I have an '02 YZF426, and I race Hare Scrambles. Our series seems to be brutally tight this year, and I'm having some stalling issues. I can't quite figure it out completely, but the symptoms are as follows:

1. It seems to stall whenever I seem to idle for any length of time, such as down a nasty hill.

2. It seems to do it more after a really tight trail section, where the speeds are slow, and the engine hot.

3. It fires up only after using the Hotstart to re-start the bike. Sometimes, it takes 20 kicks to get it re-lit!

4. And sometimes, after those 20 kicks, I'm just sitting there trying to re-catch my breath, when it will just die again!!! Needless to say, this BITES!!!

I am using the Rev-Loc automatic clutch, and have been without troubles for over a year now. And I also want to make sure you understand what I mean by a "Brutally Tight" Hare Scrambles race. Over the last 8 races, combined, our speed average is less than 16 mph!!!

I never had any problems like this last year, although the courses were faster last year too. I was wondering if anyone has any ideas? I won the first two races, and have had major stalling issues since then. Like to the tune of stalling a dozen times during the 2 1/2 hours. As you can imagine, this is killing my championship hopes! I must admit though, my right calf muscle is surely well developed!!! Thanks for your help! Mitchell "Maniac" Marks

Maniac ....wish I could help. I could state the obvious like the fuel screw adj or jetting but it I know that sounds too easy. Maybe there is a vacumm leak causing it run rich. Wish I could be of more help.

1. It seems to stall whenever I seem to idle for any length of time, such as down a nasty hill.


2. It seems to do it more after a really tight trail section, where the speeds are slow, and the engine hot.


3. It fires up only after using the Hotstart to re-start the bike. Sometimes, it takes 20 kicks to get it re-lit!
Using the hot start actually creates a lean condition.

4. And sometimes, after those 20 kicks, I'm just sitting there trying to re-catch my breath, when it will just die again!!! Needless to say, this BITES!!!


With the exception of using the hotstart (which creates a lean condition) it a sounds like the bike is lean on the bottom. The place to copy your post is in 'jetting discussion' with a post for ATTN: John Dean. He is the resident jetting guy, and does a great job.

I'd be willing to bet if the bike ran well and just started this, it's something else looking like a carburation problem. You don't want to change the carb until you are sure its the problem.

Probably better luck if you look for "James Dean". How about gearing down for the tighter tracks. So the rpms are higher?

I hear ya about the jetting concerns, but it just doesn't seem like the problem. The bike runs absolutely perfectly thru all of the jetting circuits up until the motor is quite hot and I'm coming thru a nasty section which means idle or near idle amounts of throttle input. When racing last weekend, after one of the stalling sessions, I reached down and turned the air/fuel screw in alot, which leaned it out considerably. Around the whole course, the bike would backfire and snap and hesitate, and even stalled less. But in those nasty sections, the bike would still stall some(not as often though) and it still took the Hotstart button to fire the beast back up! Keep sending me your idea's though, as I certainly haven't figured this thing out yet! Thanks for all of the reply's! Maniac

Fastest, I've tried turning my idle up higher also, and this didn't help either. On this one nasty downhill, every lap, my bike would just die. And you really needed a run to make it up the other side too. Anyhow, I even tried putting the bike in neutral and reving the bike up to try and keep it off of the idleing circuit. Yep, still stalled! The most perplexing thing is that it works awesome except for in the tight, slow stuff! Keep those idea's coming! Maniac

It just dies at idle possibly? Could it be electrical? Maybe a bad cdi? Mechanical problems are usually pretty consistent. I have had street bikes that would just die at idle, they would start right back up just fine. Always ended up being one of those damn black boxes. Can you borrow a friends? Let us know.

Fastest, I will have to give a buddies CDI a try. I loaned him a ignition case cover once, and now it's my turn to need a part. I sure hope I figure this out quickly. It's driving me crazy! :smashpc:Thanks. Maniac

Maniac,
I had a similar problem earlier this summer. I borrowed a carburetor from Kirtewll while he did a rebuild and repaint. I had bought a new ignition and did a top end job while searching for the problem until he stopped me.

I used his carburetor and found my bike ran perfectly. I had a bad TPS and a clogged jet at the bottom of the intake side of the carburetor. I was also jetted way to rich when I thought things were perfect. I dropped from a 162 to a 155 main and now it runs great. I will loan you a CDI and a coil if you need. Just P.M. me or email me at whb-mdmb@worldnet.att.net and let me know where to send them if you wnat to try them out.

I would clean every jet in the carburetor out and try to drop the main down a couple of sizes to start. Remember to change only one thing at a time and write down the stock settings and each change as you go. Good luck and let us know what you find out. I learned a lot from my mistakes.

Bill

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MANIAC,
Check this out first, it's cheap. Look at your sparkplug cap. I think they still have a small resistor in all the four strokes. A straight screwdriver will tell you if it's tight. My friend had this retaining screw loosen and it drove him nuts. He traded parts with everyone having the same bike and he was ready to sell it as 'cursed'.
The resistors themselves are good for a long time. I replaced one after 5,000 trail miles and kept the old one as a trail spare. Nippendenso sells them for a few dollars, Kawasaki wanted hundreds.

