Yesterday I tore apart my BR-600 in order to replace a broken valve. I replaced the valve and put it back together. Now it won't run. It turns over just fine. And sometimes it will sputter like it wants to crank. I'm thinking air may be getting in somewhere. I redid the sealant at the bottom of the block where the cylinder comes apart, so I'm pretty sure that's sealed up. However, I did not replace the gasket at the valves. Is that necessary to change when you take the valve cover off?