06 YZ450 I give up

This thing has beat me. A little history, I traded for the bike with the problem. The top end has new timing chain, stock piston and rings, cams and exh valves. New head and cyl have around 25 hours on them (head and valves good I check everything). I got the bike from a friend so I know the history since it was new. The bike was running fine and my friend was just running the valves and thought now would be a good time to freshen up the top end. From that moment on the bike will not start by the kick starter. I have gone over all the stickies on the web site and didn't find any that match the problem I have with this bike so here I am looking for an ideal to chase. The bike has been tore down 5 times. I can not find anything wrong. I got fed up and drag the bike and it busted off with in 20 feet and runs strong. I have put close to 3 hours of riding on it with no trouble at all (just don't fall down), but it won't start with the kick starter. I started chaseing electrical and every thing check fine, so I replaced the CDI box and wiring harness (had a wire that tested bad when you moved the bars left to right), didn't fix it. I have taken all the hot rod crap off and the bike is back stock with a JD jet kit. The one thing that I keep thinking about is the exh cam. The timing is dead on, but the cam lobes just don't look right. They seem to be retarded one tooth, but the cam marks are dead on with the head. Intake cam look good, but the exh cam lobes just look a little wrong. Has anyone replaced a stock OEM cam and the sproket was just off? One more thing that got me to thinking the problem is in the exh cam is, with the I mark on the fylwheel lined up and the cam on the marks the auto decompression mechanism does lay on the bucket. I keep thinking about the auto decomp, you can kick the bike and it will backfire every time, so I have to be getting fire while the exh valve is cracked open. But as I said the timing is good. I need some ideals to chase or this bike is going to hang on the wall of shame. Not realy I do need some help, I gave this bike to a young man just starting out and I can't give up. Thanks for any advise.

This thing has beat me. A little history, I traded for the bike with the problem. The top end has new timing chain, stock piston and rings, cams and exh valves. New head and cyl have around 25 hours on them (head and valves good I check everything). I got the bike from a friend so I know the history since it was new. The bike was running fine and my friend was just running the valves and thought now would be a good time to freshen up the top end. From that moment on the bike will not start by the kick starter. I have gone over all the stickies on the web site and didn't find any that match the problem I have with this bike so here I am looking for an ideal to chase. The bike has been tore down 5 times. I can not find anything wrong. I got fed up and drag the bike and it busted off with in 20 feet and runs strong. I have put close to 3 hours of riding on it with no trouble at all (just don't fall down), but it won't start with the kick starter. I started chaseing electrical and every thing check fine, so I replaced the CDI box and wiring harness (had a wire that tested bad when you moved the bars left to right), didn't fix it. I have taken all the hot rod crap off and the bike is back stock with a JD jet kit. The one thing that I keep thinking about is the exh cam. The timing is dead on, but the cam lobes just don't look right. They seem to be retarded one tooth, but the cam marks are dead on with the head. Intake cam look good, but the exh cam lobes just look a little wrong. Has anyone replaced a stock OEM cam and the sproket was just off? One more thing that got me to thinking the problem is in the exh cam is, with the I mark on the fylwheel lined up and the cam on the marks the auto decompression mechanism does lay on the bucket. I keep thinking about the auto decomp, you can kick the bike and it will backfire every time, so I have to be getting fire while the exh valve is cracked open. But as I said the timing is good. I need some ideals to chase or this bike is going to hang on the wall of shame. Not realy I do need some help, I gave this bike to a young man just starting out and I can't give up. Thanks for any advise.

Maybe there is nothing wrong with your engine.... how about a worn/dirty carb inlet needle and seat allowing a tiny amount or raw fuel to enter the engine ? All YZF's do NOT like raw fuel when trying to start.... does it backfire through the exhaust or the airbox ?

