I'm guessin that the existing muffler is mounted
above the deck, so that is what the engine is
designed for, and the old style muffler would not
work, even on the other deck. Why not buy a NEW muffler from one of the
many online parts places? And while you're at it,
order the parts to get your self propel working
properly. because once you get used
to self propel, you'll never go back to the push
type mower.

"I'm guessin that the existing muffler is mounted
above the deck, so that is what the engine is
designed for,"

Best not to guess in these case. NO "D" series LB muffler was mounted above deck. He is talking about the round steel under deck muffler, he says, "Bowl" shaped muffler.

Walt Conner

You are correct Walt.
I have already picked up a scrap D600 mower with what appears to be an aluminum muffler mounted below. I am going to remove the D600 engine & drop my D400 engine onto the deck this weekend (I hope) & see how it goes.

I also found a 1980 model 4531/20" with that stupid steel deck & the only crank I have ever seen with a bolt instead of a nut to secure the blade. I am hoping I can remount the engine to an old offset wheel D400 deck with the kidney bean muffler, the way the bricktops were meant to be. I just don't know if a 19" blade will clear the muffler with this weird crank. I may have something to report by the end of this weekend.