# top dead center mark is off, any ideas?



## skid (Jun 18, 2009)

OK heres the deal, i'm working on a single cylinder engine (yamaha 450 wolverine) and when i line up the top dead center mark on the fly wheel through the side peep and then put a screw driver into the spark plug hole, the piston is at the bottom and not TDC. The only answer I have for this is the woodruff key has sheered off causing the timing marks to be off by 180* approx. of what they should be. Any other thoughts on this as to what this could be?


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## NMKawierider (Apr 11, 2009)

Pull it down and see. It can't be that the valve timing is off that much or the chain jumped. Case/main bearing may be bad enough to let it make contact with the coils and seer the key..which would be bad enough. Funny its exactly 180 out though...is there any other marks on the flywheel?


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## skid (Jun 18, 2009)

the only other mark is right before TDC and the mark looks like this FI and the mark for TDC is TI. When the "I" mark of the "TI" is lined up with another mark on the side of the casing its suppost to be TDC but when i check where the piston is at its at the bottom of the stroke. I'll give you a little history in the atv, it wasn't running the greatest and i had to move it out of the shop one day to pull out my other atv, and i let it idol in the driveway for a bit then went to move it back in and it stalled and would not restart. It will turn over and occasionally back fire, thers lots of spark, fuel and air, but i'm positive its timing which has lead me to where i'm at. Either way i'll findout tonight if the flywheel has shifted. Thanks nmkawierider for the feed back, when theres not much feed back i know its not something alot have come across and any ideas help.


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## Mr.Scruffy (Jun 25, 2012)

I have forgotten a lot about my mechanic days but it seems like on a 4 stroke engine the TDC mark comes around twice, once on the firing stroke and again on the exhaust stroke. If you are working on a 4 stroke it may be that the crank needs to go around again for the firing stroke. We use to have to watch the valves when we were setting the timing on the old small block chevy's to determine the distributer position. Of corse I may be mis-remembering.


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## NMKawierider (Apr 11, 2009)

Well.. at TDC on either rotation, either the compression or exhaust stroke, the piston is always straight up. The cam rotates at half the speed of the crank so it will be either one position or the other but the top of the throw is... the top of the throw regardless. If he's sure on the marks, then something has moved....a lot! Just pull it and see.


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## Mr.Scruffy (Jun 25, 2012)

Thanks NMK. Now that you have refreshed my feeble mind, I think you are right. I was trying to find something simple for him. Really seems odd that it would be 180 out like you stated earlier.


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## skid (Jun 18, 2009)

It is odd thats why i am baffled, if the fly wheel rotated it'll be a new one for me, but i just don't see any other explanation, i'll let you know what happened and if i get can get a few pics as well.


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## brute for mud (Jul 15, 2010)

some times the flywheel key just shears and when it shuts down the flywheel has enough momentum and rotates when the crank stops this from what i have seen


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## NMKawierider (Apr 11, 2009)

brute for mud said:


> some times the flywheel key just shears and when it shuts down the flywheel has enough momentum and rotates when the crank stops this from what i have seen


Yep...and I have seen on some engines where it didn't get the proper torque and it was just the key keeping in position but in time floping back and forth it gets looser and cuts the key on both sides until it sears the rest of the way. By then its loose enough on the crank to rotate quite a ways.


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## skid (Jun 18, 2009)

Well we came to a hault last night while trying to pull the fly wheel. My gear puller wouldn't pull it off the shaft so now we are ordering a fly wheel puller designed for this atv, but its a few days away. Somethings for sure a miss with the fly wheel cause we put alot of pressure on it to try to get it off and it just wouldn't budge.


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## tate (Jul 29, 2012)

Have heard that a back fire can shear the woodruff key and then the flywheel can turn a little till it binds up on the crank again. Just pull the flywheel and it will tell you weather u have ro go further. This came from a Yamaha dealer.


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## skid (Jun 18, 2009)

yeah the problem is getting the fly wheel off, i had the tool ordered off ebay so i probably won't see it for a week or 2.


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## tate (Jul 29, 2012)

I just used a 3 jaw puller and left the bolt in the end crank so the puller wouldn't go into the crank. Put a good amount of pressure onto the puller and hit the end of it with a hammer. Should pop right off. Thats how I did mine. Wasn't going to spend 30 bucks for a puller when I had something that would work.


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## skid (Jun 18, 2009)

well the tool to pull the fly wheel came in yesterday and it sure is easy to pull the fly wheel with the proper tool, what i found was the woodruff key sheared in half so it was an easy fix.


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## NMKawierider (Apr 11, 2009)

skid said:


> well the tool to pull the fly wheel came in yesterday and it sure is easy to pull the fly wheel with the proper tool, what i found was the woodruff key sheared in half so it was an easy fix.


 
Wow...great news. Now the question is...why did it shear?


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## skid (Jun 18, 2009)

I'm guessing it had something to do with all the grass wound around the cam sprocket. The crank case vent is on the unfiltered side of the air box and there was a fair amount of swamp grass in it.


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