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mtpkts

Hard starting after valve adjustment

9 posts in this topic

I bought a used yz426 (02) and it seemed to run fine. I did a valve check and the intakes were fine but the exhaust were way out of spec going from a 1.8 to a 2.05 shims on both. Now clearances are great and it seems to run better, but starting it sucks. I have a 45 pilot jet screwed out 1 1/2 turns which seems good. I have to crack open the throttle to start it and it will sometimes backfire on restarts. Thoughts?

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if i am reading this right you took your 1.80mm shim and exchanged it for a 2.05mm shim.

Valves get tighter with wear. Going up in shim size is going the wrong way. Recheck your clearance and adjust accordingly.

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I double checked the clearance. The clearance was nearly double (loose) of what it should be when I originally checked it. How spooky when I could slide a .450MM feeler under the cam? The shims were original from factory b/c one was 1.80 and one was 1.79MM. It is correct between .200 and .250. I have an 08 KTM 530EXCr too and have become familiar with valve adjustments on the XC4 motor. The KTM valves loosen with use too. I don't know why. My XR400 tightened and so do the KTM RFS motors.

I had put a .200 on the left exhaust and it was a little loose with the .278 sliding with tight resistance. I tore it down and put in a 2.05 shim on the left exhaust and it was .205 with light resistance and not fit with a .225. The intakes valve clearance was good.

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KTM's loosen because they use finger rocker valve lifters, which wear much faster than bucket tappets do, and faster than the valve faces. Thus, clearance increases. But the reverse is true in YZF's under any normal circumstance. The lifters virtually don't wear in this configuration, so when the clearance changes, it's because the valve face wears, and the valve moves up into the head as a result, reducing clearance.

Look for wear or damage in the top half of the cam bores or mushrooming at the tops of the valve stems.

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I rode it at the track for a few hours and it ran fine but was hard to start. I tore it a part and checked the valves and timing. See pic. It seems good. There was no change in the clearances and the valves looked ok (as if I could tell).

I am going to switch to a 42 pilot from a 45 and see if that helps.

photo.jpg

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Fixed! it turns out that this exhaust cam is a hot cam and should be 14 pins off the intake cam and I set it to 13 per the manual which is for the stock cam. the cam was put in by a previous owner and they didn't re-shim the valves. The original factory shims were still installed which accounts for the .5 clearance or loosened valve clearance instead of the expected tightening.

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Fixed! it turns out that this exhaust cam is a hot cam and should be 14 pins off the intake cam and I set it to 13 per the manual which is for the stock cam.
Unless the cam was an OEM Yamaha 450 cam (which is actually what it looks like), or an aftermarket cam made for a YZ450, counting pins is the wrong way to time it.

But the Yamaha 450 cam is marked for a different cylinder head, so the timing must be set first on the intake cam, and then the exhaust cam set at 14 pins from the intake. This ends up with the exhaust cam appearing to be one tooth advanced. The pin count between sprockets on a Hot Cam, or other aftermarket cam, could be anything, but the ones made FOR the 426 are intended to timed to the 9:00 and 3:00 marks just as the stock cams are.

Read:

http://www.thumpertalk.com/forum/showthread.php?p=7467845#post7467845

http://www.thumpertalk.com/forum/showthread.php?p=3706078#post3706078

http://www.thumpertalk.com/forum/showthread.php?p=5378040#post5378040

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Thanks. I buttoned it back up this morning and it works GREAT! I can't wait to go to the track next week!

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