YZ450f 2005 won't kick start after rebuild

can you not install new valves with out re cutting seats ... ?
Not if you want them to seal, no. Assuming there was any reason to replace them in the first place, the faces new valves will be perfectly concentric with the stems, perfectly round, and ground at a perfectly flat 45 degree angle. None of this will be true of the used, worn seat, so how do you expect the valve to seal against compression and combustion?

Cutting the seats is imperative in order to get them to seal, and correctly finishing them is critical to making the valves last as long as they should.

Which brings me to the question: How is your cranking compression, anyway?

Thanks for that link PADGETT, very good reading I have the head off and valves out checking and re seating them so can't get an exact comparison to to lobe angles shown in the photo in that posting but I did do on the bench held the cams set to where they would have been when installed and it appears the exhaust cam lobe would have been pointing higher than the intake cam lobe I certainly will cross reference when installed again but it is showing a difference from the photo - Gee wiz I never new the cam sprocket could move never even considered checking alternate side just thought if the two dots were in line with head it was correct - have the cam sprockets been know to slip? - This is a whole new angle to consider when installing your cams ??

Thanks for the reply GRAYRACER the compression when kicking over felt better than ever before - I not equipped to do leak down or compression test from home to confirm it ? I didn't re cut the seats but I did fill with fuel and got no leaks from the new valves straight from the box so thought just lapping in would be enough to seal them even better ?? maybe I have missed this crucial step and all issues are in there ? I posted earlier to PADGETT regarding link he sent showing picture of cam lobes and it does appear mine would have been showing the exhaust lobe higher than that photo even though they were lined up with all the markings being correct - there possibly some answers in there also - I had know idea the cam sprocket could slip so never considered that could be a problem? all your thoughts more than welcome the top end is in bits on my work bench now so anything is an option if you feel is necessary I will try it regards J

As I was reading your problem it just sounded so much like what I went threw on mine. I went over the timing about 6 times and then grayracer513 posted the photo of the cams and I saw what was wrong as soon as I looked at the photo. The exhaust cam was retarded about 1-1/2 teeth. That was allowing the decompression pin to hold the valve open enough that all it would do is backfire and the plug was coming out wet. What upset me the most was I had installed brand new factory cams straight from the box. The cam sprocket was not pressed on correctly and no I did not get my money back on the exhaust cam. The dealer would not give my money back or store credit, because I had installed the cam.

That dealer is out of business now.

Also if your bike suddenly locked up, there is a chance that the cam sprocket could slip.

Thanks PADGETT was a interesting point one I will never forget to cross reference with when installing cams again. thats shocking you could buy brand new and still be wrong (what The!) I mean if any could be wrong you would understand on mine as they are 5 years old but brand new thats just insane...

It has a lot of bearing in my problem and is definitely suspect now I know to check both sides I may stand half a chance with timing...

My question to Mr Yamaha is why are they even making a cam that could slip? surely it could be permanently set when manufactured?? I mean it was never designed to be adjustable so why make it so it potentially could just decide to adjust itself?

Oh well thats whole different debate no doubt - I just want my bike back so if needs be I will set cams off the marks to make them correct and I have almost finished re seating and course and smooth lapping in of the valves so hope fully my next posting reads cheers all for your help we fixed it... if not it may read YZ450f for sale needs TLC.. lol

i recently rebuilt my 2004 yz450f and it was also doing the same thing. i installed new wiseco piston and pro x stainless steel valves. all it would do was back fire also. put fresh gas in it, cleaned the carb, and put a 48 pilot and 172 main in it and it runs really good now. starts second or third kick. good luck just thought this might help

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