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yama02yz250f

03 cam swap problem??

7 posts in this topic

I have now installed the 03 cam in my 250f and 426. Te 250f runs reat and is a breeze to start. The 426 is much harder to start. I know that it will obviously be harder to start ten the 250f but there seems to be places that "stick" while kicking it. Sometimes while kicking it he kick starter will get stopped at a place. I can then kick it from there. For te majority of the time it kicks fine, but just with a lott of resistance. I guess I am just worried that there may be something wrong with the cam or its timing, but when I installed it it looked exactly like te picture on in the 450 cam thread, there was the right amount of links between the top dots, the engine was at TDC, and the lobaaaaases were positioned right. I think that I just need to kick it harder, but I wanted to make sure that something isn't wrong. What do you guys think, am I just worrying about nothing and just need to kick it harder and forget about it? or could something be wrong?

thanks and sry for the long post

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The timing is important, not only to the engine performance, but to the operation of the compression release. The compression release is designed to release pressure on the first half (or so) of the compression stroke, but allow it on the last half. If the cam is too far advanced, it will not release enough compression to be effective.

Go over the timing again for good measure. One point to be certain of is that the cam you are using is in fact a cam originally made for a 450. If you are using an aftermarket AD cam built for a 426, you cannot follow the timing instructions in the 450 DCM thread. Those cams are marked for use in a 426 head, and should be installed exactly as if they were a stock cam, same as with your 250F.

A 450 cam should be timed as follows:

> With the engine at TDC, and an exhaust cam in place, pull back on both cams so that the front side and top runs of timing chain are taut.

> Move the engine back to TDC if necessary, and verify that the intake cam is correctly timed according to the factory timing procedure.

> Using the two "12:00 o'clock" marks on the cams, position the 450 cam so that there are 14 pins of chain between the two marks. This is not the same amount that the stock cam would show, but it's right when using the 450 cam in the 426 head.

> Install the tensioner and recheck the timing.

Again, the AD cam does not release all of the compression, it just reduces it enough to allow a small electric motor to spin the engine while leaving enough that it can start and run. The bike is still a 426, after all, and if you find you've done everything right, then it's probably just the way it's going to be. When we had our 250F, it had much lower resistance to cranking than my '03 450.

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would it run properly if it wasnt timid correctly? we went riding today and it ran great. It took a few kicks when it was could but for the rest of the day it started great. I will pull it of and check it but I think its just that fact that it is a 426 and that it is not a full release of compression like with the lever. If the valve measurements were slightly to big, could this be a factor?

I think I may be trading my 250f with my dad for his 426 as I am 6'3 and he is 5'10. He is feeling like it is just to big for him

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I doubt the clearance wold be a noticeable factor unless they were hugely out of adjustment, but if they are bigger than spec, you need to adjust them.

And the bike would run OK with the exhaust cam advanced one tooth. Not great, but OK. Before auto decompression, that was how you switched a YZ to WR cam timing, but with AD, doing that will make the AD system less effective, as I said.

So, it doesn't hurt to double check, in any case.

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for sure. i will double check tommorrow. i have my doubts since my dad didnt say anything about it running any differently. if it was off one tooth, will there be any damage done?

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i kind of have the same "problem" on my 01 wr250. i had an 03 cam installed many years ago and could just kick away and it would start. it did work better if i took the time to find tdc then kick fully, but i would turn over real easy. now i put in some hotcams and a new chain. i seem to have the same "hard" spot now and then. if i just past that spot, it will start 1st kick cold, but i still need to be on the stand for a full kick, none of that foot down stuff. when hot, omg its a chore, end up bumping it most times. double checked the clearances and timing so many times. always in spec, as it has been since day one, never had to shim yet (very low hours). runs like a champ, but cant figure out the starting issue yet?

any ideas?

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