Cam chain slipped YZ426 02' why? What do I replace?

[Brett1981]

I have an 02' YZ426 which was running beaultiful all day on my last riding day. When I get home I clean it, start it up and take it for a spin to dry it off then it siezes.

I took it apart and found the cam chain had jumped, causing the intake valves to hit the piston, bending the intake valves a lot and putting a mark on the piston but no damage to the pistion or bottom end.

I want to know how the hell could a cam chain jump a tooth? The tensioner seems very solid. I want to admit that when I put the cam back in last time I changed the head gasket, I didnt have a small torque wrench and am thinking maybe did it too tight and the camshaft had too much fricton because of that, thats the only cause I can think of. I also want to know what people think I should replace? The intake valves, head gasket and rings are on the cards.

I want this never to happen again, I have been a 2-stroke man all my life until now and first impressions aren't the best of the 4-stroke reliability so far but I enjoy riding them. Thanks

[qwik12beat]

Over torquing the camshaft caps could have been the cause. Since when things get hot they expand. I guessing that the exhaust cam temporarily seized causing you to jump time. Or the cam chain might have been out of spec.

I would check the cam journals condition, and if they checkout i would have the whole head redone(magnafluxed,new valve seats,valves,valve guides,cam chain, and a new tensioner. I would also replace the piston since not all damages can be seen. It could have a hairline crack in it, and cause big problems down the road.

[642MX]

The first step is to remove the mag cover, flywheel and stator. Have a good look at the crank gear and see if the teeth are worn, broken ect, ect...

[grayracer513]

I want to know how the hell could a cam chain jump a tooth?

Usually, a worn crank gear or a seized intake cam. If the cam caps have been off, then either one could have been improperly torqued, leading to a seizure.

[Brett1981]

yeh very good advice thanks. I think inadequate oil supply to intake cam plus inproper torquing caused it. A journal on the intake cam did look a bit roughed up so I can say I think it had an intake cam seizure.

Its going to cost me a fair bit though so I'm a bit pissed off and miss my two strokes already.

Would you say that if things are done properly four strokes will last as long as a two stroke? I feel like selling the thing now.

[642MX]

Would you say that if things are done properly four strokes will last as long as a two stroke? I feel like selling the thing now.

Absolutely not. A 2 stroke will outlast any performance 4 stroke.

[husqy360]

Absolutely not. A 2 stroke will outlast any performance 4 stroke.

4t win here !

[grayracer513]

Absolutely not. A 2 stroke will outlast any performance 4 stroke.

I can't agree with that at all.

[642MX]

I can't agree with that at all.

Dollar per hour ratio, a smoker is far ahead of a stroker. You would agree with that.....right?

[grayracer513]

Dollar per hour ratio, a smoker is far ahead of a stroker. You would agree with that.....right?

Having done nothing whatever to an '03 YZ450 for 5 years, no, I'm afraid I can't. On an inch per inch basis, I'd agree, but comparing a YZ426/450 to a 250 pinger, even a YZ, I don't see it.

[mike_dean]

530 hrs on a '00 YZ426, original valves, 1 piston ( it didn't need). My CR500's and CR250's cost me much more and never ran 9 yrs. My 426 still runs fine, I just don't ride it much anymore, the '06 is more fun. mike

[642MX]

Having done nothing whatever to an '03 YZ450 for 5 years, no, I'm afraid I can't. On an inch per inch basis, I'd agree, but comparing a YZ426/450 to a 250 pinger, even a YZ, I don't see it.

Maybe I replace too many parts?..., but my 2 strokes didn't cost me near as much money to own/operate as my 4 stroke.

I can almost smell the premix of my next bike already.... I think I'm going to get a YZ250 in the near future. And I'm not saying the 426 hasn't been a great bike, but I not too sure the newer 450's are as reliable as the older YZF's. I have so many toys that its hard to keep them all properly maintained, so thats another reason for me to get a smoker.

I'm still keeping the 426, that bike is like part of the family.

