Jump to content
Sign in to follow this  

2001 WR426 & ProCircuit T4 - Jetting...

Recommended Posts

What an ordeal...

I had already performed the airbox mods (no lid/baffle removed), cut the throttle stop, removed the stock pipe's baffle and the bike went great. I wanted more 'oomph' up on top of 1-2-3 gears for hill climbs so I slapped a bigger rear sprocket on the bike (+3 teeth) and bought a ProCircuit T4. ProC told me to go with the stock YZ426 jetting - which I did.

While the bike was down I also did the YZ cam timing. When I fired it up (it would only start with both the hot & cold chokes out) it bogged from idle to 1/4, ran OK to 1/2, ran but seemed very lackluster 1/2 all the way to full throttle, and popped/backfired when shutting of the throttle (deceleration).

So, next I went back to the stock WR426 jetting (I did leave in the 162 main jet based upon my dealer's advice). It will now start after only one/two kicks with the cold choke out. Runs good idle to 1/4, stumbles slightly 1/4-1/2 throttle, pulls like hell from 1/2-3/4, slightly after 3/4 throttle gets choppy and sputtering and finally flattens right off like I am hitting a rev limiter.

My first question... Is this abrupt shut off related to doing the YZ426 cam timing? Was this the right thing to do? From prior posts, I've never seen this mod 'not work' or cause this problem.

Second question, where do I go from here? Should I do the rev limiter thing (yes, I know the politically correct thing to do is do modify the assembly at the end of the wiring and not to cut the 'gray' wire itself). Could this be causing my problem.

Third question, seeing as I am still using my stock needle am I starving on top - hence, the rev limiter-type effect?

Or lastly, have I simply run out of patience too fast with respect the jetting issue?

To recap, here is my 'carb blueprint':

162 Main Jet (YZ)

42 Pilot Jet (Stock)

200 Air Jet (Stock)

75 Slow Air Jet (Stock)

Stock Needle (#4 position)

3.5 turns out on Fuel Screw


[ March 21, 2002: Message edited by: tmgolden ]

Share this post

Link to post
Share on other sites

well now you've been to pro-circuit before us, are we offended or what (what) :)

just change the needle to an N427-OCEMM (or EMN)

it'll be OK after that and even if it isn't it'll only needa a light fettle.


Share this post

Link to post
Share on other sites

Your first mistake is listening to the dealer. BAD BOY!!!!

I'm no jetting whiz here but I think your problem is your lackluster 162 main jet. These bikes come lean from the factory so bump it up to a 170. That will probably solve your lack of top end. You should have one that came in your factory pack of goodies. When you put on the t-4 it opened a whole bunch of air flow and it needs more fuel at upper rpm than the 162 will dish out.

I have the pro t-4 and it rips! The jetting i used is

170 main

100 pilot air

48 pilot jet

ekp needle 4th clip down

i think.

i am at sea level and this was the jetting suggested to me by Dr. James Dean.

I do seem to have the whack it open bog but i don't notice it when i am riding, only when i try to make it do it in neutral. trust me, if you whack it you better hold the F on!!!

btw, get behind the bike about 5 ft. and listen to the pulse of air go by. it sounds wicked, like someone is swinging a rope by your ear. also, stand directly behind it and let the pulse of air punch you in the face. it never hits the same place twice.


Share this post

Link to post
Share on other sites


I have to ask the obvious...

Did you advance or retard the exhaust cam timing?

The exhaust cam is supposed to be retarded by 1 tooth. It wouldn't be the first time someone went the wrong direction.

See: www.off-road.com/dirtbike/jun99/hobbs/


[ March 22, 2002: Message edited by: James Dean ]

Share this post

Link to post
Share on other sites

OK gang,

Today 'is attempt to fix the problem myself day.'

First off, some great news (for me at least)! After I made this post, I had an epiphany... I realized I was spending money to get YZ-like performance. So, I decided to take Thursday off and ran promptly to my dealer and bought a '02 YZ426. You will not believe the deals they are offering because their inventory is backing up due to Honda's robust CR450 sales. I got out the door for $5,700. This bike is wickedly fast and I sh*t thee not... the wider you hold open the throttle over the whoops, the more the bike wants to float across the crests like its floating on glass. Still figuring out how to work 1 & 2 in the tight twisties. I am being reintroduced to 'arm pump' for the first time in four years.

