yz400 stumper...

well...i have a cross country race this weekend, and at this very moment my crankshaft is sitting on the workbench. like all racers, i have the occasional mechanical catastrophe. this engine failure on my '99 yz400 makes 4 over the course of twenty years of riding and racing. i guess one every 5 years isn't a bad average, but that's not doing much to ease my stress level. i'm reporting all this because i have an unusual situation- at least for me. i can't figure out what's wrong.

while i'm not going to claim to be the world's best mechanic, there are few places i haven't had my hands in on a modern dirt bike. i've built wheels, repacked linkages, rebuilt shocks, revalved forks, split cases and done countless top ends. to find myself unable to diagnose a mechanical problem on one of my bikes is...well...disconcerting. anyhow, i'm hoping that somebody else will have some insight. here goes...

the patient: 99yz400, 55 hours on the motor, about 15 hours on a new set of rings and new head and base gaskets. fmf pipe, 8 oz. flywheel weight. barnett clutch plates. no other parts that are not original.

the symptom: idling down the road to the starting line, the bike suddenly makes an odd sort of grunt/cough and dies. no amount of kicking will start it. i discover that i can kick it all the way through without activating the decompression lever- i have about half compression. we pull it. no luck. the bike has been running perfectly since the rebuild this winter, and it was running fine before that.

the first examination:

- all valves can be moved freely by hand, and return easily to their starting position without binding.

- no broken cams.

- no visible damage to cam chain.

- all teeth on both cam sprockets and on crank sprocket.

- the cams are nearly 180 degrees out of time. this is the only obvious anomaly.

after much confering with thumper racing and stroker, i replace the cam chain and the cam chain tensioner. when i reassemble the top end and kick it through, the condition is the same- only about half compression.

the second examination:

- leak test on valves shows the seats are...well...seated.

- there seems to be lots of carbon on the bottom of the head and around the perimiter of the piston.

- there is about 0.006" lateral play in the big end bearing (there should be zero, right?).

- there is about .075" wobble at the little end of the rod. yamaha specs about half that, maximum.

- slight play in crank bearings.

- ample spark.

- ring gap on top and middle rings is acceptable, but at big end of tolerance.

- no cylinder scoring other than the usual at the front and back (typical, especially with slipper pistons).

- no evidence of char on head gasket.

- plug looks perfect, maybe a bit lean.

regardless of what it's contributing to the problem, i'm obviously putting in a new rod (actually, it's most economical for me to put in an entire new crank assembly). i'm also putting in a new piston and gaskets.

i and my buddies are baffled. any thoughts?


will pattison

racer, engineer


About the cam timing: Are you saying that both cams are 180 degrees off from the timing mark? Or, is one cam on the mark while the other is 180 degrees from where it should be? I'm assuming that you corrected this before doing the leak test on the valves. Did you do a complete leak down test? From what I gather, you've described a serious loss of compression, right? I'm curious as to what is going on with the cam timing. Is there a chance that one of the cam sprockets lost its press fit and spun out of time? It's unlikely, but not impossible.

I know you say the teeth on the crank look good but I’m thinking the cam chain skipped some teeth. If so I guess you’re lucky this didn’t happen at higher rpm :)

On the (barely) bent 426 crank on my dresser with about 75 hours on it there is absolutely zero lateral play at the big end. When I removed it there was zero crank play at the mains, but of course it IS bent…

Another thing you might check before you put everything back together is both cam chain guides, especially the rear one that bolts to the cases behind the flywheel. This guide is what keeps the cam chain on the crank sprocket (as well as chain tension of course), maybe if it is sufficiently worn enough clearance could result to allow the chain to skip teeth. I think what Boit mentioned about a sprocket spinning on its camshaft is another good guess (and easy to check for).

If you are getting a new piston you may consider converting to a 426, all you’d need is the barrel (about $250 I think) and of course the piston and 426 crank assembly (larger wrist pin).

BTW how’d you R&R the crank? The shops I asked were no help at all and the tool(s) recommended in the manual were too expensive so I made my own puller out of angle iron and some threaded rod (I didn’t want to use a press).

the cams were 150-180 degrees out from EACH OTHER at tdc.

as to the leak test, i probably used the wrong nomenclature. i actually just did the basic test where you pour gasoline into the ports and watch the combustion side to see if you get seepage around the valve seats. some people use alcohol and pour it onto the combustion side.

unfortunately, my compression tester doesn't have an adapter small enough for the plug hole on this motor.

as to the sprocket spinning on the cam: this occured to me, but when i put them in the correct tdc position, the lobes point the right direction- fore for the exhaust and aft for the intake. russell at thumper racing (a guru) says there is not a key way or spline on the shaft, but that he actually broke a cam gear trying to press one off. that's a tight fit!

what's wierd here is that AFTER i put in a new cam chain and tensioner and properly timed the cams, i still had lack of compression.

hick, do you know steve fox?

regarding crank removal (if that's what you mean by r&r...). i love yamaha. they have toleranced this motor so that i could easily pop the cases apart and lightly tap the crank out of the ignition side bearing with very little effort. thumper wanted $300 to split and reassemble the cases, and central yamaha quoted 8 hours!! it took me all of 5 minutes once i had all the extraneous parts removed. note: i even left the countershaft sprocket on. i have all the tranny guts secured with big rubber bands so i don't waste any valuable time mucking about with gears. the only thing that required any force was removal of the main bearings. i'll use a little heat when i reinstall 'em.

as to converting to 426, that's compelling, but not in the 2 weeks between races. i feel like i would need testing time to dial in the jetting, etc. maybe this winter...



