What would cause a broken valve keeper?

So I've been building a dr-z 400 engine with used parts i bought online and i finally got  a chance to start it last week and it actually started, then died. Got it to start 4 more times and it died again soon after while idling, and now it won't start at all. Now the starter keeps turning but it won't fire and you don't get the "starting" sound/feeling from the exhaust as you would normally(you only hear the starter and flywheel turning). So i checked valve clearances and the exhaust valve measurements are completely off. The left-side exhaust valve could fit .100 mm feeler gauge easily. So i ended up finding a broken valve keeper under the right sided exhaust shim. This causes the tappet/bucket to sit higher than usual and not flush like the other tappet/buckets.

What would cause a broken valve keeper? Also, does this mean i have to do a complete valve job?

Thanks in advance!

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Edited by Uro pieces

Not installed correctly 

Not correct cotters for tops used / valves used... Valve cotters are not a "universal" fit
Worn or damaged cotters , valve, top

Valve strikes piston and unseats cotter

Too thin a shim used, bucket depresses valve spring top unseating cotter 

BTW, I dont see a "broken" cotter in that pic, all I see is one not locked in fully

Thanks for response, very much appreciated. I've never done the top end before so this is all new to me.

19 minutes ago, Uro pieces said:

Thanks for response, very much appreciated. I've never done the top end before so this is all new to me.

So unless your seeing something in person Im not seeing in the picture.. what you have is an unseated valve cotter (keeper, call it what you will, that is a common name for them as well)

So how did it come unseated...
Did you assemble the head? If so, perhaps the cotter was not seated properly and just pupped out.

Was it a used head shipped to you and just installed as it came out of the box? ....If so, the valve spring top may have been depressed in shipping and allowed the cotter to come unseated 
If this was a fresh head build with new parts id be concerned it was assembled wrong or mismatched parts were used.
If it was a used head that was disassembled and then reassembled, id be looking closely at the valve and cotters to make sure there is no wear..cotter and valve have nice square shoulders to fit together, OD of the cotter does not appear to be worn, still have a clean taper, and same from the spring top. 
 

I basically bought a used head with all accessories for a quick bolt on and go. As you can see and this zoomed photo the spring has so two nicks/damage on it. I going to order a cheap spring compressor to reinstall the cotter and go from there . Thanks again

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1 minute ago, Uro pieces said:

I basically bought a used head with all accessories for a quick bolt on and go. As you can see and this zoomed photo the spring has so two nicks/damage on it. I going to order a cheap spring compressor to reinstall the cotter and go from there . Thanks again

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That is not the spring in that pic, it is the spring top, or cap if you will,,,separate part than the spring.

Those marks ...something hit the edge of the top and likely allowed the cotter to come unseated. ..Might have been during handling of the used part, might have been from a tool used to remove the cotters... Lots of folks hit the spring with a socket or other tool to depress the spring and top, allowing the cotter to dislodge ...Hitting the spring top gets you what you see.
Replace the spring top and cotters and you are likely good to go..

You can probably clean up the mark on the ID of the spring topper.  Or buy a new one.

Agreed with comments from others, I don't see a broken keeper but rather they do not appear to be fully installed.  They can get wedged like that if they are not set into the valve stem correctly.

Is the cylinder head currently off the bike?  If so, you may want to just remove the valve and post photos of the valve stem and keepers on a towel.  Make sure they fit together as expected and are not damaged.

Do not remove the valve without making sure the top of the valve is not damaged or mushroomed. If the top of the valve is mushroomed at all take a dremel to it and grind it to the proper size to prevent damaging the valve guide.

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