Before all you 'WR' purists grimace in pain... here is the upshot. I've decided to 'street' the WR as I already had a Baja Designs kit on the shelf and I just came into a full set of Excel/Talon wheels in perfect condition for $400 (clean living folks). And here is the better news... I will be selling my '00 XR650R - a bike that I never really liked anyway! Don't tell those dudes over on the XR forum.

So, now I have the 'dirt bike' I like. The trails/scrambler I like. I have a bike to tool around town on (whilst trying not to wheelie). What's more I have a spare bike (that someone can actually ride) to loan a friend when we go to the off road park. What's not to like.

Now, back to the matter at hand...

Here is my plan:

1) Check slide cutaway

2) If wrong fix and test; if fine, END. If still messed up, go to step 3

3) Try 170 main

4) Test; if fine, END. If still messed up, go to step 5

5) Try 165 main

6) Test; if fine, END. If still messed up, go to step 7

7) Consider results of 162 (baseline), 165, and 170 jetting changes. Put in main jet that yielded best result

8) Order 45 & 48 pilot jets and EKP & EMM needles for next disappointing round of testing.

9) Go ride new YZ426 (which is still working fine because I haven't had the bright idea to dismantle it yet!)

Thanks for all the great advice guys. I'll let you know how things work out.

Share this post

Link to post
Share on other sites

Oops... forgot to mention that I was going to have a reputable dealer (one that I trust) verify the YZ timing before testing anything...

Share this post

Link to post
Share on other sites

OK... here is an update. I removed the upper frame brace and valve cover and checked the YZ timing myself. Here is what I discovered as a result of this ordeal. God, I hope the following analysis is correct or you guys will roast me alive!

On a 2001 WR426F ('I' centered in timing window), the exhaust cam's 9:00 timing dot CAN actually be about a millimeter below the milled top surface of the cylinder head. Apparently, those third-world factory workers that Yamaha exploits/employs to punch out timing marks on 10,000 cams a day can get a little sloppy such that the timing mark positions can vary from what is pictured in the service manual.

The good news is that the intake cam is usually spot on (3:00 flush with head) most of the time.

The bad news, if you strictly follow timing advise that uses statement like, "When done the cam, relative to the 12:00 timing mark will be in the X o'clock position", these statement become relatively ambiguous. The best you can hope for is that the intake cam will always appear [more or less] in the one o'clock position. The exhaust cam will always appear [more or less] at the twelve o'clock position (once YZ timed). Pay close attention to the 'more or less' in my last statement. Believe it or not, things can vary slightly between same year/make/model bikes.

The best method I could come up with is to set the 'I' to center in the crank window, note the position of the intake cam. verify that there are 13 pins between 12:00 timing marks, move the exhaust cam one tooth clockwise, verify intake came is in the same position, count again (now there should only be 12 pins), and you're done.

Here are some visual aids I have prepared:

Link to WR Stock Timing Graphic

YZ Timed:

Link to WR YZ Timing Graphic

OK, so I verified that my WR's YZ cam timing was correct... I think. Here is what else I discovered... I am ashamed to admit that I installed the carb slide valve cover upside down. Here is a visual aid for others who may have bought second hand bikes [with no service manual] and for those who take sloppy notes when they tinker (like I do):

Link to Carb Slide Graphic

And lastly, I went from 162 to 170 on the main jet. Bike idles fine. Trace stumbling from 1/4-1/2 throttle. 1/2-7/8 pulls strong. 7/8 and beyond = rev limiter.

Question, in stock trim (plus free mods), I never used to hit the rev limiter except when winding out in 2nd gear on LONG hill climbs. I assume I'm hitting it now because I added a rear sprocket that is three teeth over stock, YZ cam timed the bike, and installed/jetted for a ProC T-4 pipe. E.g. The engine is simply spinning up faster. Is this a correct assertion, or am I smoking something?

Also, any advise that may clear up the small traces of stumbling when at 1/4-1/2 throttle roll-on would be greatly appreciated.

[ March 24, 2002: Message edited by: tmgolden ]

Share this post

Link to post
Share on other sites


JD you are absolutely right. I forgot to thank jamracing for the great advise. jamracing, thanks, and rest assured - this is a mistake I will never make again!

JD, I'll try clip #3 and report back in.

Share this post

Link to post
Share on other sites

The configuration in my signature has never failed me.

The following is the latest and greatest (developed by taffy and James Dean) baseline for bikes like ours (YZ timed and open exh) at 0-3000'

MJ 160

MAJ 160

PJ 38

PAJ 55

SJ 60

EMM #4



Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this