03 YZ 450f fourth gear slip or lean surge or jetting problems PLEASE HELP

[Mxthug]

Hello,

I'm having a problem with my 2003 yz450f. It has stummped most shops. I'll start with the history.

Last ride was at Red Mountain a year back. My bike started slipping in fourth gear while under a load. I thought it might be my clutch, so I replaced it with a oem one. While replacing my clutch I noticed my case had a crack in it. So I ordered new cases. While my engine builder had my engine a part I had L.A. sleeve bore out my head to 97mm And I bought A new JT High Compressin piston. I should also mention I'm running a Dr.d pipe and VP fuel.

So to the present..got my bike all back to gether and fired it up 1st 2cd and 3rd gear work great after I uped my main jet to a 172 and my piliot to a 45. But fourth no matter what I do with the jetting fourth kinda slips..I guess would be the best way to describe it. or maybe like it's running out of fuel. From quater throttle to half it hesintates but if you crack the throttle full open it does it a little less. I was told that the gear would either be good or bad. I thought maybe I missing a tooth but it's not constint enought to feel like that. I really thought I was lean surging due to it wanting more fuel but no matter how high i go up on jets it does not effect the problem. Also I would think if my fourth gear was messed up my engine builder would have seen that when he split the cases.

Please offer me any help or advise..I m pretty much at the point of tearing my engine down again but would like to avoide that cost.

2003 Yz450f

97mm bore

97 mm high compression piston

vp fuel

172 main jet

45 piolet jet

Dr. D pipe.

New clutch

[grayracer513]

If it is gear wear causing the problem, the symptoms will be:

> The bike will run normally in all other gears.

> In the suspected gear, the bike will again run normally UNTIL a certain amount of throttle is applied. At that point, the gear will slip out of engagement and then immediately back in, producing a jerking sensation. More throttle will make the disengage/re-engage cycle occur more rapidly and violently.

If you say that the bike goes through a phase related to throttle opening that clears up when you open the throttle farther, I would guess the transmission is not at fault, and it shows up in 4th because of the increased load.

[Mxthug]

That sounds pretty close to what it does. It's a jerking feeling when in 4th gear. Well I guess the engine comes back out.

[tom nolan]

It sounds like fuel starvation. I would check and make sure that the petcox screen is not pluged up. Maybe the fuel can't keep up with the demand under heavy loads. If it was in the trany, you would see metal in the oil filter.

[jeffboyd47m]

I would bet that 4th gear is out. There wouldn`t be any gear teeth missing but the dogs on the sides of the gears would be slightly rounded. Also pay close attention to the shift forks, if they are marked or scratched at all they should be replaced. If it is 4th gear you will have to replace 4 different gears to repair it.

[jimmyg]

shift fork is probably bent it happend to my 03 the same way i would replace the shift forks & 3rd & 4th gear at the same time.you might save yourself from splitting the cases again.

[grayracer513]

If it is 4th gear you will have to replace 4 different gears to repair it.

This is not correct. Unless there is damage to the teeth of the gears, only the 4th pinion and the 3rd pinion (on the main shaft) need to be replaced, along with the #2 shift fork. These are the two elements that have the locking lugs that hold 4th gear engaged. The fork will have been damaged by the force of the gears jumping apart from one another.

[Ga426owner]

If it was me I would replace all worn items in there and be done with it...

[jeffboyd47m]

This is not correct. Unless there is damage to the teeth of the gears, only the 4th pinion and the 3rd pinion (on the main shaft) need to be replaced, along with the #2 shift fork. These are the two elements that have the locking lugs that hold 4th gear engaged. The fork will have been damaged by the force of the gears jumping apart from one another.

My experience is limited to 2 bikes. The first one was my 06 450 that lost 5th gear with the same symptoms as mxthug describes. When I tore it down and inspected the tranny I found that the side lugs on 3rd pinion and the matching slots on 5th pinion were rounded on the edges, along with marks on the shift fork that engaged 3rd pinion on the main axle. Also I noticed there were marks on the shift fork that engaged to the 5th wheel gear on the drive axle. That lead me to check out both the 5th wheel gear and the 1st wheel gear on the drive axle. I found that the side lugs and the corosponding slots for those gears were rounded also. That called for the replacement of 4 gears total, plus 2 shift forks.

The second bike was my friends 04 YZ450. He lost 4th gear just recently with again the same exact symptoms. I tore it down for him and basicly had the same scenario, the only difference being a 4 speed as opposed to the 5 speed. We needed to replace 4 gears and 2 shift forks. Neither bike had damage to the gear teeth, just the side lugs and slots on the sides of the gears. I guess what will be correct is whatever he finds that he needs upon tear down and inspection.

[grayracer513]

Naturally, any transmission that has accrued enough wear to cause it to bounce out of one gear will have other wear unless the rider has a very odd pattern of gear box usage. The entire assembly needs to be inspected critically and objectively, and everything that looks like it will be a problem sooner than you want to tear it down again should be replaced.

However, my point was and is that the specific symptom of hopping out of one gear dictates the replacement of the freewheeling element of that gear set, the gear that locks it to the shaft it runs on, and the fork that shifts it, and only that. He may very well find that second and third gears also show significant wear, just as you say, but the reported symptoms themselves don't call for their replacement.

[jeffboyd47m]

He may very well find that second and third gears also show significant wear, just as you say, but the reported symptoms themselves don't call for their replacement.

Fair enough, that makes sense to me.