YZ450F 07 Wont start after top end rebuilt, please help!

I should start by saying that is my first top end rebuilt and nothing was to complicated as I was folowing the book. But I might have forgot something... hope not

So after my top end rebuilt, valve adjustment, cleaning air filter, changing the fuel because it sat there 2 weeks, checking spark plug... I am now out of idea. When I kick the starter it feel alright but except for few back fire, it wont start and not even close to. I did try with or wihout choke and 2 blips. It just dont want to start.

What I should look for ? any idea would be great.

Thanks for any help. I just want to ride now that the new track is open :thumbsup: Hopefuly I will be able to try it this weekend.


Double check the flywheel first:

Pull the spark plug and bring the engine to TDC. Put a Philips screwdriver down the hole to contact the piston. Rock the engine back and forth as you hold the screwdriver against the piston to determine if TDC is really where the mark says it is.

Then, if that comes out right, recheck your timing. If that is also good, try fresh gas. Drain the carb before trying to start it with the new stuff.

Thanks for your answer :thumbsup:

I just checked my TDC with a screw driver and it does fit with the mark on the flywheel. I do have a question about the timing thought. Is it normal that after aligning the mark on the cams with the cover I endup with 12 pins between the 12 o'clock mark ? I know that the book call for 14 and that a lot of other thread say that it is 13. But if I put 13, my intake cam mark is a little below the surface of the head.

Also, I did drain all the fuel of my tank and put fresh one... still don't want to start.

I'll try to play with the timing again tomorow. Maybe the mark do not have to lineup perfectly with the cover.

If you do have anything else, thanks again :busted: Could I have do someting wrong when I did rebuilt my top end ? I did'nt mentionned that I did change the piston and rings to, making sure that they are not aligned.

12 pins is wrong on a 450. When both cams are timed correctly they will have 13 pins between the two.

When timing the cams, be sure that when you check them with the tensioner off of it that you pull first the exhaust, and then the intake cam backward very slightly, then slowly turn the engine forward again to TDC. This replicates the condition the chain would be in while the engine runs; front and top runs of chain tight, all the slack on the backside.

The timing marks will very rarely line up exactly right. The question to ask yourself is whether moving the cam tooth in either direction will make it look closer top right or not. (visualize the mark by the next tooth). The timing marks are the important thing. Counting pins can be used as a cross check where you are really uncertain, but if it gets down to a situation where it looks about the same whichever way you time it, you most likely need a new cam chain.

Your timing has to be correct with the tensioner in. Often you will need to set it so that it looks like the intake is one tooth too far counter-clockwise. Whey you put tension on the chain it will often roll the intake clockwise a tad revealing the actual timing. Are you checking the timing with the tensioner in or out?

You have spark?

Thanks to all,

Yes I do have spark. I do check my timming with the tensioner in. I'll try again tomorow and I'll put it back to 13 pin making sure all the slack is on the back side. I'll report my finding.

So I guess it will be a timming issue, I hope I will be able to ride tomorow...

Thanks again.

if its backfiring that usually means its timed late - as in the the spark is happening too late in the cycle - 13 pins not 14

It still wont start...

I did recheck my timing. With the chain slack all on the back side I did align the intake cam and after check for the good spot of the exaust cam, 13 pins between the 12 o'clock mark.

Here is a picture of my timing, I know the view is not perfect


I also check my pilot, main and needle jet. Cleaned them even if they were good. No change.

Is it possible that the compression would not be good ? Like I said, I changed the piston, rings, valve seal and reseated the valve.


I finaly had it to start! After reading about the stroke on this site


I figure out that I could be on exhaust stroke, so I did a 360 on my flywheel and after few kick it did start. It could be a beginer mistake, but the important thing is that it work now :busted:

That being said... I tought by changing the rings and the valve seal that it would resolve my smoking problem. BUT NO! I have the same problem :thumbsup: After few sec of running, it did sarted smoking again... I really need your help on that one.

Being "on the wrong stroke" is not possible with a YZ450. The reason is that the ignition is triggered at the crank, and it gets a spark just before TDC on every stroke, whether it's the compression stroke or the exhaust stroke. The fact that it started after you did that is pure happenstance, my friend. But, I'm glad it did. :thumbsup:

Like you said, I am glad it did :thumbsup:

By the way, my smoke issue is resolved. It was probably residu... because I went ride it few minutes, and when I was back, no more smoke. I'll check my valve clearance again because I hear a "clack-clack" more then before, but except that, everything seems to be alright !

Thanks again for all your help!


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