'08 YZ450F WON'T START

You have to take the carburetor apart and clean the pilot jet and accelerator pump passage ways. You should always drain the carb before you let it sit for any extended period of time.

I had the same prob on a YZ400F. It turns out the fuel line rotted and clogged the carb. I had to take it to John Burr Cycles to soak the carb. You might be able to find carb cleaner to do it at home. Black gooey stuff came out and it started on the 2nd. kick after that. I also one time had the rings stick. It was noticeable because of the manual decompression release. Yamaha made a fuel additive at the time to prevent ring sticking, probably the same stuff as Seafoam or Stabil. It's all because of pump gas. That shit is nasty if it sits more than a week.

Needs four things to run, spark, air, fuel and compression.

Start with the easiest check for a spark, then if you have one move on to air+fuel see if you can get it running with starter fluid then mess with the carb. If all else looks good run a compression test

I just have to check, did you try new gas. Gas goes bad even after 6 weeks so after 8 months it's crap.

Hopefully it is one of the solutions offered above. If the bike is much easier to kick over there is a chance you have an exhaust valve hanging open. I have two low hour 08~09 yz's have this happen while sitting for awhile with the engine not at TDC. Seems to be a build up on the exhaust valve from the reformulated gas. You can scrape it off with a finger nail and get it to work but the same problem reoccurred. Only replacing the valves stopped it from happening again. The gas theory was given to me by Yamaha tech support because whatever was on the valve came right off without much effort yet would keep the valve from closing. The old valves checked out perfectly straight. The guy with the bike I didn't replace the valves in said if he just bump starts it the compression comes back. It did for about three rides then the spring broke and valve hit piston. So if they are hanging up replace them.

Ethanol fuels are killing small engines like this. I work in the out door equipment business and man these fuels are tearing things up! Always drain the carb!

The gas seems to break down faster in the floatbowl than just sitting in a large container. It must seperate faster in smaller quantities. I've noticed with snowmobiles the older carbed models will be tempermental with season old gas in the tank and empty float bowls while the EFI pressurized fuel rails with old gas tend to be less finicky. My hypothesis is what allows the fuel to atomize is what evaporates first. Plus the nasty corrosive solvent that is left behind gives you the nice green color from the brass jets.

Smell the gas in the gas tank. Does it still smell like gas? If so drain the float bowl then Turn the gas on. Twist the throttle 3 or 4 times to prime with gas and kick. It will take several twists to pump good gas through the accelerator pump.

If it sputters ...... kick it over with the hot start to clear it and repeat with 1 twist....then kick it several times. Repeat till it starts.

The only thing that goes bad from just sitting is gas related components. (Assuming its in a weather controlled environment).


Remove the 17mm cap off the bottom of the carb, clean your pilot jet and main jet with a piece of guitar string.. put the cap back on and see if she fires.