Back fire from exhaust. Went thrue the carb this weekend for the 3rd time, its good. Check the hot start from the bars to the card and its all good. This bike may test all of us. Thanks

Have you checked the compression when using the kick starter. It should be less than normal due to the ADC, however if the timing on the exhaust cam is off then the exhaust valve could be held open longer than normal explaining the backfire through the exhaust. Also explaining why it ran good when the engine speed was above 700rpm(disenguaging the ADC). I claim to be no expert on these engines, but I am a master auto technican and it seems like a logical possiblity.

I keep thinking ADC and if I advance the exh cam one tooth every thing looks right, cam lobe degree will look more intime with the intake cam and the ADC will have clearance off the bucket, but the cam sprocket would then be out of time by the marks. Advanceing the cam would seem to work, but I sure don't want to drop a few hundred dollars for some valves.

One thing about auto decompression in the YZ is that if the timing is off by a tooth either way, you'll know it immediately when you try to start the bike.

Timed correctly, the AD pin holds the exhaust valve off its seat to around 80-90 degrees BTDC so that the compression stroke is effectively shortened, and you can actually kick it through, but it still has enough compression to start. If it were a tooth retarded (which would be 22 degrees), the valve would seat at 60 degrees or so, and you'd have too little compression for the engine to run. If it were a tooth advanced, the valve would seat at 110 or so, which is not much different than the point where the intake closes, and you'd basically have no reduction of the full compression stroke at all; you couldn't kick it.

Normally, at TDC, the AD pin will be near the bucket on the back side of the camshaft, but not actually in contact with it the lifter. The check is to move the flyweight. If it moves freely, retracts the pin, and returns when released, it's not touching anything.

Post a picture of the cams in place.

Popping off through the exhaust usually means it's pumped fuel through but hasn't ignited it, and is quite often a fouled plug.

i had a friends that did that. we could push it and it would pop once in a while. we found out it to be that the cam spacers fell out of place when the head was turned upside down so they were not gapped correctly. fixed that and it ran perfect. just a thought

Thanks everyone for the ideals. I know for fact that everything is installed correct. However, I have tried to move the flyweight to retrack the pin at TDC and it didn't move. Roll it just a little pass TDC and it will move. I know the shim on that exh valve is correct and seated in the bucket. I also know that all the valves are seated and pass the liquid test (sit all night with gas in every valve chamber and no leaks or wet spots at all). I think I will pull the head one more time and check that exh valve, even tho it seated and does not leak, the fylweight pin has got my attention. The problem must be in that area. Thanks again for all the information and if someone comes up with a new ideal, I will be happy to check it out.

The AD pin should be free at TDC. Since it isn't, it sounds like one of three things:

1. You're wrong about the cam timing marks being lined up.
   
2. TDC indicated on the flywheel is not actual TDC because the flywheel key is sheared or out of place. Check the TDC mark against actual piston position by probing down into the spark plug hole.
   
3. The exhaust cam may have seized and slipped in its sprocket. With the intake and exhaust cams in the timing position, the lobes should look roughly as the lobes in the picture here do. If your exhaust cam lobes are pointing more upward than this, that may well be the problem.
   

Thank you gray and I do see a problem. 1st timing marks are in time, 2nd The flyweel key is one of the things I was going deeper into this weekend. I have not had that off yet to check it, but my freind did put the new timing chain in and I do trust him, but we all have are moments of weakness when the wife call to do honey doos. I do use a straw to see TDC on this bike, my dial indicator I use on the 2strokes is too short and I'm to cheap to buy another one. 3rd is the photo you have....thats the part when I said it just does not look right. I exh lobe on this bike looks one tooth retarded, even tho the cam marks are correct. The biggest problem is the cam is brand new OEM. Now some light is comming on and every thing makes sense. Could the cam sprocket been installed wrong? could it have spun? maybe anything can happen, but its brand new. Well I got some things to look over anyway so thank you all for your time, you put some logic into what I suspected. I will post back on what I come up with.

Could the cam sprocket ... have spun?

3 The exhaust cam may have seized and slipped in its sprocket.

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I know what your saying gray. The cam has not seized no marks on it or the head. But I will give it another look over to satisfy curiosity it has been run for about 3 hours, but the problem started when the cam was put in new out of the box. Thank you.

