Surging jumping ?

I have an 08 yz450 that seems to surge or jump real bad in 1st gear. Then it works just fine in the rest of the gears? Feels like the gas is going on and off. Any suggestions on what that could be.

Think about this, test it if you have to. 

 

Are all of these conditions true?:

 

  • It happens ONLY in low gear
  • It does not happen unless or until you go past a certain amount of load on the gear.  That is, if you open the throttle only so far, it won't do it, but if you open farther, it does
  • The jerking is harsh and violent, like rapidly fanning the clutch lever.

Think about this, test it if you have to.

Are all of these conditions true?:

  • It happens ONLY in low gear
  • It does not happen unless or until you go past a certain amount of load on the gear. That is, if you open the throttle only so far, it won't do it, but if you open farther, it does
  • The jerking is harsh and violent, like rapidly fanning the clutch lever.
ya pretty much. I cant really test it right now cause i got it taken apart right now. But that sounds like what it was doing. I just bought it and i bought it to take it apart and rebuild it. Only paid 1300 for it. Bone stock except for they did some thing weird to the stock muffler. Looks like they cut it down and took the guts out and re ribbitted it. I also noticed that the idle positioning sensor was plugged in. Could that have something to do with it?

Oddly enough, the throttle position sensor is supposed to be plugged in.

What your lurching sounds like is a worn or damaged first and fifth gear on the output shaft, and. Bent shift fork. This would be a very unusual failure on a YZ. It's possible that it was reassembled wrong, to, if it's ever been apart. Look for broken or rounded over locking lugs and slots on those two gears.

ya pretty much. I cant really test it right now cause i got it taken apart right now. But that sounds like what it was doing. I just bought it and i bought it to take it apart and rebuild it. Only paid 1300 for it. Bone stock except for they did some thing weird to the stock muffler. Looks like they cut it down and took the guts out and re ribbitted it. I also noticed that the idle positioning sensor was plugged in. Could that have something to do with it?

 

The 07-09 mufflers were super short as it was like 8" or so, I cannot imagine anyone thinking any shorter was a good idea. I suppose your talking about throttle position sensor when you say "Idle positioning sensor" or possibly the neutral switch on the side of the crankcase below the counter shaft sproket. In either case they should be plugged in but does not really affect anything if one or both are not plugged in. So in short I don't think anything you just said could have anything to do with your problem. 

 

I had a possibly similar experience with my current bike when I test rode it before I bought it, an 09 that came with a FMF jet kit installed. It kinda picked up real slow off the bottom and had a hit of power in the mid then was smooth from that point up to red line. It kinda felt like I was on a 2 stroke. I removed the jet kit and set it back to stock and the problem has been gone since.

Edited by f150jokerstyle

The 07-09 mufflers were super short as it was like 8" or so, I cannot imagine anyone thinking any shorter was a good idea.

What he likely did was to remove the cone baffle from the end of it. Remember seeing that mod?

What he likely did was to remove the cone baffle from the end of it. Remember seeing that mod?

 

I have not heard of it, I will search it for some reading material though. 

The "shorty" was really cute, and everyone said how cool it looked.  Everyone also said that the '08-'09 had weak low/mid range power.  Both things were true, and both were caused at least partly by the muffler. It was so bad that virtually any decent slip-on could add 4 to 8 horsepower at 5000 RPM.

 

Here's a picture.  The yellow cone is the big problem.  The smaller one in front of it not so much, but it doesn't help.  Usually just the rear cone gets removed, mostly because the front one's harder to get out.  And yes, it's a little loud. :thumbsdn:

 

89103034.y06uA26j.YZ450FMuffler1.jpg

Well heres a picture of what it looks like when a dummy tries to mod a muffler. I dont know if the ribbets came out at one time or if their blowin through these. But it melted holes in the plastics.

Sorry heres the pics

1395990416228.jpg

1395990443390.jpg

1395990463116.jpg

Well thank u jokerstyle and grayracer for the feedback. Im gunna break this sucker down. Ive never done a bottom end before so i would appreciate it if u guys would walk me through it. Let me know what to look for. I have the haynes repair manual and it has alot of pictures of how to do things but it really doesnt have good diagrams on how things go back together incase i forget or get lost. I think i should have got the yamaha repair manual

You will also need a flywheel puller.

 

Not sure of your mechanical ability but the way I keep track of parts is once the complete motor is pulled, everything from the top end gets piled into the valve cover as far as nuts bolts washers ect. Everything from the right side of the case into the clutch cover and everything from the left sde of the crank case into the flywheel cover. Once you have the case striped down I get a box for the internals if it's going to be apart for a while.

 

If it's your first time threw you could go with the ziplock bag and labeling method of where parts came from. It's just tedious to me and time consuming. Also take note of the Yamaha manual as you tear it down to see what the book is trying to tell you. If you cannot understand the text while it is still in once piece it will be more self explanatory.

 

The hardest thing for me is dropping the dam trans assembly back in. It usually takes me about 5 or so try's to remember how the heck I did it before then it all clicks in and it is like o yeah that was easy dummy.

 

Good luck.

 

Also I strongly recommend all OEM parts for internals minus clutch hard parts and maybe a few others i'm not thinking of. If you shop around online you can find most OEM parts for 30% less than your local dealer will sell to you. The last one I did for a buddy I think we saved close to a grand by shopping around, again not fun but worth it.

 

I personally don't like messing around with the heads I send them out. I really have a hard time doing this though because I trust very few shops and most send the head out to a third party as well. Again some due diligence goes a long way. For me living in southern C.A. there is a lot of shops that sell machine work and only a few that really do any.

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