Looking for More Suggest > 2003 YZ450F Won't Start

Might wanna check out the yamaha 450 section. You'll get ur answer over there.

Sounds like you've been wandering in the dark. The impeller seals cannot have been the problem since the water pump is designed to leak externally rather than internally when the seals fail so that the two fluids can't mix.

Regarding the missing lash pad, you'd better be absolutely certain it was missing and that you did not instead drop it into the engine. You didn't mention what the clearances are, but I'll assume you checked them.

One thing you should check to begin with is the flywheel indexing. Use a probe such as a Philips screwdriver in the spark plug hole and check that the TDC mark is actually at TDC. If they don't agree, you need to replace the flywheel Woodruff key.

Start with a NEW spark plug. When there's been this much trouble, it's possible that the plug is fouled enough that it shorts under compression pressures, but not in the open air.

The carburetor is the usual source of most starting problems, and the usual area of the carb that is at fault is the pilot, or idle circuit. Very often when a carb sits for an extended period, the drying fuel will leave a coating of "varnish" on some parts, particularly in those with little tiny holes in them. The metering orifice in a #42 pilot is 0.42mm in diameter (.0165"). If that orifice gets coated with only a .002" film of dry varnish on one side, it turns it into a #35, which is way too small, and varnish deposits are often bulkier than that.

See this post: http://www.thumpertalk.com/forum/showthread.php?p=8021726#post8021726

Also, check that pilot mixture screw again. You failed to mention whether there is an O-ring present, and if there's not, that's an air leak into the idle circuit. The ring may just be stuck in the carb body, or it might be gone. PN is 3TJ-14562-40-00.

Also check the starter jet, be sure the choke works freely, and that the hot start plunger is sealed and closes completely when the lever is released. Check the float level, too.

To cold start an '03, turn the gas on, pull the choke, and give the throttle 2-3 quick twists to prime the engine. Push the starter until you feel the resistance of the compression stroke. Bring the starter back to the top and deliver a smooth full stroke with the throttle completely closed. If it hasn't started by the second or third such kick, try opening the throttle VERY SLIGHTLY off idle, or take a turn in on the idle speed screw.

Download a manual if you need one: http://www.yamaha-motor.eu/eu/services/owner-manuals/index.aspx

Browse this thread for info: http://www.thumpertalk.com/topic/378677-common-threads-look-here-1st-mods-maintenance-common-issues-and-faqs/

Let us know what happens.

Thx for the options, greyracer. The spark plug is brand new and there is good spark. I did the timing 3 times. It's right on. I did test it again the top position of the piston and it's on. The fuel mix screw is brand new as well and it has the o-ring, washers, and spring. Starter jet works great too. I've pulled the carberator and cleared (and double checked) all the pathways. The float works good and the pin slides freely.

The choke does tend to pop back in when I'm kicking it over, unless I turn it slightly to a certain point then it will stay in place.

I haven't tried the exact method you described for starting though. I'll give that a shot. Rolling/compression starts haven't worked either.

Thx for the suggestions.

I would toss your pilot jet and put in a brand new one...blowing it out with air is not going to get rid of any varnish. How long did the bike sit?

I would also check all the valve clearances, especially if you just used a 170 pad for no reason other than that's what the exhaust valves had.

How does the compression feel?

If you're not familiar with the FCR carb, double check that you don't have the vacuum release plate in upside down. It's a common error, because it looks like it should go that way. The "square" end with the hole near the edge goes down.

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