is an 02 which is the last year they made 426s, there are not really any "mods" to update the engine/components besides the 03 cam mod, and the 08 carburetor?

(just out of curiosity, why a "03" exhaust cam? is that just the only/best year that matches for a 426?) how much is the mod's total cost? is it hard to do?

Thanks for the replies!

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  • grayracer513

Posted December 16, 2011 - 08:05 PM

#8

Whats a leak jet?

A leak jet prevents the accelerator pump from working when the throttle is opened slower than a certain rate in order to make the engine run cleaner during part throttle maneuvers. The '05 and later YZ450's had them.

Read:

http://www.thumperta...330#post4415330

  • Spartan426

Posted December 16, 2011 - 11:04 PM

#9

A leak jet prevents the accelerator pump from working when the throttle is opened slower than a certain rate in order to make the engine run cleaner during part throttle maneuvers. The '05 and later YZ450's had them.

Read:

http://www.thumperta...330#post4415330



WOW that was thee biggest post I've ever seen/read, but thee most informative!

thank for sharing!

  • ktmm

Posted December 17, 2011 - 09:36 AM

#10

I have a rebuild kit that came with my 2000 426 bike.You pay shipping its yours.

  • RasmusDK

Posted December 17, 2011 - 03:28 PM

#11

So it just makes it easier to kick over? how so? you don't need to do "the trick" anymore just kick it trough? or still do the trick but with better odds of it starting? And that's true the beast has enough HP as it is, haha :bonk:

Whats a leak jet?

so I'm guessing sense mine is an 02 which is the last year they made 426s, there are not really any "mods" to update the engine/components besides the 03 cam mod, and the 08 carburetor?

(just out of curiosity, why a "03" exhaust cam? is that just the only/best year that matches for a 426?) how much is the mod's total cost? is it hard to do?

Thanks for the replies!


He yea you can then kick all the way thru, insteed of finding TDC, decomp, kick it a bit further, and then kick. Guess you still got find TDC to get the best result.

What i ordered to my when i did the carb the clutch lever an mount, to get the hotstart fitted as it should by your left hand. The newer lever and mount made my clutch easier to pull too. I think you old throttle house will for you but the cables are diffrent so get then too :smirk:

  • Spartan426

Posted December 17, 2011 - 09:11 PM

#12

I have a rebuild kit that came with my 2000 426 bike.You pay shipping its yours.


What kind of a rebuild kit? carburetor, engine, or somthing else?

He yea you can then kick all the way thru, insteed of finding TDC, decomp, kick it a bit further, and then kick. Guess you still got find TDC to get the best result.

What i ordered to my when i did the carb the clutch lever an mount, to get the hotstart fitted as it should by your left hand. The newer lever and mount made my clutch easier to pull too. I think you old throttle house will for you but the cables are diffrent so get then too :bonk:



Well that sounds like a pretty cool mod, when I have some $$$ ill look into it =) I'm sure after riding my bike for awhile ill be in... "situations" when it would be nice to kick it over easier lol, (does it help w/ flooding? if you flood it, how do you clear it without a normal decomp release to clear the cylinder?)

Thats pretty cool I'd like my hot start to be on a lever mounted on my bars, ive seen kits you can do it w/

Thanks for the info on the carburetor swap mod, sounds pretty cool as well ill keep that in mind, is an 08 the best one or "thee" one that matches w/ my/our modle bikes? how do you find out?

(roughly how much does the cam mod cost? whats the price range on an 08 carburetor?)

Thanks for the responses!

  • board9898

Posted December 17, 2011 - 09:35 PM

#13

A leak jet prevents the accelerator pump from working when the throttle is opened slower than a certain rate in order to make the engine run cleaner during part throttle maneuvers. The '05 and later YZ450's had them.

Read:

http://www.thumperta...330#post4415330


A leak jet DOES NOT prevent the accelerator pump from working. The leak jet controls the volume of fuel from the accelerator pump to the engine. Excess fuel from the pump is allowed back through the leak jet to the fuel bowl.
The larger a leak jet the lower the volume of fuel pumped to the motor. And not surprisingly the smaller the leak jet the great the volume supplied.

The acclelerator pump operates on throttle opening by squirting additional fuel just behind the throttle slide. It has no function at constant or closed throttle, only openning.

  • grayracer513

Posted December 18, 2011 - 12:50 AM

#14

A leak jet DOES NOT prevent the accelerator pump from working.

I'm afraid it does, in fact.

The accelerator pump is a positive displacement pump, meaning that if the piston moves at all, it will displace a certain amount of fuel, based on the bore and stroke of the diaphragm. In an AP circuit without a leak jet, regardless of how slowly the throttle is opened, a fixed amount of fuel while always be discharged into the carb throat through the pump nozzle. But there is a certain amount of pressure to raise fuel that far and squirt it through a little nozzle, so what happens when you put a leak jet into the bottom of the circuit is that fuel flows out into the float bowl rather than rising to the nozzle until the throttle is opened quickly enough to create the pressure necessary in the circuit to lift the fuel the rest of the way. So it prevents the AP from working at slower throttle opening maneuvers like controlling speed in a corner.

Notice that the first bikes in the line to get leak jet carbs were the smaller displacement YZ250F. The 37mm carb bore is 15% of the displacement, so naturally air velocity at low speeds will be a lot lower than with roughly 9% of displacement that the FCR 39 is on a 450. The high strung little thumper needed a lot of help from an accelerator pump to make it respond to sudden throttle openings at lower speeds, but that big s squirt being dumped in the intake during more precise throttle work made them stumble. The cure was the leak jet. It allowed the use of a pump with enough capability to boost the big, quick yank on the throttle without fouling the engine while picking through the rocks. As the big YZ's became more and more powerful and high RPM oriented, the more they needed bigger pumps. The bigger the pumps got, the more the issue of low speed control of the pump became, so they finally got leak jets added in '05.

Automotive carbs made for street use have had leak jet circuits since the mid sixties to make engines run "cleaner" during city driving, not to adjust the gross output of the pump. Of course, the leak jet does reduce the maximum delivered volume, that's a secondary result, and the pump pressure and stroke usually have to be increased to make up for the addition of the leak circuit.

Read the post linked above.

  • 2grimjim

Posted December 18, 2011 - 09:57 AM

#15

A leak jet DOES NOT prevent the accelerator pump from working. The leak jet controls the volume of fuel from the accelerator pump to the engine. Excess fuel from the pump is allowed back through the leak jet to the fuel bowl.
The larger a leak jet the lower the volume of fuel pumped to the motor. And not surprisingly the smaller the leak jet the great the volume supplied.


A better way of summarizing the function of the leak jet is that it simply limits the functional range of the accelerator pump to situations where the throttle is opened rapidly and diminishes the effect of the AP circuit when throttle opening rate falls below a certain range.





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