In that case, there is the possibility that the cam was not indexed correctly, or is in fact the wrong cam for the engine.

You don't feel any compression when you crank this thing, do you?

It has compression, but it feels weak to me. It has enough that you realy don't wan't to kick it over in sneakers . I put my mx boot on. The old SC500 yamaha and the Honda 250 elsinore taught many years ago that sneakers are not a good ideal (I still can't dance right, hopping on my right foot) ?. As for the cam it was an over the counter Yamaha replacement part from the local dealer. Yes the bike has hand grenade when it was 8 days old. Yamaha or should say the dealer replaced all the parts from the cases up exept the valve cover. My freind did all the labor, he is sponsored by this local dealer and he and I have work for the dealer for many years in the past, so this is not are first rodeo. The bike did not get the factory cams installed at that time, thats when all the hot rod stuff was bought and put in. The problem started at 25 hours on this replacement top end for service and that is when the stock cams was installed. I took the valve cover off last night and rolled it over to look at the timing again and everything lined up. That exh cam lobe is up and thats the problem, but I rreassure you the timing is correct. I will dig deeper this weekend unless something else comes up around the house. Thank you for your time.

It was the cam. I replaced the exh cam and the lobes look right. Gray your photo told me what I suspected . Before I took the new cam out of the bike I check the timing marks and the lobes for the last time and the cam sprocket was 1 1/2 tooths off. What ever the reason the cam sprocket was not in time with the cam lobes. Replace the cam and the bike started the first kick. We took it out and put 3 hours on it with no problems. Thanks everyone for the ideals, we did fix it, I just wish the parts didn't cost so much. By the way, the new OEM cam that came out of it does not even deserve a place on the wall of shame.?

SWEET Another one on the trails

Ok I know that this is an old thread but I'm having the same issue and I didn't want to start a new one. I think that one of my cam sprockets has slipped.

I've got pic's of what it look's like. This is at top dead center with the dot's on the cam's lined up on the opposite side per manual instructions.

My camera refused to focus on this one but you can see the dot lined up with the block.

Intake

Exhaust

And I stuck an allen wrench in top of cylinder and moved it up and down until I found TDC.

So with the lobes being off from one another like this, could I have bent valves? it doesn't feel like it has enough compression basically same deal as Padgett's earlier problems.

I have hot cam's in it so I had my mother pull my stock ones out of storage and mail them to me. So I can try that out. I took the cam chain off of the cam's so that the decompression wasn't running and still doesn't feel like a bunch of compression. Should I take this in and have it leak tested?

I know they aren't a long way's off but they are a little.

That's pretty much what mine looked like, Sledtrash, after I replaced the cam chain, & a valve pad/shim or two.... ?...... Sure looks like it's timed correctly to me....

Jimmie

I've got pic's of what it look's like. This is at top dead center with the dot's on the cam's lined up on the opposite side per manual instructions.

...So with the lobes being off from one another like this, could I have bent valves? it doesn't feel like it has enough compression

...I took the cam chain off of the cam's so that the decompression wasn't running and still doesn't feel like a bunch of compression.

What does that tell you about the cam timing? If it doesn't have good compression with the valves shut all the time, cam timing is not your problem.

What's your valve clearance right now?

There's nothing wrong with the way that's set up that I can see.

What does that tell you about the cam timing? If it doesn't have good compression with the valves shut all the time, cam timing is not your problem.

What's your valve clearance right now?

Well it didn't tell me anything about the timing but it did tell me that I didn't have compression. After I posted I got to investigating and I found that my valve caps were binding a little due to the shim not being where it needed to be. I looked it over no damage that I can see. Because those little "shim's" (not sure if that's what they are I've never been into my valves before) were sitting higher up my exhaust valves were cracked open all the time causing all of my compression to exit via exhaust.

So got those set up and I retimed it and it fired up first kick!

It's about time I'm so ready to ride this thing!!! Thanks to everyone with their help on this!! Gray you've been a life saver throughout this project.

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and best of all my lights work!!! ?