Spark plug blown out there is no more threads in hole , plug falls into the head of the engine. How does this happen.

Go back to basics. What EXACTLY does it do when you try to start it? Just spins it is often No spark. 'Coughs' and tries to start is possibly a fuel supply issue.
An engine needs 3 things to run. Air, Fuel, Spark. Did you check for spark on both cylinders?
Have you checked the air filter is clean. You will be surprised at how much a dirty one can affect an engine.
Have you checked for fuel supply at the cylinders. If you can get your head near the plug holes try this. Remove both spark plugs. Crank the engine for 2 seconds then remove the ignition key. Sniff the spark plug hole. Does it smell of fuel? Do the same for the other cylinder. Use the comment button below and I'll see if I can help you more.

On the top left side of the engine screwed in to the cylinder head. Here's a link to a pictue. The spark plug is in the head.
You have to remove the plastic side cover to access it.

Make sure the kill switch is in the run position.

AN engine need three things to run:
Compression
Timed spark
Fuel
now.....
1*. Check the spark plug to make sure it is firing.. remove the spark plug.. make sure you have metal to metal contact and then turn the engine over.. (this might be a two man job).. the spark plug should have a nice big blue white spark,,(some electronic's will have a weak orange and still fire fine!)
2. Make sure you have gas in the tank and (going into the carb/FI)... (have to ask-actually have had 3 bikes in my career come in complaining -won't start...!! Add gas and they ran.. WOW!.. never charged any of them....the embarrassment on their face was payment enough!)
3*. While the spark plug is out... place your finger over the spark plug hole firmly .. keep the spark plug cap away from your hand..turn the engine over and see if it will blow your finger off..(it should blow it off the hole easily).

If any one of these three are not nearly perfect.. you have an issue!!

Check these and repost with the results...

engine dead? has the cambelt /timing chain snapped?
has the engine run out of oil.

I am assuming you checked the compression with a compression tester

Has the head gasket blown or the head cracked.
If you have no or low compression tested with a compression tester the unit has serious or terminal issues.



check timing first....belt or chain I mean, really would not do anything like attempt to start it until I had adcertained whether the camshaft driver is intact or not

Too much compression will cause overheating, detonation and possible piston failure. Luke's Racing Engines suggests that you C.C. the head which means the process of finding the cubic centimeters of the area above the mating surface head-block. Its accomplished by having a burrett, filling it to a specific amount then getting a flat plate with a small hole in it and sealing it to the head then pouring the fluid in the burrett into the the following: ""C.C. ing the head" is done by placing a piece of Plexiglas over the head (Drill a small hole in the Plexiglas). Put grease or vaseline on the head gasket surface to prevent leaks. Use a syringe (or any measuring device marked in c.c.'s) and fill the head chamber with water. (don't forget to put a spark plug in the head)! This will give you the stock volume of the head. Mill the head to the desired squish band thickness then open up the combustion chamber to bring the compression back down to the stock or preferred c.c.'s.

This procedure is for 200 Kawasaki Prairie 400
1:on the flywheel cover, remove the pull starter asembley so you can turn the engine over by using a wrench. on that same side, remove the timing inspection plug. it will be on teh side of teh flywheel cover, and have a large flat blade screwdriver mark or allen hole you use to unscrew it.
2:with the inspection plug removed, slowley turn the engine untill you see a "F" and a line. from there, rotate the engine very slowley untill you see a "T." JUST after the "T" there is a line. you want the mark inside the inspection plug hole to line up with that line after the "T."
3: with the bottom end now timed, you will install the cam into the head. when you set the camshaft into the head, do so with the lobes pointing directly down. after the cam is installed, then install the cam sprocket and make sure the lines on it are directly left to right. the lines will end up lining up with the level surface of the cylinder head surface. ater you have installed the cam and sprocket, look again at the inspection hole in the flywheel cover to recheck that the bottom end is still on time.
4: once the cam is installed, and the sprocket is installed on it, you will see the lobes point down, and the cam sprocket lines are from side to side. then yo uwant to install teh cy;inder head cover.
5after the head cover is installed, install the cam chain tensioner, and slowley rotate the engine to ensure the valves and piston do not contact. USE HAND TOOLS TO ROTATE ENGINE, NOT ELECTRIC STARTER!! the electric starter will cause damage if the timing is off.

Turn off the fuel tap the spark plug should be on the motor has a thick wire going to it, remove the plug and move the wire away from the hole to avoid a fire then crank engine over to push all the fuel out of the cyl, then fit a new plug .connect wire and see if it starts then turn fuel back on.

ok.. clean around the spark plug really well.. you do not want crud falling into your engine when you install the compression gage..
Make sure your spark plug wire is out of the way..
Once you have the gage screwed in..(MAKE SURE THE GAGE THREADS ARE THE SAME LENGTH AS YOUR SPAK PLUG!!).. hand tight! Hold the throttle wide open and turn the engine over for about 3-4 seconds.. it should read about 120psi+.. I have seen a single cylinder engine run with 88 lbs. before..but if you have less than 100psi.. its time for some investigation.. it could be the valves not seating (warped - too tight)..or just normal wear for a 20 year old atv.
New piston/rings.. complete valve job,. carbon clean out.... you should be ready for another 20 years.. KEEP the air filter clean and the box sealed!!

check fuel flow and check spark at spark plugs

According to an "ATV Rider" 2009 article, "When the Grizzly was introduced in 1998 as a 600cc machine, the top was blown right off the displacement wars of the time."
It was replaced in 2002 with Yamaha Grizzly 660, an even bigger engine derived from the well-established all-terrain vehicle (ATV) Yamaha Raptor 660R.
Grizzly's 660 cubic centimeter (cc), four-stroke, liquid-cooled single overhead camshaft, five-valve engine performs well in open country, but sometimes it causes serious problems for owners.
Before you call a Grizzly dealer, however, trying a few tricks in your garage can save you a lot of time and money.

Check the fuel level.
Locate the fuel meter indicator on top of the multifunction display.
If it does not work properly, open the fuel tank, and push the Yamaha 660 Grizzly side to side.
Ensure the fuel is not contaminated by water or rust.
If you have not driven your Grizzly 660 for a long time, drain the tank completely, and fill it with a fuel tank cleaner.

Check fuel hoses for cracks or leakage.
The hoses should be soft and simple to bend.
Replace those that show any sign of wear.

Check the compression.
Screw a compression tester into the spark plug hole, and press the electric starter button.
If no compression exists, contact a Yamaha dealer.
Wipe wet or dirty electrodes with a dry cloth.
The spark plug gap should be checked regularly

Correct the spark plug gap.
Measure the gap with a wire thickness gauge.
If necessary, adjust it to 0.8mm using a screwdriver or replace the spark plug.
Use only a Yamaha-specified spark plug.

Check the Yamaha 660 Grizzly's battery.
Operate its electric starter.
If the Grizzly starts quickly, then its battery is in good condition.
If the engine turns slowly, check the battery lead connections, and you may need to change the battery.
If the engine still does not eventually start, your Grizzly is beyond home repair.