450 not running smooth. Jetting??????

New to this site. This is actually my first post and can't be happier to of found this site.

I am in the hunt for a new bike and my neibour has a 2003 YZ450 for sale for $2000. He had the motor gone through, with hight comp piston, new cams, new exhaust. Not sure if he jetted it or not, but when I rode it last night, it isn't smooth. There is allot of ride time on this set up, so it's not like this is all brand new.

1) Idel is real hi, so I screwed the idel screw in all the way, but still hi.

2) Under power, the bike kinda pulsates, and isn't smooth.

3) Back-fires when you let off the throttle and one when kicking to start.

Are these things all an indication of bad jetting, or could there be something wrong with the motor re-build?

Thanks for any help.

first step. pull the carb and give it a good cleaning. see what jets are in there and post the results.

also take a look at the jetting thread (the sticky at the top of the forum) and see what other people are running on their 03's at your climate and elevation.

does it have fuel screw adjuster? make sure your fuel screw is at 2 1/2 turns out as well.

welcome to TT. Grayracer will probably be online soon and he will provide with links and a very detailed explination of what to do.


Well, the bike is not mine, just for sale and It's what I have been looking for, and at a great price. I just need to know if this bike can be fixed without replacing motor parts. Cleaning carb, rejetting, stuff like that I am capable of doing.

It sounds as if it's excessively lean on the pilot circuit. The only item of concern here as far as engine parts are concerned is the valve clearance. If the intakes are excessively tight, they can cause some of these symptoms, and any valve in one of these engines that needs more than a single step down in shim size to correct is something potentially expensive to worry about.

If the seller is willing to let you fiddle with it, you might try backing the fuel screw out 1/2 to 3/4 turn just for fun to see what that does, then go from there. If in doubt, pass. You can find a good bike that doesn't have these problems to buy instead.

OK, Thanks. He will let me play around on it. Is the fuel screw under the carb pointing strait up, or the black one on the side of the carb under the choke?

Also, if I pull the plug, will this indicate the lean condition??

If he will let you, pop the valve cover and check the valves. It doesn't take that long to do and is a good idea anyway when buying a used four-stroke.

Is the fuel screw under the carb pointing strait up, (?)

2) Under power, the bike kinda pulsates, and isn't smooth.

That sounds like the symptoms of the bent shift forks that some 426s suffered from. Was that a common problem with the early 450s as well?

Eventually, it's a problem with all of them, or can be, but the bent forks were caused by, not the cause of, the problem. It starts with wear at the locking lugs, or dogs, on the sides of the gears that lock them together to engage the different ratios. These are normally cut to cause the gears to hold to each other under a load, but as they wear, the opposite begins to happen, and the gears start to force away from each other under power instead. Since the forks cannot move out of the way, it gets bent, and the problem is exaggerated.

If this is the problem, the symptoms would be severe jerking under a load, and only in one or two gears in particular. Easing up on the throttle will reduce the severity, and changing gears will make it stop.

I don't think this is the case then, since the pulseing is in every gear and under all different amounts of power. I did turn the fuel screw out one turn and rode it last night. Nothing changed in teh way it ran. I have it loaded up to take to the coast for the weekend, and will go down and get the size jets alot of you are running and try them.

Other things that might cause it: Blocked jets, low float level, old gas. It could even result from an excessively rich condition such as flooding, or from an electrical issue, such as loose connections or a faulty TPS.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Similar Content

    • By Zack Ailing
      hey guys just picked up a new project bike. Its a 2008 Yz450f with a big bore kit and a stroker crank. The total cc of the engine is now 499. It runs on race gas and will be an absolute monster when shes ready to rip. I will continue to make more videos on the bike so stay tuned Link to the video: 
    • By High On Octane
      2005 YZ450F, Athena Big Bore, Power Bomb, Q4.
      Ok.  Last fall after a decently hot day at the track, I noticed coolant leaking from the weep hole on my water pump after pulling into the pits.  Ordered an Athena re-seal kit, put it all back together and didn't really get a chance to ride it.  3 weeks ago, went out to the track, and after about 20 laps, it started leaking again.  Kept filling it with water to get thru the day, never ran anywhere close to dry or empty, jut a cup or 2 low.  Ordered a Hot Rods re-seal kit with bearing and also ordered a new w/p shaft.  Did it all again and got it back together, ran it, and as soon as it got to temp, it was leaking out of the weep hole again.
      So, what the heck am I missing?  I'm totally stumped on this one.  Is something wrong with my case cover?  Do I need to replace the w/p itself?  I'm feeling like I should've just ponied up for the Boysen in the first place, and now it's biting me in the butt.  Any help or suggestions is greatly appreciated.  Thanks!
    • By Awheeler25u
      Alright guys, my 04 YZ450f runs fine minus some slight backfires when decelerating (need to adjust fuel screw). The other day when I was done checking my valve clearances I changed the oil and filter and put the bike back on the stand, kicked it a few times without the ignition coil in to get some oil flowing got off and noticed a little drip spot on the ground, it was no bigger than a dime so I thought nothing of it. Started the bike with no issues took it for a quick 10-15 min ride, came back to the garage to find out my whole case was soaked with oil and it started pooling in the low spots on the clutch basket side. I cleaned all the oil off and took it out again for 5 mins to try and see where it was coming from, here are my three possible places but nothing directly leads me to believe it’s any of the three either.
      1. Decompression plug area
      2.  Valve cover gasket leak (not really sure because the amount of oil leaking Looks to be a large amount
      3. Oil tank (bad weld or cracked weld somewhere
      what are you guys thinking?
    • By bofar
      Hey guys. I could really use your help diagnosing an issue that came up riding today. Bike is a 2013 YZ450F. Well maintained and haven't had any issues until now. Both the oil and coolant are new and levels are to spec. 
      After riding for about an hour in the sand dunes, I noticed a grinding going on on the right side of the bottom end (from what I could tell). When I looked to see what was going on my kickstarter was vibrating and grinding really loud. This was a different sound than if you engaged the kickstarter while running. It sounded less "clanky" and more "vibrating/grinding" if that makes sense. Grabbing the kickstarter and moving it didn't help the issue.
      When I noticed it, I shut the bike off and gave it a second. Being a few miles deep into the dunes, I had to get it out of there so I started it back up and it seemed fine- no grinding. I headed back to the car and after a few minutes, it came back so I shut it off again, started it back up and nothing. That process happened one or two more times before I made it back to the truck. Seemed to run fine otherwise. 
      Once I got it back to the truck I started it up and tried to pay more attention to what was going on but it wasn't doing it. I did seem to have more of a metal on metal noise coming from the motor. I started and stopped the bike a few times and after the motor stopped there was a distinct metal clank/grinding noise that happened right after the motor shut of for maybe half a second. This doesn't happen every time. 
      I tried doing some research and have seen things that make me think it could be the spring that connects to the kickstarter, or the kickstarter stopper plate- but really I have no idea.
      Seems strange that the issue is off and on.
      Thanks for the help!
    • By Goof331
      Does anyone know if there is any differences between the 2 radiators? Will the yz450fx radiator bolt on a yz450f? What I am wanting to do is wire up a fan to my 2016 yz450f. I know how I am going to do it I just don’t want to use a “universal” fan kit. What I would like to do is get a 450fx radiator and the direct bolt on Trail Tech fan for that radiator to have a good clean look.