Thanks for the update. Curious to hear about the muffler situation when you get time. Also, this engine has the potential in inject a LOT more heat into the engine compartment, which is already overheated in my view. You may want to consider adding a fan somewhere.
Also, I'd consider adding some screen to cover the air intact holes at the back of the tub. The screen will quickly get clogged with debris in some situations, but the main air intact shroud should still be free to pull air from the engine compartment. I need to do this to my tractor because the engine is getting clogged with leaves.
The muffler is whatever the local muffler shop had in stock. I asked for the smallest inlet/outlet size, oval shape, QUIET and 18-24" overall length. With an elbow on the outlet to point the exhaust noise away from my ears, I can now hear the actual mechanical noise from the engine. Turns out there's quite a bit of that, too.
The Robin big block comes with both a stationary inlet screen and a rotating screen that attaches to the flywheel/fan. Between the two of them, they add at least an inch to the length of the engine. Once I re-drill/slot the engine mount holes in the bottom of the tub, I should be able to move the engine forward enough to accommodate the rotating screen. I'm hoping that the rotating screen will throw debris off.
Regarding the heat issue: I have an idea that the bigger engine may actually run a bit cooler because the airflow and cooling fin surface area should be greater to be in proportion to the displacement. The 720cc engine was maxed out most of the time while mowing, and was running on the ragged edge of it's designed cooling capacity. The 999cc engine should be relatively loafing under the same load, and have plenty of cooling capacity in reserve. That assumes that the tub allows the hot air to escape adequately. If not, I'll take appropriate measures.
I did get to play just a little bit after work today. With the old engine at full throttle, climbing the steepest parts of my front yard wasn't any problem as long as the PTO was off and I feathered the treadle with a bit of care. With the new engine, it's still not any problem if I feather the treadle a bit...but today, I did it at about a quarter throttle.
It's still early in the game, but my impression at this point is that the big block engine hasn't turned my PT425 into a hotrod unstoppable beast. It will just let the PT do everything it should be able to do without panting and gasping desperately. I'm pleased so far. We'll see how it all works out.