[642MX]

530 hrs on a '00 YZ426, original valves, 1 piston ( it didn't need). My CR500's and CR250's cost me much more and never ran 9 yrs. My 426 still runs fine, I just don't ride it much anymore, the '06 is more fun. mike

Mike- Did you not feel any improvement with a fresh top end in the 426? I rering mine once a year and can always feel a difference. It doesn't surprise me that your valves are originals, the SS valves are the way to go. My 426 is on the second set of valves and my wifes bike is on the third set.

[qwik12beat]

I'm thinkin of getting another smoker too. My valves are always on the move, and its got me so paranoid i check them after every ride.The last ride i was on i had shimmed my valves right before, and then when i get home i see that both of my exhaust valves opened up .04mm putting them at .28mm(out of spec). I hate checking them all the time, but it is easy.

[BASSic]

Mike- Did you not feel any improvement with a fresh top end in the 426? I rering mine once a year and can always feel a difference. It doesn't surprise me that your valves are originals, the SS valves are the way to go. My 426 is on the second set of valves and my wifes bike is on the third set.

Do you really feel a difference or is it more of a psychological effect that you improved power dramatically after replacing the rings? I'm not poking fun here, but I'm asking a serious question. I only ask because my 01 YZ426 has no problem keeping up with modern (07-08) kawasaki, KTM, and suzuki 450s, on straightaways at least (throwing them around a track is another story).

I bought my 426 last December and checked the valves, and they were all within spec. I checked again last weekend before doing the 450 cam swap and found that they tightened up a bit but weren't too bad. The exhaust valves were just out of spec, but two intake valves were still good, and the third intake valve was also just outside. I don't know the history of the bike, but in the year that I've owned it it's seen a fair share of hard aggressive riding, hitting the rev limiter more than necessary. I was very surprised to see that the (Ti) valves weren't further outside of acceptable limits.

When you replaced valves on yours and your wife's bike, did you get the head machined? Did you replace with SS or Ti valves?

I was going to install new rings, but I'm going to try to put that off another season. (a fork rebuild and new tires for the 426, and new linkage bearings/bushings and a top end on my KX250 are enough to keep me busy before the ice on my trails melts)

[642MX]

I have no dyno results to prove anything, but yes, I noticed more snap (hit) after reringing them. I really can't feel the power dropping off like a 2 stroke does when the rings need changed, but I can feel the difference after the rebuild.

My wifes bike had the seats touched up before the valves where installed. The second set of valves where OEMs, the third set where Faction SS. Same with my bike, except I'm running OEM Ti valves. If they need replaced again, I'll go with SS.

[mike_dean]

Mike- Did you not feel any improvement with a fresh top end in the 426? I rering mine once a year and can always feel a difference. It doesn't surprise me that your valves are originals, the SS valves are the way to go. My 426 is on the second set of valves and my wifes bike is on the third set.

There is nothing like the rush of a 500 2 stroke, but the 4 stroke is easier to ride and faster for me.

I'm with you on the SS valves, no need for titanium on a dirt bike.

I did feel an improvement, but the bike always had enough power. I always use the middle of the power band and have not hit the rev limiter except in a panic on a jump, and change oil and check valves too often. I remember the price of my 2 stroke piston and rings once or twice a year, and that it was a big improvement in power, my 500 needed crank bearings, my 250 had constant power valve problems, I still changed the trans fluid after every race along with the spark plug and 2 stroke oil, I think my 4 stroke was way cheaper.

If I did the recommended 4 stroke maint. it probably would cost more, but I think Yamaha's schedule is extreme from my experience. Mike

[Brett1981]

I've just started off with a bad 4-stroke example maybe, I mean I had one ride, I now need new intake valves, rings, camchain (I think), gaskets.

Which I think will cost a lot more than I ever spent on my KX250 '93 I had for 4 years.

I love the power delivery on the 4-stroke but reasonable reliability would make me love it a lot more.