blown 4th in an 04 450?

so i started noticing this on my last 2 trips to the dunes. when im goin along in 4th and get on it, it doesnt accelarate smooth. it acts like the gears are grinding every like 2 seconds. i installed a new tension spring. its the spring that holds this little roller in its spot. im pretty good friends with the local dealership owner here and he said it would be lucky if this was the problem and that spring was just wore out, and it was popping in and out of gear real quick, but that wasnt the problem. if my 4th gear is out, im not sure what im gonna do.

Well, I'm not either, but that's what's going on, more than likely. The lugs on the 3rd pinion that lock it to the 4th pinion to engage 4th are worn so that they push the two gears apart rather than holding them engaged. The #2 shift fork will also be damaged.

so i started noticing this on my last 2 trips to the dunes. when im goin along in 4th and get on it, it doesnt accelarate smooth. it acts like the gears are grinding every like 2 seconds. i installed a new tension spring. its the spring that holds this little roller in its spot. im pretty good friends with the local dealership owner here and he said it would be lucky if this was the problem and that spring was just wore out, and it was popping in and out of gear real quick, but that wasnt the problem. if my 4th gear is out, im not sure what im gonna do.

You have to tear it down and inspect it. Like Gray said, the lugs on the gears are warn. The associated shifting fork is also bent (damaged by the gear moving over). While you are in there you will have to replace several bearings, gasket and seals. If you plan to keep the bike a while longer, take your time and don't overlook some other things that might be due for replacement on a bike of that age, like the crankshaft and other gears.

ill pry end up keeping it for quite awhile. love the bike. i dont weigh a whole lot, and my buddy( the dealership guys son) has the 2010 and has the gytr tuner and i can about keep up with him on my bike so it suits me just fine. ill be porting and polishing the head and putting a high compression piston in in while i have it apart.

I know it is "normal" for the older YZs to round off the lugs on the gears. But is it normal to just straight up break them?

That can happen, especially if you let it skip in and out too many times under a load.

ill be porting and polishing the head

The three valve intake port is exceptionally tricky to work with, and extremely fussy about having the shape and contour right. Far more people who embarked on a DIY port job have lost significant horsepower than have gained any at all. Unless you know what you are doing based on experience with the 5 valve heads, or are being guided by someone who fits that description, don't do anything more than lightly clean up the roughest areas of the ports.

anyone had those type of motors torn apart like that? sounds like it might be a bit extreme for me? and is it possible to put a 5 speed in my tranny? cause that would be awesome.

That can happen, especially if you let it skip in and out too many times under a load.

thats the thing, it works perfectly then breaks. I had the same problem with my 04 yz125 that i bought brand new also

anyone had those type of motors torn apart like that? sounds like it might be a bit extreme for me? and is it possible to put a 5 speed in my tranny? cause that would be awesome.
If you've ever done the bottom end and trans of a two-srtoke, the only real difference is the top end. Download a manual, and pay attention to details.

The only 5 speed that can be bolted into the 4 speed YZ450 is the wide ratio WR450 ('03-'06), but the ENTIRE trans has to be replaced, including the shift cam and forks. Read:

http://www.thumpertalk.com/forum/showthread.php?p=2761046#post2761046

ive never done a bottom end on any bike ive owned. the bike i have now runs awesome, but its definitally time for a rebuild on the top end. and checking the crank bearings. and it just happenes 4th is gone. i had the side cover off the clutch side doing that torsion spring, and that was about enough for me. there was gears and springs all over the place. i felt like i was gonna screw something up for sure.

Find a local riding buddy who's done it before to help you. Otherwise, you're going to need a mechanic.

im in a real small town here in montana. its not quite like california. there aint much riders here. the ones that are here buy a new one every year, or are redneck with the early 90s haha.

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