Fouled plugs = foul mouth!

Hey guys thanks for spending so much time working on your bikes and giving us all insight. Since I don't have all that much time to work on the darn thing I have only been able to ride 3 times since I picked it up in Nov/00, and that was all in the first week. After it ran perfect for that week I was amazed that everyone else was having all these probs. I had to leave it sitting for about a month after that and have not been able to get two laps out of it since. Anyways to get to my question - I was just wondering about you guys that are always taking your bikes to the dealers, how much money are you forking out to them?? Any time that I go talk to the place were I bought mine in Colorado Springs, CO they just want me to drop my bike off for a few days and are not to helpfull with giving me any advice. Now I don't want to be draining my wallet on service for a brand new bike - especially service that can not guarantee a fix for the problem. So what are they charging you for their time??? Hey Merfman who are you talking to at Apex?? and how well do you know him??

Originally posted by blbainb:

It looks like no needles will be available for the '01 until April so I have to hold off on trying leaner needles.

The stock ’00 needle is a few sizes leaner on the straight portion so will be leaner initially or just off the pilot. If I am wrong about this I’m sure James Dean will notice and correct me :)

Here are the ’00 Yamaha part numbers to some needles that should be leaner at lower throttle openings:

5JG-14916-EP-00 This is OBEKP, same as stock ’01 needle (OBEJP) but ½ clip position leaner (edit).

5JG-14916-E1-00 This is OBEKQ, one size leaner initially

5JG-14916-ER-00 This is OBEKR, stock ’00, two sizes leaner initially

That surprises me about the ’00 diaphragm, I would’ve guessed that would deliver more fuel than the ’00, not less. I guess another aspect of the pump operation has indeed been changed…

But wait, if you are saying that the ’00 part has a shorter post then the stroke will be LONGER and MORE fuel will end up being delivered by the pump. Right? Maybe the area under the diaphragm is different as well (or what else am I not accounting for?).

The spring under the diaphragm will ensure that the beginning of the stroke occurs at the same time with either diaphragm (as soon as the actuating rod protrudes below the carb body), the longer post will contact the bottom of the pump cover sooner, shortening the stroke relative to the one w/ the shorter post.

Thanks for the info blbainb.

[This message has been edited by Hick (edited 01-18-2001).]

Hick is right about the carb needles. Don't wait, order one of the leaner '00 needles. Get the OBEKR.

The EKP is basically what you already have with the taper start shifted by 1/2 clip leaner, don't waste your money on that one. OBEKQ is too close to tell a difference.

If the '01 diaphram post is longer, then Yamaha has simply made a P-38 Lighting setup. This REDUCES pump flow by shortening the duration.

The timing of the pump start is determined by the screw adjustment on the side of the carb, NOT by the diaphram dimensions. There is about 1 turn of play before it starts to pump. This allows the slide to open far enough so the pump squirts UNDER the slide. Be sure to check that it does. You can FEEL the freeplay if you push on the accel pump adjuster screw. This feels like light tension before hard tension. It is a very small amount. It can't really be measured.


Originally posted by Haro:

Hey Merfman who are you talking to at Apex?? and how well do you know him??

Well, I spoke to the service manager when mine first started acting up. I don't know his name as I don't do much mechanic business with them. ALL my mechanic (none warranty anyway) is done by Slavens. It was through Jeff that I got the info on the TPS. No update on that BTW. The one mechanic I know and trust at Apex is Paul Zinke. He used to work for Jeff before he worked for MCY and then Apex. According to Paul, the bike they had in the shop to jet was giving them fits.

The service manager gave me some jetting specs that didn't make sense. When I called him on it he claimed they didn't have a 40 pilot and said they were only 1/2 turn out on the fuel screw. I'd just left there and bought a 40 pilot from their parts dept. BS

number one. He also said the bike was running find. BS number two 'cause that ain't what Paul said. Bottom line? I'm kinda waiting to see what the problem is and then confront them with a fix. I'm pretty sure Yamaha will stand behind the fix. I fragged the tranny on my 98YZ after it was out of warranty and they fixed it for free after the cause was revealed to be a press-fit pin that fell out and into the bottom in.


Oh lord, please excuse all the typo's in that

last message... Never try to post from work...

Just an update on my bike. It isn't fouling

plugs except on starting after a day of riding. It runs like a scalded dawg and the jetting (I'm convinced) is spot-on. No burbles, no boowa's, nada, just good clean tongue-bending speed. Runs fine all day but won't start the next day. My jetting:

6000' ASL

40 pilot

1.5 turns out FS

3rd clip (I think)

168 main

I'll double-check next time I have it apart since I forgot to write everything down.


Most of the people north of you (5000') are running either a 152 or 158 main in their 426s. I am running a 158 and finally am not getting a jet black sooty plug (after a carb and CDI swap). The plug is dark brown and black, but no soot/carbon buildup. My jetting is now a 158/3rd clip/42/1.5 turns. Note-this jetting gave me fouled plugs with my old CDI. Clearly your bike must be quite different than mine to run all day at 6000' with a 168 main!

Hey Merfman, thanks for the inputs. Yeah I know Slavens is a good place, I use to bring my CR250 there. And like you said I am hoping that Yamaha acknowledges the problems and comes up with the solution. It would be nice to ride often again...

Originally posted by mikeolichney:


Clearly your bike must be quite different than mine to run all day at 6000' with a 168 main!

That's why I said I'd double-check... It is

a 158 main. I checked my jets last night

and have a 168 and a 162 in the box so that

means the 158 is in the bike.

