Alright so a week ago i did the cam mod on a buddies yfz and rejetted it accordingly. So we rode it around all was fine the bike ripped like A MOTHER. sO YESTERDAY WE GO OUT TO THE DUNES AND ITS KICKIN SOME ASS THEN ALL OF THE SUDDEN IT STARTED DYING LIKE IT WAS OT OF FUEL. sO MY FREIND STARTED IT UP AND WAS IDDING IT BACK TO THE TRUCK AND IT JUST DIED.sO I BROUGHT IT BACK AND PULD THE SPARK PLUG IT WAS GOOD. cHECKED THE CAM TIMEING AND ITS GOOD. But it is realy hard to turn over. There is no metal n the oil filter. And the only thing i havent checked is the valve clearences could thes cause that.

the valves could have closed up to zero tolerance.. i dont know a thing about yfz's though

Thanks for the quick reply ill check them. Anybody else?

Ok the exhaust valves are good. But my feeler gauges only go to .15mm. But that doesnt go in the intakes so i take it they are zero or very close to it.

dont do the cam mod! ur talking about setting the timing to the dirt bike timing right?

thats a really good way to grenade the motor

It's the thing to do on the yfz. It improves performance.

i did this cam mod to my friends yfz a couple months ago...works perfectly fine.

The cam mod is perfectly safe.
One of Yamaha's GYTR tune things is to "degree" the cam, by turning it back one tooth to match the dirtbike timing.
If it was bad, the bikes would explode.

I adjusted the valves to spec and still nothing. So any body got any suggestions to why it wont start and is realy hard to turn over?

check the battery and the CDI

I checked those. But its even hard to turn over by hand. But it seems like its sucking from the exhaust and intake . And is building a lot of crankcase pressure . What about piston and rings would this cause anything?

you're saying it sucks air in through the exhaust instead of the intake?....check your timing again, as this is the only thing that'll cause the scenario i think you're describing. the cam lobes should be facing outwards and a little up. here's a diagram for how your timing should look.

its apart in my garadge right now and they are 100% perfect.

but you're sucking air through the exhaust instead of the intake? like if you cover the exhaust with your hand, it'll suck on your hand? instead of pushing it away?

alright so i just went and tried cranking it again. When it sounds like its realy hard to crank it sucks from the exhaust then after a while it will change and sound normal and blow like normal

yea im pretty sure your cams are just backwards, if you can, take a picture of your cam timing and post it on here.

Ok so i reset the timing to tdc on the compression stroke everything is perfect according to the pic in the owners manual and still the same results.

are you sure you counted 15 pins from the top of the exhaust cam to the top of the intake cam? and you also need to have the "1" mark of the exhaust cam right where the head meets the head cover...make sure its not where the gasket meets the head cover cuz the difference there would be about the same as 1 tooth. and also make sure you have your cam chain tensioner installed before you turn over the engine...i just recently didnt realize that mine wasnt installed before i turned it over and the chain jumped a few teeth.

and as many times as you've tried starting it without it firing, you could have drained your battery depending on how long you tried starting it. as long as your valves aren't hitting the piston at TDC, then a FULLY CHARGED battery should have no problem at all turning it over.

k ill charge the batt tonight. This is on a yfz so there is only suposed to be 14 pins according to the owners manual and the guys at yamaha. thanks for the help ill give her a nother try tomorow.

oh yea...sorry, forgot it was a yfz. i actually have a yfz450 manual on my computer too. give me a minute and i'll try and look for the timing diagram.

Check the decompression mechanism and make sure it's working properly.
Sucking through the exhaust isn't unheard of.
I've had a few that sucked through the exhaust while turning over slow.
Mainly stretched timing chains caused it, but it also depends on cam profile.

Check that decomp setup, and have you tried pulling it yet?

Yes i have tried pulling it with no luck. Could the gear on the cam be spining?

It could. But they are all either pressed and welded or just pressed on pretty tight.
Check the lobe positions on the cam.

Alright so i went outside after school and she crankd and fired right up!!! But the bad part is she has a clankin noise in the top end somwheres near the right side. And if it idles for about a minute or so it just flat out dies.
Any suggestions?

How slack is that timing chain?

there is no slack its pretty tight. and the chain is on the left side of this bike anyways.Also after it dies it will start right back up and do the same thing.

That is very odd.
Yeah, I was thinking Z not YFZ... but same principal.
Check your plug or exhaust temps and see if it's leaning out on the idle, or flooding.

Would that cause the knocking.How do i check the exhaust temps.

Your hand will tell you enough about the exhaust temps.
If you can hold your hand within 1/2" of the muffler tip and it's warm, but not burning the crap out of you, it's about right.
If it's barely warm, you're a little rich.
If it's burning your fingers off then you're lean.

Checking the plug will also give a better idea of what and where the issue is.
If you don't know how to get good results from plug checking PM me and I'll give you a good method, that has always worked for me.

ok so the temps seem alright but now its getting realy hard to crank over and wont start again.

Does choking help at all or just make it worse?
Go blow the soot out of it and pull the plug.
I'm curious what you see.
Sounds like a fuel issue for sure.. but could also be ignition related.
The knock is harder to explain.

if you pull the choke it makes it worse to crank as running the soot out of it its not starting again. Its just barely cranking agian. It seems like it gets worse the hotter it getts.

It sounds rich.
When it sits for a while and the temp is up it will prolly try again

so you think it being rich would do this. The hard cranking and not starting.

If the Decomp is not working 100% and it's rich yeah it can.

The more dense the A/F charge the harder it is to compress.
With Cold weather it could cause the jetting difference.. and also by doing the cam mod it will change how much vacuum is pulled through the intake when the valves are opening.
Lots of factors could do it.

If it has an adjustable fuel screw you might try going in a little bit.. Or just barely crack the throttle while cranking. Don't pump it cause it will flood it worse.
Slowly open the throttle to about 1/8th or so and turn it over to let it clear out some of the gas vapor.

Thanks for the help but nothing is working and ive tried everything so i think ima pull the head off and see what i can see. But thanks for the help.

Ok so i pulled the head and i found that there is piston to valve contact. its not heavy impact but ther is contact so im thinking rod bearing?

could you post a pic of what the damage looks like? and then maybe we might be able to tell what caused it depending on the severity of the damage?

very possible. unless the timing has been off and turned over by the starter.
But i would check the crank play. with the knock it would make sens that the rod bearing is probably shot.
Pull the jug and see how much slack there is if it has any then its worn. If there is enough that you can hear it. Like mine was then it could definitlety cause piston contact. Especially with a compression ratio over 13:1.

Yep pulled the jug off and the rod is done. Thanks for all the help you gave me.