The Emerson String Quartet is engaged in a Mendelssohn blowout. Deutsche Grammophon has released its recordings of the complete string quartets. The Emersons are playing those quartets at London’s South Bank Festival next month. And, Tuesday night, they embarked on a four-concert series at Zankel Hall called “A Vision of Mendelssohn.” They will play a variety of Mendelssohn – including the complete string quartets – and works by other composers related to this output. All in all, it’s Mendelssohn madness, as an ad writer might say.

On Tuesday night’s bill was the Quartet in E-flat Major, Op. 12, followed by a Beethoven string quartet – one that heavily influenced Mendelssohn. After intermission were a couple of Bach pieces – more influences – and then a Mendelssohn fugue, and finally that composer’s Quartet in A Minor, Op. 13. This was a musicologically thoughtful program, but concerts aren’t classroom exercises. They are concerts. How did this evening go?

It went poorly in the first half, and far better in the second. The Emerson String Quartet, bear in mind, is the most lauded chamber ensemble in the world.

In the E-flat-major quartet, these four played very Romantically, gobbing on the portamento, for example. Their sound was rich – but more than rich, thick. It was thick almost to the point of unclarity. You don’t have to play early Mendelssohn like Mozart, but you don’t have to lard him up, either. Forgetting lard, the entire first movement was heavily draped, like certain Victorian furnishings. And the first violinist, Eugene Drucker, was repeatedly flat, souring the music.

The second movement – labeled Canzonetta: Allegretto – also suffered from a heaviness, a thickness. This music need not be dainty, or prissy, but it should have a lightness and grace. Think of those words “canzonetta” and “allegretto.” Moreover, the players were often out of coordination, as well as out of tune. We don’t expect live performances to be studio-perfect – even from the Emerson String Quartet. But this was substandard. And Mendelssohn’s whirling G-major section did not breathe its delight.

In the Andante, some of that thickness paid off, although the Emersons would still have benefited from a somewhat thinner sound. The group seemed to be saying, “This isn’t just teenage stuff. This is Great and Important Music.” A little demureness wouldn’t have killed them. At times, the Andante resembles the slow movement of a violin concerto, and Mr. Drucker acquitted himself well.

The closing movement, fortunately, had some Emersonian bite and brio. This music might be described as furiously merry. But continuing flatness of pitch was a detraction.

Beethoven’s Quartet No. 10 in E-flat Major, Op. 74, is called “The Harp,” owing to an impression of that instrument in the first movement. The Emersons’ “harp” was not exactly heavenly – it was a little thick-fingered – but it was passable. What the players did best was capture the conversational, you might say questioning, nature of this music. They seemed to be talking to us, which is what Beethoven wants.

The Adagio of this work – Adagio ma non troppo, to be precise – is a small masterpiece, and the Emersons’ flatness nearly spoiled it. This concert proved, unnecessarily, how important pitch is in music-making: This is so in opera, in symphonies, almost anything. Mr. Drucker was so flat, he was painful. Not even the Zankel subway could cover this offensiveness. Pitch aside, the Emersons were sweet and elegant in the Adagio, but they rather missed Beethoven’s nobility. Gratifying about this movement was that David Finckel sang beautifully – and in tune – on his cello.

The Presto was decisive enough, with Beethoven’s mad counterpoint well-handled. And the variations that close the quartet were adequate.

So, that was the first half. And the second? The group started with Contrapunctus 1 from Bach’s “Art of the Fugue.” They were a new group – or rather, an old group, more like their excellent selves. They played the music – so familiar to them, and to us all – with poise, intelligence, and a dose of spirituality. Also with proper intonation. Eugene Drucker was especially fine. Next we had a fugue from Book II of “The Well-Tempered Clavier,” arranged for string quartet by Mozart – and the Emersons gave us more of the same: They were in balance, in tune, and in their groove. What followed was the fugue by Mendelssohn himself, a fugue in E flat, catalogued as Op. 81, No. 4. The Emersons played it exquisitely.

That quartet in A minor, Op. 13, incorporates a Mendelssohn song, “Ist es wahr?” This can be an enchanting work, embodying the best of early Romanticism. Our four were, yes, a little thick in it, but they were far more nimble and adept than they had been in Mendelssohn’s E-flat quartet. The Intermezzo was favored with a very steady pulse. And the final movement – a Presto – began almost operatically, so intense and theatrical was it. This movement had a beautifully persuasive ending, too.

But there was more Mendelssohn to come: The Emersons’ adoring fans asked for an encore, and they got one, the capriccio from that group under Op. 81. The players brought it off with accuracy and style.

In a review the other day, I quoted an old saw: “Well begun is half done.” And how about well ended? Half mended? In any case, the Emerson String Quartet did not make a liar of its reputation.