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Too hard to start bike due to high compression

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Hey guys,

So I recently just went through the bike and replaced the exhaust cam on my 2002 yz250f with an autodecompression cam out of a 2004 model.  I then shimmed the valves, cleaned the carb, and replaced the spark plug.  Now it won't start regardless of whether or not I try to kick it or bump it.  It seems like as I keep kicking it it goes through periods of extremely high compression where it is very hard to kick and then periods of low compression where it is a little easier to kick.  Either way I can't get the bike to start.  Any ideas?

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Sounds like to me that the cam is out of time. Bring everything back to top dead center on the compression stroke and make sure your timing marks are aligned properly. However, I'm not familiar with the YZ250f so hopefully someone with more hands on experience will have some helpful suggestions.

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10 hours ago, SS109 said:

Sounds like to me that the cam is out of time. Bring everything back to top dead center on the compression stroke and make sure your timing marks are aligned properly. However, I'm not familiar with the YZ250f so hopefully someone with more hands on experience will have some helpful suggestions.

Yeah I had a feeling that might be it.  I guess I'll need to open it back up.

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While the valve cover is off, check that the spring on the flyweight of the Auto Decomp mechanism on the exhaust cam is still engaged. Move the weight out; it should snap back toward the center, and it should move easily.  I had a spring break once.  I just bent the end back into a hook and hooked back to the flyweight.

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I'm having the same issue with a 2002 WR250F. I'll kick it three times and will be nice and soft and then it hits top dead center and it's kind a like the decompression cam not working is what it seems like but I know it is because I checked it when I had it out. When I bought the bike from the guy, it had been sitting for a while, he said he was having a hard time starting it. The carburetor was in 1000 pieces So I rebuilt the carburetor. Just a note, I also tested the entire electrical system as well even swapping out parts form my 02 YZ250 just to make sure.

 

From what I was told and what It look like it just got a fresh valve job and I know for sure he put a brand-new 290 big bore kit in it. The cams also look brand-new, the black coating on the cams lobes is barely worn. I think it was after all that work that was done is when he couldn't start it and dug into the carburetor thinking that was the problem. That being said I talked to a mechanic because I can't get this thing started and he was telling me that hot cams may have a different valve shimming spec then the OEM. He's saying if you're on the high side of the clearance and the clearances are tight then that might be the issue. I sent an email to hot cams to see if they had something they could send me but I haven't heard back yet. It looks like I have a stage two intake cam. In any case I ordered a YZ/WR 250 hot cam shim kit. I'm just waiting for it to come in now but I think I'm going to try to go towards the low-end of the clearance and see what happens.

 

If anybody has any info on hot cam valve clearances please share them! Hot cams doesn't give you a phone number so you can't call them which sucks!

 

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12 teeth between cam top dead centres from memory. id check you havent moved your crank and timing is ok. 

as your cam lobes are the same height (old cam and new one) i would have thought they would be shimmed to the same specs? usually they just change the time the came opens or shuts the valve, not how far the cam pushes down on the valve. they do this by altering the shape of the lobe on the cam.

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while all cams have the same designed lift and duration, they are all not manufactured the same so the valve clearance msut be checked each time a cam is replaced.  Futher, the tiiming marks for the 2004 Yamaha cam will not work with a 2002 head; as I understand it the head design and cover are diffferent requiring the marks to be in a different place on the gear.  Counting the teeth would not work for the same reason.  You would basically have to eye it up.

The link below is for swapping the cam in a 400/426; I have used these exact instructions successfully 3 times; so if the 250 heads are different  this should be very similar for the 250. .  https://www.southbayriders.com/forums/threads/106199/

Edited by tcmII

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