RE: TH .46 ran fine upright, not side-mounted!
Been here, done that.
Been through this with the TH .46 on my biplanes, both side and inverted mounted engines... with and without the Slimline Pitts, the Bison Pitts, the Sport Scale Pitts, the TH .46 muffler, and the O.S. P-Box muffler... see below...
The engine runs a bit richer mounted sideways and still more when inverted even when you have the tank to carb relationship correct.
Making things still worst, is that the TH muffler (and the pitts muffler, been here too!) has no baffle. While this maximizes power, it also provides for almost no tank pressure so you will experience tuning changes based upon engine orientation. With the TH muffler and pitss muffler there is almost no difference running the muffler pressure to the fuel tank, or not.
The good news is that the TH engine will accept a good range of tunings, especially once it has been broken in.
Your best bet is to use a muffler with a baffle plate (the O.S. P-box .46 muffler is a good choice) or add one to the TH muffler.
You will need to retune the low speed and then the high speed needle.
Sideways your engine is running a bit richer throught the low range. You get an initial "burst" of power as you throttle up, because you are quickly permitting more air into the carb. The mixture leans a bit because of the initial airflow. As the fuel is sucked in however the engine goes rich and slows down, but does not shut off.
Lean out the low end a bit, and re-tune.
Then make sure that you check the engine in all aircraft orientations, nose down, nose up, etc.... waiting at least 10-15 seconds for each orientation change to kick in...
Pay particular attention to when you have the cylinder head pointed downward (inverted) as the engine goes very rich at this point.
Lean it out enough so it does not stall out.
Then check the nose up performance. Make sure that the engine does not go too lean on nose up.
A muffler that maintains a higher pressure will cure all of this, but you'll loose a few hundred RPM's on the prop.
With a baffled muffler you'll need to retune the low and high needles as the engine will be initial very rich, but fuel flow will be far more consistent in all orientations.
BTW: Blocking one of the Pitts exhaust ports had little effect with both of the TH .46 engines I own.
Also this engine seems happiest with 10% Nitro not 15%.
Once you do have it tuned, this is a POWERFUL .46 engine...