If you pulled the engine apart and found the cams spun around that far, you obviously had some sort of a serious failure, and you should have seen very obvious evidence of this somewhere.

If you didn't spot any thing obvious, like a very lose chain or sheared or spun gear, then perhaps we need to verify how it is that you're determining cam position.

And Boit may have something with the spun gear issue. How are you verifying this has not happen? What about the gear on the crank?


the crank gear is integral to the crank itself. no keyway. no spinning.

i determine the position of the cams like so:

with the flywheel at the tdc mark, afix the cams such that the "e" dot on the exhaust cam is lined up with the valve cover gasket surface, and the "i" dot on the intake cam aligns with same. this is the correct timing position. in that position, the cam lobes on the exhaust and intake cams point fore and aft, respectively.

can't truly verify that a cam gear did not spin other than to check the lobe orientation when the cams are positioned as described. there is also no metal scarring that might indicate such a failure.



Don't overlook Boit's sugestion regarding the spun cam sprocket being the cause. It has been the cause of serious frustration for a friend of mine. After replacing all 5 valves, having the seats ground, and replacing the rings his 99 YZ400F still had little to no compression. He was timing the bike dead on according to the marks on the cam gears. I don't know how many times he has had the bike appart and back together. Each time he had no compression.

As a last resort he took my bike apart to compare how the cams looked at TDC. He just called me to say the lobes on his were not even close to mine (maybe a tooth off or so). Anyway it looks like the the gear spun on his exuast side cam. End result -- No Compression!!!


ok, you've inspired me to pull the valve cover on my 426 and have a lookie.



The thing about the cam sprockets possibly spinning, is that the sprockets have punch marks that should line up at 12,3,6,and 9 o'clock positions with the cams at the TDC marks. If one of the sprockets has spun, then the punch marks won't line up properly. Unless, or course, if by some miracle the sprocket spun at exactly a 90 degree increment. At any rate, this is a pretty interesting problem and I would like to know how it plays out.

I didn't even consider that the gear on the crank could have spun...or even broken a tooth or two cause the cam chain to skip at times. Yes, this is grabbing at straws...but sometimes, the strangest things fail. Sometimes, it's the most obvious we overlook. Like earlier this evening when I wanted to change my main jet to one step leaner. It took me all of 10 minutes to do this when before(when my dain was bramaged) it took me an hour and a half at least. I simply removed the float bowl plug and the main jet was RIGHT THERE. All that wasted time of unbolting the coil...unclamping the carb...twisting it over...removing the float bowl...etc....Geez!

Will: back to your problem. Just for argument's sake, let's suppose that one of the cam sprockets has lost it's press fit. From what you've described, you should have normal compression. If a sprocket has spun...it may not necessarily spin freely and continuously. It might just have a small range of movement and then bind again, which sounds like what jbird's friend experienced. When it comes to building the proper compression, 4-strokes aren't very complicated. The valves must seal...the piston rings must seal...the cylinder head gasket must seal...and that's it. Checking each one and making sure it's doing what it's supposed to do is the way to go...in my opinion.

Oh...and I forgot one...is your spark plug tight?

[This message has been edited by Boit (edited 04-19-2001).]

Well my friends bike is back together and running strong. The cam gear was spun on the exhaust side. It spun two teeth. He timed it by comparing the cams on my bike and making his look the same at TDC.

I guess the next thing is why it spun and if it will spin again? He only tried it out for a short period around the house.

Any thoughts on this??????????

Will, I almost hope this is your problem so you can get it fixed and not spend 3 weeks messing around like my friend.


that's why this thing has been so wierd- you guys are all correct, but the evidence isn't there to indicate that a gear slipped on a cam. at any rate, i got the motor built back up again last night, and i have enough compression that i can stand with my full weight on the kickstarter and not so much as budge the piston. that's more like it!

here's my theory as to what happened: i don't know how the slipped cam chain fits into the picture, but i believe i blew a head gasket- instantly and catastrophically. don't ask me how. my only other blown head gasket was on my '98 as a result of the head bolts around the exhaust coming loose, and it was gradual. i had symptoms like high rpm cutting out and very hard starting that i didn't have in this case, but nevertheless, i think that's what happened. there is no soot on the gasket, or any other obvious signs of damage, but i can't figure it any other way.

now...if i were superstitious, i would say that the beast did it to itself out of self preservation. see, i was having suspicions of looming crank trouble because of of some slight knocking at certain rpms. further, the race i was about to start that day was a fast, high rpm affair with several straights where i would have easily been hitting 80 mph. i could say that this bike somehow knew it wouldn't survive the event without throwing the rod and doing huge damage. so, it coughed up it's own head gasket and threw it's own timing chain just to make sure i had to get in there and find the big end bearing about to let go.

IF i were superstitious. as it is, it's just a wierd mystery i'll never solve. i will be racing this weekend though, and i have to once again give some kudos. central yamaha in plano, tx was a huge help in getting all the parts on time at a good price. yamaha itself gets both a thumbs up and a thumbs down. there is no reason this crank should have gone south. this bike had 1/3 the time on it that my 180 hour-old '98 yz400 did when i sold it, and those hours were more gentle to boot. other than the blown head gasket, the only issues i EVER had with the '98 motor were leaky water pump seals. on the positive side, i am totally pleased with the tolerancing on this engine. i put the crank in the freezer for about an hour, then heated the left side bearing with my electric heat gun for about 5 minutes. even without grease, the crank would have dropped in easily. additionally, the cases practically put themselves back together. good engineering.

so ends the story, i hope. it's not completely back together, but at this point i have no reason to believe it won't run like a charm.

thanks to all youse guys for the moral support.


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