Carb setup--what do you think?

Since we have such knowledgable experts on this list, I thought I'd dip into the fountain of wisdom once more. I think I finally figured out the right question to ask. I've posted a couple of questions on this list before and appreciate the great advice I've received on the subject of, you guessed it, JETTING. I bought my 99 WR400, used, in January and I just realized this week that it has been deoctopussed. (I never saw the octopus, or should I say, "air cut valve". It was like that when I bought it.) I have been experiencing the usual phenomenon of intermittent sputtering at constant speed with the throttle between 1/8 to 1/4. I ride mainly within 300 ft of sea level and avg temp is around 80° F.

Here's what I now understand about my carb/exhaust setup: stock header, FMF Megamax II muffler, 11 plates, quiet core installed, 180-main, 45-PJ, #75-pilot air jet (PAJ) 1, DTM needle, clip in 4th position, #90-PAJ 2, #200-main air jet, no octopus and the air vent on the intake air horn has been plugged. The PAJ 2 has NOT been plugged. It had a 2" vacuum hose attached to it that was open to atmospheric pressure. That's where my bike seems to differ in carb configuration from others who have been deoctopussed. Last weekend, I took off the hose for the (cringe) first time and realized that the jet was clogged (not completely) with some mud. I cleaned it with some carb cleaner, blew it out and reinstalled it. I took it for a ride and it ran like crap! It seemed to be running way too lean, so through trial and error I adjusted the pilot screw to about 3 turns out and it ran great. It had a lot more power that I just about ever remember it. After I got home, I checked my manual thoroughly, took my carb apart, and figured out the whole octopus thing. I just finished installing the 48 PJ and reset the pilot screw back to 1-1/4 turns out. I also ordered a #100 PAJ 1 in anticipation of reconfiguring the carb to the settings I read about on the technical section of the owners forum.

Now, here's my question: What's the problem leaving it like it is? Should I go ahead and plug the PAJ 2 and install the #100 PAJ 1? I did notice that dirt had gotten into the PAJ circuit through the open (PAJ 2) vent port. I cleaned it out, put a long vacuum hose on it and ran it through the bottom of the chassis like all the other carb hoses. I think that will fix the dirt problem. Do you think it will perform even better with the reconfigured carb, maybe solve the sputtering problem?


I don't understand the pilot air jet 2 location. The '99WR is different than the 2000 WR (no octopus).

The 1/8 - 1/4 throttle sputter sounded like a simple switch of the needle clip to the #3 clip position because of the throttle range.

A #45 pilot jet and 3 turns out sounds like a rich setting. It may be compensating for a hose that needs to be closed off. After which the pilot screw would need to be turned back in. Just an idea.


[This message has been edited by James Dean (edited 11-10-2000).]


Thanks for the feedback. The PAJ 2 is located on the lower left side of the carb just forward of the air intake horn. It opens up into the PAJ 1 chamber. Clark Mason refers to the PAJ 2 as "the exposed secondary start air jet input--on the left side of the carb" in a message he posted on Oct. 24, 1999 titled "Re:Factory jetting for YZ400?--Eliminate Vacuum module".

I have tried needle settings ranging from 2 through 5 and none of them seemed to fix the problem.

The pilot screw was only adjusted out to 3 turns when I unclogged PAJ 2. Before that, it was at 1-1/4 turns, but when I unclogged PAJ 2, it ran WAY too lean. Turning the pilot screw out to 3 seemed to fixed it and it ran really strong (but it still sputters).



Make sure you completely BLOCK OFF the secondary air jet PAJ2 with the approiate cap or you will have a direct shot into you intake system for airborne crap / crud. I would furhter recommend you go to the 48 Pj and the fuel screw at 0.75 to 1.25 turns out. The stock DTM needle I have never liked but at your altitude it may be ok. Go with a 175MJ a minimum probably with WR timing a 178 would be a better choice for your altitude. The 180 will be close. At your altitude the 75 air jet should be fine but you can experiment with the 100.

The purpose of the octopus is two fold 1) it alows the PAJ2 to operate during starting an immediatly shuts it down after the engine starts and 2) during decelleration the octopus vacuum module modulates under the differential pressure across it (between the intake port at the head and the carb port on the intake to the carb) and it in turn leans the mixture via the PAJ2 for emissions compliance to USA Federal Standards. After eliminating the Octopus the bike is easier to jet and a more predictable prefromer.


[This message has been edited by Clark Mason (edited 11-10-2000).]

Thanks, Clark. I figured that you and James were engineers and I was right. (I looked up your profiles.)

