Riding Lawn mowers - My lawnmower's motor will not turn over.

I have a Honda 3813 riding lawnmower. It's 10-12 years old. The motor will not turn over. I hav..

I have a Honda 3813 riding lawnmower. It's 10-12 years old. The motor will not turn over. I have taken the spark plugs out and motor tries to turn over just fine. Then put spark plugs back into motor and the motor acts tied up. I have a new battery and new plugs in. What is next?
Bought new battery and replaced spark plugs. Still not turning over. Can't even jump start it.

Ok sounds like it needs some valve work done on it they most likely are worn or the cam is worn not letting them open enough, I would also check the muffler to make sure it is not plugged up any as it could cause it also.

How do I check to see if the muffler is plugged? Is the valve work something I can do myself? And if the valves are shot, am I looking at a motor that needs to be rebuilt?
One more note - last week I was able to jumpstart it and once it's running, it works just fine.

On the muffler just remove it and see if it will let it crank over if it does then you know the muffler needs replace, On the valves I would first do a compression test and see what reading you get, if they are just worn a little some have an adjustment on them witch there would be a small cover on the side of the engine and some you have to replace them and grind them down till you get the right setting. Most likely it does not need a complete rebuild. If you have never worked on the valves before I would suggest taking it in to a shop and have them check them.

I'm not qualified and don't have the tools to check the valve compression. I'm heading out right now to see if I can get the muffler off. I'll let you know. Thanks.

Removed muffler. Didn't start. Removed spark plugs, still won't start. Checked battery on meter and it has 12.6 v power. Any idea how much a valve job costs? Or what would be a fair price for that?

If it can be adjusted I would say around $65 1 hour of labor But it all depends on if it can be adjusted or if they will have to replace any parts with out having it and checking it out it is hard to give a good estimate on this.