epilogue to the '99 yz thumper stumper

will pattison

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#1
so what was it i said last thursday? "i see no reason why it shouldn't run like a top.", or some such foolishness, if memory serves. well...it didn't. here's the last chapter of the story.

after buttoning everything back up thursday evening i jumped on and tried to start it. no dice. zip, zero, zilch, less than nada. i checked spark again. no worries. i started fiddling with the carb to make sure i was getting fuel. in that process i discovered that the pivot for the accelerator pump linkage was a bit grungy and it wasn't moving smoothly through its full travel. "aha!" i said, while scratching my head about how so many things could go wrong with a bulletproof motorcycle at once. i cleaned up the carb, put it back in and...it didn't start. by this time it was friday morning at 8. time to go to work.

the part of this story you don't know is that my brother was in town for the supercross. our plan was to also hit the tccra race at red river, which would give us a very rare opportunity to ride together during saturday practice. it was something both of us had been looking forward to for a long time, but on friday morning it looked like the only way we would be riding together was if we went double on my 426. the whoops at red river are a bit deep for that...

since it was the one thing i hadn't done yet, i suggested to my bro that while i was at work, that he open the valve cover on my 426 and compare the cam timing to the 400. by noon, it was obvious that EVERYONE WHO SAID A CAM GEAR MUST HAVE SLIPPED WAS RIGHT. turns out that it was the intake cam - the most expensive of the two, of course. the gear is about 30 degrees clockwise from where it should be. so much for expert opinions, eh?

as i suspected, central yamaha didn't have one in stock, and since we had to be at practice by 8 the next morning, a saturday delivery wasn't gonna help. luckily, i just happen to know a guy in my class who has a yz400 motor just sitting on his shop floor doing nothing. he lives about 40 miles from dallas, so friday afternoon my brother drove up and got it. by 7:30pm we had swapped the borrowed cam from the borrowed engine and after 2 solid weeks of extreme stress, my steed was all back together.

it started with the first kick.

the irony is that after an excellent ride the next morning and an excellent supercross the next evening, my race was a bust. after overcoming fatigue on the first lap and finally getting my mind right, i had moved into 4th place and was feeling better every minute. unfortunately, i popped my front tire at the beginning of the 3rd lap. after riding the next 9 miles on a motorcyle that handled like the u.s.s. new jersey, we borrowed the front wheel from my 426 and i finished the race...9th.

ain't that racin'?

oh, by the way, since i had a direct comparo between the 20" front tire i'm normally running on my yz400 and the 21 incher on my 426, i can say unequivocally that the smaller, fatter front tire is a definite advantage off road. it sticks better and it's plusher in the rough. very nice.



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will pattison
engineer, racer
ignition
www.ignitioninc.com
 

Vic

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#2
Glad you got it worked out. Would it be possible to spot weld the cam gears to prevent slippage?
 

will pattison

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#3
i thought about that. i think a carefully placed tig stitch would do the trick without overheating anything.

wp.

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will pattison
engineer, racer
ignition
www.ignitioninc.com
 
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#4
Will - Glad to hear you can finally sleep again :)
In one of your earlier posts you had discounted gear slippage because you felt there was no external evidence to support it. After comparing the slipped and replacement cams did you see anything other than orientation that would change this view? Was there any evidence on the cam lobes or bearing surfaces that would point to an unusual load being applied? Was there any thing specific about the events that led up to the final failure that would point to this type of failure?

Inquiring minds :)

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Rich Rohrich
==

"Wisdom is not a product of schooling but of the life-long attempt to acquire it."
- Albert Einstein
 

TexKDX

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#5
Wow, Will, what a story. Glad you were finally able to get to the truth. Such a bizarre story finally has a rather simple ending, one that was not wasy to find. Better start calling you Fox Pattison or Will Mulder.

I can remember back in the high school days trying to get a friend's car going after we re-did the motor. It acted like it had no compression but would crank over fine and fire (just ask the guy sitting on the windshield with the can of ether and a carburetor between his legs - me - if it would fire). Yep, turned out we mis-red the timing chain markings. Pulled the cover, lined it up right, and it fired up and ran the first time we touched the key.

Now I know who to email if I ever have to get into the guts of one of these beasts. I'll be sure and get a king-sized can of ether before I do though ;).

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TexKDX
 

will pattison

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#6
rich-

here is what i figure:

1. too much drag on the intake cam plain bearings.
2. engine idling for too long- not up to temps, which might normally have relieved that drag.
3. the drag overcame the friction from the press fit, and boom, no more cam timing.

anybody who has ever been into this motor knows that getting a torque wrench onto the middle bolts on the intake cam cap is a real pain. if, like me, all you have is a 1/4" drive 8mm socket, but your torque wrench is 3/8" drive, you are hosed. unfortunately, i learned about snap-on wobble sockets a little too late. what i am trying to say here is that i probably over tightened the cam cap bolts a bit when i re-ringed the motor over the winter. when i first opened the motor after the disaster, i noticed that i couldn't budge the intake cam by hand. when i disassembled it, i found very slight galling on the surface of both plain bearings. polishing fixed that.

i should say, however, that this same thing happened on my '98, but the galling was worse, and the cam gears never slipped. i also polished that problem out, and then ran the motor another 120 hours with zero issues.

i also had a buddy trash his head on a '98, and i think it was for the same reason. the moral: get whatever tools it takes to put the correct (10nm) of torque on those cam cap screws!!

wp.

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will pattison
engineer, racer
ignition
www.ignitioninc.com
 

NVR FNSH

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#7
Thanks a lot, Will. I just adjusted the valves on my WR....... I don't have a 1/4" drive torque wrench either :(

On another note - I did pick up a strange sound after doing the valves. I don't know if it was there before & I just didn't notice it or if it started after the valve adjustment. From what I've read on Thumpertalk it sounds like the counter balancer drive gear nut/key are loose :(

Back to turning more wrenches....

Brian

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'99 WR400 - dual sport
'90 KX500
'92 XR250R - dual sport, wife's bike (I ride it more than she does because it goes ANYWHERE)