I think I'm seeing the light here. The octopus is basically an emissions control device that operates by measuring the pressure differential across the carb to modulate the opening of the PAJ 2 in order to meet emissions standards, as described above. This explains why the YZFs don't have one, since they were designed for use on closed courses where emissions aren't an issue. This may also explain why, as some have said, that the 2000 model WRs can't get a California green sticker while 1999 models can. What that means to me is that I feel I should be able to expect better performance without one as long as the holes are plugged and the right jetting is used.

Surprisingly, the 180 MJ is the best for my bike. Anything smaller makes the thing feel weak above about 3/4 throttle. I'll be testing the 48 PJ tomorrow and I'll get a plug for PAJ 2. I'll try it both ways-with and without the plug. I have a feeling it may be too rich with the plug in, but we'll see. I may have to go back to a 45 with it plugged, but I hope not. I'll keep you posted if anything interesting happens.


Well, I hope this is the last post I'm doing on this subject, because I think my bike is finally fixed!!! I plugged PAJ 2, went from a stock PAJ 1 (75) to a 100, and I set the pilot screw at 1-3/4 turns out. I also lowered the needle to the #3 position from #4. This made the throttle response much more linear, smooth and definitely made it easier to start. I've put about 70 miles with it like that and I may have just about had it perfect...

Except for one thing, until this afternoon. You know that the low/mid-throttle sputtering the bike had? Well, that's now fixed, too, but it had nothing to do with the carb. It was a corroded/dirty spark plug cap. Starting about 5 months ago, I began having problems with weak spark. It sometimes wouldn't start because it wasn't getting any spark. You'd get a real good arc from the open end of the high tension wire, but it has been intermittently not so good with the cap on while grounding the spark plug. A mechanic friend of mine insisted that dielectric grease would fix it, so I followed his advice. That got it running, but I had to regrease it regularly because it kept happening. BTW, a new stock, cap, which is the only cap that fits, is outrageously priced at $54! I decided to take the cap apart and clean it before I had to spend that kind of money. I used contact cleaner, Q-tips and pipe cleaners to get out all the old grease and corrosion and it worked. I also clipped off the last 5 mm from the high tension wire to give it a better bite on the screw. Testing it now produces a hot, clean spark. I think the grease was actually inhibiting the electrical contact. (It shouldn't surprise me because dielectric material is used as insulators in capacitors.) I used dielectric grease this time too, but only as a seal around the porcelain base of the plug and around the high tension wire insulation to keep water out. (Which I think is what it's really supposed to be used for.) I had a suspicion this may also fix my mid throttle sputtering and I was right. I rode about 10 miles this afternoon and it now runs better than it ever has before. I may move the needle clip back up to #4 later on, but I'm pretty happy with it now.

Thanks to those who helped me with this. This list is by far the best thing out there to help maximize the potential of the bike.

Next step--respring and revalve the suspension (when I get a spare $700 lying around.)



How’d you get the cap apart (a good yank)? A related question would be, how’d you get it back together?

Mine feels like its starting to come apart at the elbow area and, at $54, I’m a bit relieved to bump into someone who’s successfully reattached the wire to the cap.


Try MX-Tech for your suspension. They seem to be the up and coming leader in the industry. I was quoted $230 on each end. $460 sounds a lot better than 700.

Just an idea, I thought I'd throw out!



86TT225, 98CR80, 99WR, WR timing, throttle stop trimmed, air box lid removed, White Bros head pipe, silencer and air filter. Odometer and headlight removed. Moose hand and mud guards. YZ stock tank and IMS seat. Renthal Jimmy Button "highs" and Renthal Soft half waffle grips. AMA, SETRA.

Bill: FYI, I had MX-Tech do my suspension. $719 for both ends and that was no change in the fork springs.....did change the shock spring though. The difference is astounding! That cost might be astronomical for most riders, but this is my only passion...and I'm not married... :)


The cap comes apart with a twist and a screwdriver. The cap screws into the high tension wire (HTW), which comes out of the coil. There's a stud, about a half inch long, looks kind of like a wood screw, that screws into the end of the HTW. If yours feels like it's coming apart, screw it back onto the HTW. I cut off the last 5 mm of the HTW because the wires inside it were looking pretty chewed up and may have been preventing a good connection. That was probably half of my problem. The part of the cap that attaches to the spark plug requires a flat-blade screwdriver to disassemble. If it's corroded you may have to put some elbow grease into it to break it loose, and there's about a half inch of threads on it, too. Once you get that part out, you'll find a resistor that looks like a small fuse and a long thin spring. That's the part that I cleaned out with contact cleaner, Q-tips and pipe cleaners. I think the proper application of dielectric grease will prevent the type of corrosion that I had.

[This message has been edited by Rich in Orlando (edited 12-05-2000).]

Thanks Rich, that was very descriptive and is sure to be helpful.

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