I will loan you a CDI and a coil if you need. Just P.M. me or email me at whb-mdmb@worldnet.att.net and let me know where to send them if you wnat to try them out.

Bill, Thanks for the offer. I really appreciate it. I might need to take you up on your offer later, but for now, I'll try a few other things with my buddie's parts. I love this ThumperTalk community! You guys are AWESOME!!! I come down almost every winter to visit my father for a couple of weeks, maybe we can hook up for some riding then? Until then, take care. Maniac

Maniac,
The parts will be here if and when you need them. If you get down here please look us up. John and I will be glad to get with y'all for some riding. Our home is your home.

Regards,
Bill

Valve shims, or maybe you've got a valve thats stretching and getting ready to let go.

If the valve clearnce is legit, see if you can be certain a cam sproket didn't slip by checking the LC's while the head cover is off.

Just for kicks, can you rotate the carb by hand, especially at the head with the clamps tight? If so pull the coupling for the carb, clean it and seal it to the head, then clamp it and remove any excess sealant from inside before mounting the carb. I installed mine with PRC fuel tank sealant and narrow SS clamps, problem solved. However mine would just go into a wacky kind of idle thing - maybe like yours.

After reading this I would most likely start with carb coupling, and lastly go with the valve timing.

All this assumes the carb is clean.

PK

Guys! I just might have this thing finally figured out!!! In the June issue of Dirt Rider, I ran across a caption describing an amazingly similar problem that Russ Pearson was having with his YZ450F.

Here is what the caption said; "Pearson's YZ450F engine got so hot it heated the gas in the oversize tank and pressurized the carburetor, causing the engine to run poorly and requiring him to push in the dry-break every couple of minutes to vent the vapors."

I believe this might be what is happening to me!!! It makes sense, as my stalling problem is always right after a really nasty, slow section, where my engine heat is high and speeds are slow. This would also explain why I always have to use the hotstart to re-start my engine when I know my jetting is perfect.
And also why sometimes, while trying to catch my breath after kicking it over about 15 times, the damn thing will stall again while just sitting there idleing!!!
God! I hope I've finally got this thing figured out! I removed my gas cap "one-way valve" and drilled out the vent hose nipple as large as I dared, and tomorrow, I'll insulate the bottom of my gas tank to try and keep some of the engine heat away. I can't believe I finally found something useful from one of the MX magazines!
Man, this makes perfect sense. It also explains why at the first two races this year, no problems (fast courses), but all of the remaining races have been insanely tight and I've been having trouble since then! FINALLY!!! I'm looking forward to this weekends race, the first time in quite awhile!!! I'll report back after this weekends race!!! Thanks for all of the input guys!!! Maniac

Maniac, Why dont you wrap your header with some Thermotech? That should cool the area pretty well. It does work well. It will get ragged in an mx application I am sure but it is cheap enough. Good Luck. Never heard of vapor lock in a bike. Even though that is not exactly what this is, but close enough.

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If that helps, might want to look into ceramic coating the pipe. I dont know for sure, but I have read that it cools the area significantly. Ive seen pictures, and it definitely looks good if nothing else.

fofiveoh

Yeah, I am going to insulate my header too. Just not in time for this weekends race though. And yes, that ceramic "Jet Coat" coating looks awesome!!! I would love to do that to mine, although, I'm sure I would drop my bike and ruin the header the first day I got it back from the coating company!!!

I really appreciate everybody's help in trying to figure this problem out. It's been driving me crazy!!! Now, I just hope I've solved the problem! I guess I'll find out this Sunday! Thanks guys! Maniac

Well guys, the results look promising! I just got back from todays Hare Scrambles race, and I had no stalling issues like at all of the previous races.
All I have done so far, is to remove the one-way valve from the gas cap breather hose, drilled the cap nipple out as large as I dared, and I used some reflective insulating tape on the underside of my gas tank.
I did stall a couple of times, but this was due to a slightly lean pilot condition, and the bike would fire right back up immediately.
I sure hope I have solved this problem of mine, but today's race was finally at another fast course, so I really didn't duplicate the slow, high engine heat conditions that have plagued me recently. But it sure was neat not having to kickstart my bike fourty times in a day!!! I'm actually starting to look forward to the races again, now that my bike is running better again!!!
Again, thanks guys for all of the help, and all of the wonderful offers of loaned parts to try and identify my strange fuel delievery problems. Keep it real! Maniac

Wrap the header and put the check valve back in! FIRE!

16 MPH sounds brutal! We just had a race at "The Pearson Ranch" (Russ Pearson's family ranch) and it was tight and hilly as hell with 6 creek crossings and a sh*tload of trees. We never even got out of second gear for the entire GP!! Fortunately it was only an hour though!