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Old 03-29-2019, 08:43 AM
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Default '05 YFZ450

     

So, I bought the above-mentioned atv with the intent of flipping it (don't make fun of me, it was cheap and dirtbikes have been nearly non-existent in the market for the past month or so... So, I got desperate...). The seller ran a mechanic shop and was gracious enough to allow me to look into the engine before buying it. He had rebuilt it but was unable to get it start after the rebuild. He did a compression test and it wasn't making any compression and decided to sell and cut his losses. Anyway, I checked the timing (because he suspected that) and it was fine. I checked the valve clearances and the issue became clear: The clearances weren't right, but the primary issue was that the center intake valve was completely zeroed out.
Fast forward a few days: I finally got a chance to set the valve clearances. I was able to get both exhaust valves in spec with shims (one was slightly loose and one was slightly tight). I was also able to get both left and right intake valves in spec. However, the center one was still zeroed out even with the thinnest shim available. So, I began to wonder if there was a connection between the huge right valve clearance and the tight center valve clearance. I pulled up the parts fiche and BINGO! (...or at least I hope so) The center valve on these motors has a different part number and apparently is shorter than the other two valves.
I post this to share the info in case it serves anyone in the future. I will update what I find once I pull the head. I'd also appreciate any feedback from those of you familiar with this motor.
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Old 03-29-2019, 03:26 PM
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You will find that that center valve has streatched beyond spec. You will also need to cut the valve seat when you install the new valve.

Paw Paw
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Old 03-29-2019, 04:06 PM
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I found the center and left valve were different lengths and the longer valve had been installed on the center. The valves and piston were new (as the owner had said, which brings some peace of mind) and now I'll hopefully just need to wait for a new head gasket to reassemble and get it ready for sale.
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Old 03-30-2019, 01:14 AM
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Well, I got the valves switched over and the clearances set, BUT on reassembly I noticed the exhaust cam is binding. It will not spin by hand once the cam cap is put on. Any ideas on how to free it up?
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Old 03-30-2019, 03:45 PM
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I cleaned up the edges on the caps and it improved, but was still a little gritty. I looked around and found a Ken O'connor video where he described this exact same issue on a yz450f. He recommended lapping the caps. I did that and it was right back to normal.
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Old 04-10-2019, 11:43 AM
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Interesting: I rebuilt the carb and rejetted it (45 pilot, which is what I usually go to for our local conditions and a 165 main). I installed it without the airbox and it fired right up and idled nicely. If I stab the throttle, it responds nicely and I can see a good squirt from the accelerator pump. However, if I just touch the throttle the SLIGHTEST LITTLE BIT, it dies. It fires right back up though. I'm thinking it's still a bit lean since the airbox was off. I'll clean/oil the filter and see if the condition improves.
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Old 04-10-2019, 12:08 PM
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Accelerator pump timing is off just a bit. It is adjustable.

Paw Paw
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Old 04-10-2019, 01:00 PM
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Quote:
Originally Posted by PawPaw View Post
Accelerator pump timing is off just a bit. It is adjustable.

Paw Paw
Thanks, PawPaw! I thought of that last night before going to bed (more along the lines of doing the "ziptie" or "o-ring" mod on the pump). I thought I may be wrong though since it does it only when I JUST BARELY touch the throttle though. As in, barely just get the slide to move a smidge. I'll definitely give it a go though since its easy to go back if it doesn't work out.
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