Take the carb off, take the inspection plate off. (also easier if you remove the manifold.)
Remove #1 sparkplug (the one closest to the radiator.)
Rotate engine till both valves in #1 close. Look in the hole. & the pointer is at the notch in the crank pulley. #1 is at TDC at this point.
Check clearance on 1, 2,3 & 5. Adjust to .015 clearance cold.
Rotate engine 360 degrees until #4 is at TDC.
Check clearance on 4, 6, 7, & 8. Also adjust to .015 cold clearance.
Repeat and double check everything.
Get a new gasket from TM (one of our sponsors) or use good sealant and install the cover plate.
Install the manifold (if removed), the carb and any spark plugs you took out.
It should now run much better without the clatter.
Good luck and keep posting.