01 YZ426F Missing, stalling

I have been working on a 01 YZ426, It runs like a top for 15-20 minutes. then starts misfiring and stalling. You can restart it 4-5 times then it requires a new plug before it will start. I've heard rumours of defective carburetors, bad CDI boxes, bad coils. The plug is pretty black when I take it out. It is re-jetted for altitude-7000ft. Any help out there? :D:)

Did this start after the bike was rejeted for this altitude?

No Dave, Supposedly it started right after he got it back from the local Yamaha dealer for his 1st service. I checked the valve clearances and they are spot on so I think this is coincidental. I checked the stator and ignition specs as per the factory service manual and they are within specs except for the ignition coil which is slightly out- I didn't have another one to try. Carb setup: 152 main, 2 smaller on the pilot (38?)clip up 2 positions.

ksp

76CT70

77CR125

97CR500

92GSXR1100

FWIW, my '01 426 is jetted for Colorado Springs (6500')

and is way diff from your jetting. I'm running: 162 main, 2nd clip,

42 pilot, stock needle, WB E-series SA with 7 discs, stock

header pipe.

Merf

ksp,

Well...I'm by far no expect when it comes to jetting a carb for altitude on this bike, but these settings seem way out of line for just 7000 feet.

Were these changes made during the last visit to the dealer? Did the bike ever run well with this carb setting?

Mefman just may have your answer.

Let me know, and we'll take it from there.

DaveJ

Merf, It sounds like your pretty much running stock jetting- the factory manual says 162 main and 38 pilot but if yours runs good- great! This one has stock pipe.

Dave, when the bike first got here it wouldn't idle and felt rough and uneven througout the rpm range. I was the one that re-jetted the carb, after that it seemed to run excellent for the first 15-20 minutes of riding and then it would start to act up, which would make me suspect an ignition problem. Merf is running almost stock jetting at the about the same altitude and having no trouble. If anything I would think I would be on the lean side and the plug should come out white or very light. However it comes out black, (fouled).

Let me double-check my 6500' jetting. I may be out in the weeds.

I'd just rejetted since I'm heading to Texas this weekend (1800')

for an enduro. I'll recheck and repost on Mon/Tues. I may have

had the wrong jetting stuck in my feeble brain. I know I go one

step richer on everything when racing in TX so that may be the

confusion. I'm pretty sure I'm not running the stock jetting at this

alt, I fought with it for quite a while before getting it right.

Sorry for the mixup....

KSP,

So you may be having the very same problem that is going on with Motoman in the other post.

Perhaps you should check the quality of the spark at higher temps as well.

DaveJ

Originally posted by ksp:

Merf, It sounds like your pretty much running stock jetting- the factory manual says 162 main and 38 pilot but if yours runs good- great! This one has stock pipe.

Dave, when the bike first got here it wouldn't idle and felt rough and uneven througout the rpm range. I was the one that re-jetted the carb, after that it seemed to run excellent for the first 15-20 minutes of riding and then it would start to act up, which would make me suspect an ignition problem. Merf is running almost stock jetting at the about the same altitude and having no trouble. If anything I would think I would be on the lean side and the plug should come out white or very light. However it comes out black, (fouled).

That sounds exactly like what mine was doing when I first got it. It was used and the jetting was royally screwed up. It ran great for about 5-7 laps, then loaded up, started missing and fouled the plug. I think this is what I have in there now:

Main - 158 or 160

Pilot - 38 or 40

2nd Clip stock needle.

(Sorry for the vagueness on the main and pilot, but I have not had to mess it since I fixed it)

Also, check the fuel(air) screw. Mine was way to far out, I turned it all the way in, then turned it 1 1/4 turns out. This seems to be a big problem with almost everybody that is complaining about the carb.

Also, look at the float level. Mine was out of spec so I fixed it. After that, I have not had any more problems with fouling/misfiring etc.

Just so you know, I am at around 5000 ft in Boulder.

Good Luck

[ November 16, 2001: Message edited by: Hokie ]

I had this exact problem. But it seemed temp related. I know two other guys that had it too, and many people on this forum. A new CDI pretty much cured mine, but the temps were warmer by then, so who knows. Alot of guys say race gas helped them. I did try it, didn't help mine, but its still worth a shot for something quick.

I assume the bike does not have the BK mod. If not, I would do it. Then try a 162 main at 50F or 158 main at 70F. Start with clip #2. I would try the 42 pilot at 1 turn out, if cold, or a 40 at 2.5 turns out if warm (there are many procedures for setting fuel screw, do a search).

I think the BK mod helps because you dont get that long squirt of fuel into the carb. I think that long squirt really messes up the jetting and results in that black plug. If you are constantly blipping the throttle, you get all that extra gas. If you roll it on and hold a steady throttle, you don't. I would be very interested to see if the problem goes away after the mod. The mod is easy and also reversable. The 250Fs never seem to have the problem, and they have a much shorter squirt stock, like the BK mod. As a bonus, the bike runs alot better, my guess is because the jetting is more consistent.

Do a search on BK mod for info. For reference, time your jet squirt with a stop watch. A Videocam in slo mo or frame by frame works even better. You want a 0.3 sec squirt after the mod. Nine frames on my VCR is 0.3 sec, I think most are 30 Hz. Dont let the mod intimidate you, its a piece of cake to do.

You will find that your O1 YZ426 has a 2 to 4 second squirt stock. Mine was 2.6 seconds stock.

My wr250f was 0.3 sec, on the money. My friends YZ250F was 0.15, so was Motomans. And I have never heard of a 250F with this fouling problem.

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