07 yz450f crank/cam oscillates after kickstart??tar

Please help. I have a 07 yz450f. It was making a noise I haven't heard before when trying to start it. Randomly (maybe 6 out of 10 times) I kickstart it, I could hear something moving/spinning/rotating as soon as I finished my kickstart stroke with my foot. I took the valve cover off and watched the cam and the noise is the cam is oscillating or bouncing back and forth fast for maybe a second or two as soon as I get done kickstarting it. The timing chain tensioner is fine, the timing chain seems to be fine, nothing appears to be loose or have slack in it either. I talked to a mechanic at a yamaha dealer and he told me depending on where the piston/crank and everything is at the end of the kickstart stroke that if its in a decompression stroke (no back pressure on the piston) the crank could oscillate back and forth briefly before finding its place and stopping. However I find this hard to believe. It seems like there would be or should be too much friction in the system especially with the rings in the cylinder that it wouldn't allow it to oscillate back and forth briefly. Has anyone heard of this or have any advice???? I could start the bike and it ran fine but seemed loud and seemed like it made a little different noise when I revved it up. I know they run loud just not sure how loud is normal.

Well, I called yamaha direct since no one had any input and they told me the same thing that this is a common occurrence depending on how you kickstart you bike. I guess there is a correct way and wrong way. The correct way is the ensure the piston is always at TDC before kicking it over. This is accomplished by pushing down on the kickstart until you feel a click or feel a decent amount of resistance. Once you reach this point, bring the kickstart back up and kick. When I followed this method the numbe of times I heard the oscillating noise decreased immensely. Now I have a knock in bike though and not sure what to do. I think it's a crank bearing or rod bearing because the sound is more than a tick, its basically a knock and when I load the bike up the noise goes away. So can I get away the rest of the year trail riding it a couple more times or is it time to spend some $$$? Who knows.

The correct way is the ensure the piston is always at TDC before kicking it over. This is accomplished by pushing down on the kickstart until you feel a click or feel a decent amount of resistance. Once you reach this point, bring the kickstart back up and kick.
OK, this officially makes me nuts. The crank position you have described IS NOT TDC, regardless of what anyone calls it. This misnomer has become pervasive, and the trouble with it is that when one really does need to locate TDC for any other reason, confusion ensues.

TDC is Top Dead Center, where the piston is at the top of the bore, and the crank is at the top of its stroke:

TDC.jpg

The position you find when you turn the crank until resistance is felt is the start of the compression stroke, not TDC. In an engine with manual decompression, that is around 120 degrees BEFORE TDC (crank rotating counterclockwise in the illustration):

130BTDC.jpg

In an engine with auto decompression, the resistance is felt around 80 degrees BTDC. This is also the position that a manually decompressed engine should be advanced to after finding "the hard spot" to set it up for the kick:

80BTDC.jpg

This allows you to kick the engine through a reduced compression stroke, past the ignition point to start the engine.

It's virtually impossible to find true TDC with the kick starter, but you can feel it go over the top. As you push against compression, and the crank slowly gives way, you eventually get to this point just before TDC:

BTDC.jpg

...where the piston nearly stops moving upward, and the crank will suddenly pop forward to this point:

ATDC.jpg

...just after TDC, where the resistance resumes, this time the resistance of trying to pull a vacuum in the closed combustion chamber until near bottom where the exhaust valves open.

If you told someone who knows what TDC is to find TDC and then kick the bike, this last spot is what he would look for, and he'd kick from that point, probably never to be able to start the engine. 'Seen it happen.

So call it up against compression, like what it is, not TDC, which it ain't. Not bagging on the OP, but on the general, widespread, incorrect popular abuse of the term, TDC. :smashpc:

the post '06 450's tend to be noisy at idle.

if it were a crank problem it would be normal at idling and knocking on a load

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