I think I might have found the cause of my car's lack of power. To give credit where it is due, it was Stickbuilder's thread about modifying the airbox that pointed me in the right direction. In that, he mentions that the fresh-air snorkel appears to be quite restrictive. I went out to the car and had a look at mine. As I have modified the airbox by removing the hot air duct and the flapper, and plugging the hole where it was, the snorkel is now the only source of air for the engine. The diameter of the end of the snorkel did seem to be quite small, smaller in fact, by a good bit, than how I remembered the diameter of the throttle body I cleaned the previous afternoon. I went and got a ruler, which confirmed my suspicions---the throttle body is 55 mm, the end of the snorkel is only 42. I could see how that might be a problem, so I removed the vertical part of the snorkel, which gave an intake opening of 55 mm diameter. Went for a drive, and the car did seem noticeably more responsive, though still not quite up to what I remember the other one as having been. I'll try to get the rest of the snorkel off before I go anywhere tomorrow, and see if that makes any additional improvement. If it does, I will have to figure out how to fabricate a replacement snorkel of adequate dimensions, as I like the shape of it, which looks like it would be very effective at keeping water, insects and other debris out of the airbox. The only thing I can't figure out is why it is so undersized to begin with----it seems to me that the engine would have to draw its air from both the cold and warm air intakes at all times to breathe adequately.

As a footnote, I will mention that on my 740, which ran so well, that the flap was stuck in the hot air position, and the hose was removed, so it was getting all its air through the one hot-air opening (of which I don't know the exact dimension, but it's not all that big---I'll dig up the 740 airbox and measure it when I get a chance)