I learned this from a buddy about 30 years ago, when I would try to buy old Jeeps that had 4 cylinder engines and I used it when I bought my  WD45 tractor with a 4 cylinder engine. You start the engine at idle or 1/4 throttle, with a firm grip, place a new 1 dollar bill  about 2" over the exhaust pipe, as the engine is running if it sucks the dollar bill into the exhaust, it will snap like you can't believe. I was told that this is a sign the  exhaust valve is  sticking or the seat is bad, it could also be bent, burnt or out of adjustment too. Here is a video from youtube that shows no problems with the valves  http://www.youtube.com/watch?v=l8LRhuHuerw" rel="nofollow - http://www.youtube.com/watch?v=l8LRhuHuerw  sorry I could not find a video of it sucking in the dollar bill, my friend sold his ford tractor that did this last year. Here is another link to this test scroll down to shop tip  http://books.google.com/books?id=Yn9F0RhJ0EkC&pg=PA573&lpg=PA573&dq=dollar+bill+exhaust+valve+test&source=bl&ots=S9F4AHOCyc&sig=o6U4jNIctW4upR1VjJqCyqTAMEk&hl=en#v=onepage&q=dollar%20bill%20exhaust%20valve%20test&f=false" rel="nofollow - http://books.google.com/books?id=Yn9F0RhJ0EkC&pg=PA573&lpg=PA573&dq=dollar+bill+exhaust+valve+test&source=bl&ots=S9F4AHOCyc&sig=o6U4jNIctW4upR1VjJqCyqTAMEk&hl=en#v=onepage&q=dollar%20bill%20exhaust%20valve%20test&f=false