Back from the dead - 03 YZ450F rebuild

Hi all.

First post and I figured I'd throw my problems to the pros in search of a little advice.

I bought an 03 YZ450F in pieces. The bottom end had seized and the kid took it apart. He got to splitting the case and quit as he had just done a top end rebuild on it. The kids dad did a good job labeling things and so I took it for a good price.

I rebuilt the bottom with the Wiseco crank set and finally got it back together on Sunday. Got it fired up after a few hard tries.

Seems that it's popping and backfiring, the kickstart fires back at me, and it won't idle. I'm thinking a dirty carb since it was out of commission for a good 6 months. Any thoughts?

Also, really worrying me, the bike sounds like its knocking. I've seen other posts that state the bike sounds like that but it's bothering me. Just curious as to any other owners. I haven't touched a bike since I was 12 so I want to make sure it's good to go.

Any help would be awesome.

The YZ-F should fire up after 1-2 gentle kicks, this bike is very easy to start. Does it fire up ok or do you have to put much effort into it (several hard kicks)?
I would recheck the timming. It sounds like it could be set wrong by 1 tooth. Had similar problems with an KTM SX450. It fired up hard, the engine made noise and it didn`t quite work as it should - lack of power. But it idled propertly...

Clean the carb first and if it doesn`t help - check the timing

Edited by thefex

I was pretty confident that I timed it correctly except that the manual says look for the middle vertical mark for TDC and all I could find was two mark on the stator. I got the cams in right. I fought them for almost an hour to make sure.

They were hard kicks dude. Had the kickstart come back on me. Felt like it was gonna break my leg.

I would agree with the timing, it can be a little tricky getting it right. The marks will look like they are aligned until you put tension on the chain. Did you double check your marks after you installed the tensioner? 

Better reread the manual regarding the marks (or stop using it if it's not a Yamaha manual).  There are normally three marks on the flywheel.  Left to right from your view point, the first two are connected by a vaguely visible horizontal line, making it look like an 'H'.  These are for ignition timing.  The third mark is TDC.


If ever in doubt, pull the plug and locate top dead center with a screwdriver against the piston.


Pilot jets are notorious for plugging up at least partly with dried fuel varnish when they sit for long enough.   A film so thin you can't see it can change the effective jet size downward by 10 sizes or more, and it makes a huge difference.



It's the full Yamaha manual for the specified model. I made sure of it.

Can timing issues cause the knocking sound? I'm about two seconds away from parting this thing out if there's more wrong with the motor than just the timing.

YZ450's are notoriously noisy when they're in good health.  Don't panic until you have someone familiar with them take a listen.

I have a friend at work who has the same year. He claims his is quiet. After reading all of the noisy posts I'm thinking he's full of it.

Gray I was just thinking. I timed this bike a month or so ago. can it be timed with the cover off or is there a reason, other than the notches, that I would need to keep it on. I honestly don't remember what the marks look like and I'd like to take another look.

Yes, it can.  Just find TDC with some sort of probe down through the plug hole.  TDCis, of course, Top Dead Center, the highest point the piston reaches.  You can actually feel TDC with a fair amount of accuracy.  Play around with the end of the crank once you're close to it, turning it back and forth past it, and you'll see what I mean.


Another thing to consider when noise is the question is whether you have an aftermarket skid plate.  If the plate touches the wrong thing, the noise will be somewhat disconcerting.  Remove the plate, if you have one, and see what happens.  The factory glide plate won't do this unless it's loose at one point or very bent.

My 04 is noisy as crap but mechanically great. Freaked me out the first time too. So bad that I did a complete teardown only to find everything in spec. You'll learn to love those sounds

Thanks to everyone who posted on here. I finally got a chance to get working on the bike. I gave the motor a full turn and it looked like the cams were off a tooth. After more disassembling than I wanted I managed to get them lined up. Had to fight with the tensioner a bit. It's kicking good kick back. I had some problems getting the plug wire back on the plug. It was tight and had no snap. Kicking good but no fire. I lost light and quit for the night.. hopefully I can get back on it soon.

Hi--newby here.  I read not long ago about some guys trying to install a 5-speed into their 03 YZ450?  Thought I'd share some mechanical knowledge and wasn't sure if it was helpful or not.

Since you're a newbie, I'll politely explain to you that what you just attempted is called a thread hijack.  That is, you changed the subject of the thread to your bike or what you want to discuss.  Normally, that kind of thing just disappears because it's off topic.  Start a new topic of your own instead.


And review the Common Threads topic.  The info on the '03 - '05 5 speed swap is in there.

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