This makes twice in the last few months I've seen this on these flat head engines. Apparently Briggs put soft valve seats in them. I think I posted a thread about the first on an MTD snowblower. Now the same engine on my MTD chipper has done the same thing. About 30 psi compression, removed head and can turn the intake w/ my thumb w/ it at TDC. I remove the valve, measure the length w/ digital caliper and grind the stem end 'til I've removed about 6-8 thousandths. Then put it back in the guide and check lash. The valve seats in both have a groove worn in it so that's why I think the seat is too soft. Opinions?