Do a compression check. When you pull the spark plugs, make sure none of them are fouled. Compression should be between 125-150 psi depending on your altitude and engine condition. No lower than 120 psi and no more than 10% deviation between cylinders. If you don't have a compression tester, one can be had from almost any local auto parts store for $16-$30. I paid $17 for mine and it works perfectly.