those tiny view ports are near worthless & if you can put a PMR sight glass in there as a replacement I would highly recommend it. a D3 can be a fun engine....with a ton of work. watch for port alignment from the steam chest into the cylinder assembly , mine was off by quite a margin & significantly made the engine an underperformer, a simple drill bit of the proper size slid into both pieces to check for alignment will give you the answer.
Now that you mention it, I Bet my ports ARE miss matched! I have watched a video of Bruce's modified D3 and it is capable of revving like a race car
My stock unit runs more like a really fast idle right out of the box, I had lined the Wilesco fire tray with aluminum foil to keep a full load of gel fuel from leaking out the air holes and this setting wanted to boil over the edge(to full) then once it burned 1/3 down from level with top it wanted to go out...i believed it was starving for air so I dumped the gel and made a new short aluminum foil tray liner that only went up the sides of Wilesco tray about half covering the stock air holes, then a toothpick to rub the foil thru the holes to get full air supply (and holds foil in tray well) Although this setup holds a Lot less gel fuel, it does burn nice and reliable. All this fussing with the fire tray distracted me and I was not thinking about the RPM, thanks Bruce!
On the port matching note: Many of the Mamod Roadster videos I have watched, even the person operating the steam car noted it wants to run Faster in reverse! I want to take some careful measurements and see if I can optimize my Roadster steam ports for Full speed and power in the forward position.