The golden dome that tops the old customs house at the head of the Grand Canal in Venice looks a mite tarnished this morning. The walls are encased in rusting corrugated iron. Such noble decrepitude.

Not for long though. The billionaire French luxury-goods magnate François Pinault recently acquired a long lease on this building. Soon he will transform it into a museum of contemporary art to show off a permanent collection, hand-picked from his vast holdings. The official opening is scheduled to coincide with the Venice Biennale in 2009.

Meanwhile, just a lazy bend or two down the Grand Canal towards the Rialto Bridge, his last-but-one acquisition in the city – the 18th-century Palazzo Grassi – is showing a temporary exhibition of painting and sculpture, part brand new and commissioned from a range of younger artists, and part recent – much of this is postwar American art.

The palazzo is a difficult space for new or newish art. Inside, a vast inner courtyard soars up to the height of the three floors. On each floor there is a balustraded walkway opening on to gallery spaces. Some of these spaces retain features of the original building – there are ceiling frescoes and various kinds of architectural detailing. Others are discreet, artificially lit white cubes that could be anywhere in the world.

What happens in this vast atrium sets the tone and the mood for the whole. When the building first reopened in the spring of last year, the ground floor was covered with a work by Carl Andre across which you were invited to walk. Many people didn’t notice it was there. A purple balloon heart by Jeff Koons hung in the air. The discreet met the cartoonishly colourful.

This time we are presented with what looks like a giant Christmas tree, hung with dozens of framed images. A whole heap of them has spilled down on to the floor. This work, called Jet Set Lady, is by the Swiss artist Urs Fischer. It’s not a tree exactly. More a vast, spiky-armed, climbing metal armature, and, inside that armature, rods of white neon that slash about in the air. The images as disparate as they come. What they all share is a spirit of wild, free-hand, subversive, Dubuffetish gaiety. Here’s an image of Catherine Deneuve from Buñuel’s Belle de Jour. There’s a palm tree. And that one’s an anus, isn’t it? Yes.

This vast, tapering structure stands on a thickly piled, patterned carpet, which, seen from above, looks as if it might have been whipped out of Aunt Edith’s sitting room. Except it’s monochrome. This is a work by Rudolf Stingel, and it’s supposed to remind us of an all-over colour field painting, according to the catalogue. Yes, it’s a kind of carpet-painting. It’s meant to make us think more about the nature of painting – and more about the nature of carpets perhaps. To question our cultural assumptions.

It doesn’t. It just looks rather ugly down there. What is more, it sits very oddly, and very unhappily, with Jet Set Lady. They jar against each other. The issues that these two pieces have raised come up again and again as we walk around the rest of the show. So much of the work here seems to exist to demonstrate how this, that or the other stands in relation to the often tedious theoretical debate about what painting and sculpture are. So little of what we see here is visually exciting for its own sake.

There are exceptions, of course. On the very top floor, in a light-flooded gallery that overlooks the Grand Canal, a curvilinear painting 9 metres (30ft) in length, assembled from panels of painted Plexiglass, called Flying Curve, Differential Manifold, by a young American artist called Kristin Baker, is all about the idea of speed. The painting, floating above the space, gulping light from the windows, is revving off in all directions at once. It roars back in the general direction of Futurism and Vorticism – except that, tonally, it’s much less sombre than those Vorticists, who were doomed to live beneath war-scarred, European skies. It also revs into the future, with wonderful, shard-like splinters of colour wheeling and probing.

Baker’s work is a real find – as is that of the German painter Anselm Reyle, whose restless and exciting work is paired, very sensitively, with that of Martial Raysse, who was a pioneer of French Pop Art in the 1960s. Of the older artists here, the work by the African-American David Hammons always deserves attention – look at his stone head with its topping of cornrow hair – a beautifully insightful commentary on the idea of the cultural neutrality of the sculpted stone head.

Those few names apart, much of this show, with its array of bankable names – Franz West, Richard Prince, Urs Fischer, Mike Kelley – is laboured, self-important drudgery. Monsieur Pinault needs to hone his skills as a collector.

— Sequence 1 - Painting and Sculpture from the François Pinault Collection is at the Palazzo Grassi until Nov 11

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