I ran with the '00 diaphram (which isn't the stock item - leaner/richer I'm not sure) on Friday (42 deg.F, dry, 45 pilot, 4/7 needle, 162 main) and it ran great. Seemed to have better acceleration -slightly- than the '01 diaphram. Did a full throttle run/shut off to check the plug and it looked much cleaner than usual: no soot accumulation (other than what was already there) and a gray/white insulator (looked a bit lean - I don't know how you guys can run a 158 or 153 main).

So, I thought that maybe the problem was solved (again). Then today, Sunday (45F), I went out to a local track and right off the bat the bike felt rich. It burbled/missed in low/mid throttle all day (but the plug never fouled and the condition didn't get worse with time). Now, I admit that I greased the linkage/shock/swingarm but I'm pretty sure I didn't touch anything else. Also, it would rev ok and the problem didn't worsen at higher throttle settings (so no rags in the intake, no disconnected electronics, etc.). Looks like it's back to the '00 CDI to see if the problem goes away. Now I wish I'd taken the other CDI with me so I could change it immediately. Does this point to the CDI with some erratic component causing a change in the timing or something else? I know that others have provided anecdotal info that a cdi change cured a rich running/plug fouling condition but I still can't think of why.

I swear I think some evil spirit invades my bike's electronics every few rides...

Well I got aroud to checking my ignition out yesterday. The only thing that I could find was my coil readings were off. The actual reading of the primary coil was .38 ohms (target range is .2-.3 ohms). And my reading for the secondary coil was 21.5k ohms (target range 9.5-14.3k ohms). To me these numbers are quite a bit out of spec so I will probably order the new part. It will either be a waste of 70 bucks or the best money I ever spent. Then if this does not fix it it will be time for a new C.D.I. (or KTM).

i'll be very interested mike in your result. i for one have never discounted the coil set up.

very possible you've got something, but don't lead yourself up th garden path. no change is no change.

you don't have to go yamaha for the coil. just about any will do.

on tthe road bikes kwacker have until recently still sold their bikes with the coil & lead seperate...


blbainb: Did your problem go away when you went back to the 00 CDI? My bike has been running perfectly with the new CDI, even in cold temps with pump gas. Has anyone else with this problem replaced their CDI? The mechanics at Twin Peaks want to know if the CDI was it- I am convinced it was on my bike.

I've had a problem with plugs fouling rite after a start up. at first this was incredibly frustrating( all of my buddies ride 2-stokes) . After some consideration I've come to a simple conclusion. The bike would start and run fine for a few seconds then stall and foul. I could restart the bike if I turned on both the choke and hot start. but if I turned them off I had to hold some throttle to keep it running and it would sound like crap only to die when the throttle was cut. This led me to the conclusion that the plug wasen't fouled but ruined totally. this only happened if I washed my bike between rides. what was happening to me was that when i would wash my bike I would hit the plug cap with the water. This fill up the hole where the plug is. Then when i'd go to start up next time the motor would start but the temperature would rise and the water on the plug would somehow ruin it. extreme temperature differential would probably crack it internally. what a pain in the ******* ass huh!!!!!!


Just spoke with my local dealer who is working on my fouling problem and he said my throttle postion sensor was way out of specs on the carb and that it came from the factory like that. He said that its tied to the ignition and may solve the problem. I will let you guys know when I get it back.

I'm betting on the CDI (220 bucks worth). I posted a while back that I found my readings on the coil were off...I discovered that this was due to taking the readings with the spark plug cap on. When I got my new coil in it did not come with a cap so after pulling the old one off I took another reading of the old coil and sure enough the readings were in range. Anyway my new CDI should be in next friday...I'll post my findings!

I just wanted to add some insight on the CDI box. I am a electrical engineer and can tell you this about the boxes. I also write tons of firmware for all types of small controllers.

If they are bad they will normaly fail completely. If there is a firmware problem in them then you can get intermittent results.

Swaping a CDI box with a new one would not fix the problem unless they updated the firmware, MAPs. 01 to 00 would make some since.

Of course, you need to verify all the sensor inputs into your box and the reference voltage need to be checked. The box can't work with out good info.

I will also add this. The uP in the CDI only has the TPS and RPM to determine timing. While this is good it lacks a temperture measurement. I think this is some of the reasons we see strange performance problems. I know my bike starts working better once it get up to temp an I mean temp, not just warm.

My guess is the mapping was done with the motor nice and crispy at sea level with ideal fuel. If you can't measure it you better know the value.

I have seen lots of hardware changed to try and solve a firmware problem.

Yes, I have pissed a lot of people off with a ten minute firmware fix, that the hardware guys have been working days on.



01 YZ426F #85 Vet C

I have no doubt that my problem was the CDI. After it was swapped, there have been no problems whatsoever.

Originally posted by mikeolichney:

I have no doubt that my problem was the CDI. After it was swapped, there have been no problems whatsoever.


Mike I am just curious, if Yamaha updated the CDI. I have a couple questons for you if you don't mind. Does you new CDI have the same number on it as the old one? If there is numbers? I have not checked. Also, when you removed your old one, what did the connector look like?




01 YZ426F #85 Vet C


First I have to say my dealer is being great. He switched the CDI and the plug was still sooty and the throttle response worsened over about a 20 min ride. I took it back the next day and suggested he switch carbs with the new one he was assembling. He said sure but yamaha wanted him to fix a Mapping problem first with the neutual sensor. anyway I will let you know!

I just got my bike back and the dealer changed my mapping circut and the first plug was white and clean in the electrode area but the outside dia. was still black. Dealer thinks he can clean that up with jetting. I'm going to race this weekend and we will see!

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