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The first step of any engine tuning is to ensure that valve adjustment is correct. For B18 and B20 motors, Volvo recommends checking this at 6,000-mile intervals. I make a habit of checking this twice as often (whenever I do an oil change), although adjustment is rarely needed once a new motor settles in. It's a good diagnostic tool -- if valve clearances tighten up, it's a pretty sure sign of valve seat erosion; a matter of some concern with today's unleaded fuels.
Incorrect valve adjustment is not only detrimental to performance and engine longevity, it can also fool one into thinking more serious problems exist. We recently compression-checked a motor that couldn't be made to idle smoothly with any amount of tuning and obtained readings that varied greatly from cylinder to cylinder; normally an indication that a rebuild was imminent. This turned out to be nothing more than poorly set valves, much to the owner's relief.
As an experiment on one of my motors, I tightened valve clearances just .002" under the "usual" setting and found that this had a profound effect on the way the carb needed to be jetted. Camshafts are engineered with particular clearances in mind -- these are factored into the design of the cam profile. When one varies from the recommended clearance settings, one is effectively altering the way a cam was designed to operate. I don't mean that as a "thou shall not;" just reinforcing my point that valve settings have a global effect on engine tuning.
The Haynes manual method for setting valves -- setting #1 when #8 is fully depressed, #2 with #7 down, etc. -- has worked pretty well for us until recently. Here's a method I find I like better, though, that works with any cam profile -- courtesy of David Hueppchen, who got it from Waddell Wilson years ago:
Once the valve adjustment is correct -- and only then -- you're ready to set ignition timing and tune carburetors.
I like David's method a lot better than the way I've been doing it all these years -- and I'm sure you will, too.