Ok so I got some time to work on the car today - with curious results -
First I drove about to warm the engine. All working well as normal. Then I fitted a vacuum gauge to the manifold and found it to be running at about 10in Hg. This was the first surprise as the engine is pretty much stock. I don't know what cam is in it, but it's a pretty mild flat tappet cam.
Based on 10in Hg I would expect a #45 power valve. I pulled the primary side float bowl and the power valve was... unmarked! I remembered that I had rebuilt the carb many years ago and fitted the power valve that came in the rebuild kit. I was young and inexperienced and just assumed it would be correct. Don't hate me... :-)
So I dug out the power valve that had been in the carb when I rebuilt it and found that it too was unmarked! Now what...?
I decided that since the carb was apart (and I had noted the primary jets were 68s), I would refit the 'old' power valve. I noticed that this old valve had a much stiffer spring than the currently-fitted valve. I'm struggling to think whether this makes it open sooner or later - depending on whether the spring holds it open or closed.
Anyway, I fitted the old valve that I had removed years ago (the one with the stronger spring) and I think there is a subtle improvement in smell, throttle response and smooth running at cruising speed. It's so slight that it might just be my imagination but I don't think so...
One other observation - with the engine running at idle, I disconnected the vacuum gauge allowing extra air enter the manifold. The engine speed went up noticably! I assume this to mean that it is running rich at idle, but it responds well to closing down the idle jets in the side of the metering block. Not sure what to think about this...
Anyway, a quick drive around the block confirmed that the car is running a little better with the old power valve fitted. I'll see how it goes on a longer journey when I get the chance.
Any thoughts or comments gratefully received.
David