Those valves sound way too loose. "A loud tappet is [not always] a happy tappet." I'm guessing you didn't have it at TDC when you adjusted the valves. There is a lot of confusing info out there about how to find TDC. Also, I've never had any trouble with it on any bike -- except on the 1200 8V motor for some reason. What worked for me is remembering that I'm looking for the top of the compression stroke, and then just watching the valves (and double-checking by grabbing the rockers and making sure they have free play). Intake valves open, keep spinning, they close, then slowly move the crank to the spot where the valves are all closed and you can feel the free play. It sounds like you've got 1/4" of free play, rather than .006" free play.
I watched a friend of mine do the same thing on his 1200GS. He was watching for some timing mark, and I was watching the valves. I told him it wasn't at TDC, he told me the BMW timing mark doesn't make mistakes, he made huge adjustments to the valves (that's a sign you're not at TDC -- big adjustments), and he had to truck his bike to the dealer when it wouldn't start. Guzzis, on the other hand, will sometimes start with seriously maladjusted valves.
Finally, at the first service I loosened my valves to the .006"/.008" or .15/.20mm specs suggested in various Guzzi forums, even though the sticker under my GRiSO's seat said .10/.15mm. I didn't like it, and re-adjusted the valves 1000 miles later to the factory spec of .10/.15mm. Makes for quieter valves and I like the power delivery better. On my wife's 750cc V7 I've consistently set the valves at .15/.20mm, and that's what her owner manual calls for. The owner manual on my GRiSO has no valve spec (!) -- it's on a sticker under the seat. I hope some part of this rambling helps!