It was painfully apparent one recent Friday eve that little Zento - its austere dining room nearly empty on a street pulsing with revelers - doesn't yet have the curbside bling that typically lures Old City's roving nightlife.

That thirsty crowd usually requires a mojito to get things rolling, or a day-glo martini, or a chalice of Belgian beer. Or any beer at all, really. And Zento (that's Japanese for "a future") is BYOB - a format beloved in every residential neighborhood, but still out of place in the city's party zone.

The simplicity of Zento's storefront space, formerly a no-frills cheesesteakery owned by Eagles offensive tackle William Thomas that has been transformed into a tiny white box with a sushi counter, doesn't offer much to a streetscape that's all about image. I suspect the promotional banner in Zento's window - which really is bush-league - has even chased a few customers away.

But those are the unlucky ones, because Zento more than compensates in sushi substance for what it lacks in designer flash. Of course, I probably would have walked past Zento, too, had I not already experienced Gunawan Wibisono's ability to elevate the mundane to something worthy. If Wibisono can distinguish a bare-bones sushi express counter from the typically cheap-o competition with quality fish and catchy rolls, as he has done with Kami near City Hall, what can he do in a real restaurant?

The 27-seat Zento is hardly bigger than Kami, but it's warmer inside than it looks from the exterior. The clean white walls are trimmed with bamboo wainscoting, a soft blue light emanates from a wavy ceiling cut-out, and the fare on the plasma TV ranges from animated Hayao Miyazake films to a documentary on grading tuna.

It's what the chef makes with tuna, though, that is the real draw. Wibisono, a 28-year-old Indonesian of Chinese descent, maki-ed his way up the sushi ladder from quick-service Yanagi (in the Oxford Valley Mall) to creative Kisso in Old City to masterful Morimoto, where he worked the sushi bar for a year. It was a dream job that Wibisono had applied for 10 times before he got it.

But he was clearly a quick study. Though the current surroundings are considerably less grand, the Morimoto influence is obvious in many of Zento's best creations. There are the pillars of minced fish tartare: buttery pink toro or sweet white yellowtail, that sparkle with garlic and the crunch of fried shallots. Zento's "new style" sashimi is also an Iron Chef homage - pristinely sliced ribbons of suzuki (striped bass) and hamachi (yellowtail) drizzled with citrusy hot yuzu oil.

Wibisono's signature roll is also an eye-catching variation on Morimoto's square sushi, a seaweed-free rectangle of pressed rice layered with avocado, eel and sweet plum paste, then topped with tuna (or salmon) and a streak of spicy mayonnaise.

Wibisono isn't the first chef, and he won't be the last, to borrow and elaborate on great dishes from his mentors. Morimoto, in particular, has a number of local acolytes, some more successful than others.

But making great sushi is much more about details in the doing than simply following a recipe. It's about attention to quality ingredients (from the freshness of the mackerel to the toothy bite of the rice), skillful knife work (to maximize the beauty and tenderness of each slice), tight, precise maki rolling, and an aesthetic that bigger rolls are not necessarily better.

It's clear Wibisono knows what he's doing, even in the sparest nigiri, where a small fillet of silver-striped needlefish is draped like a well-tailored sportcoat atop a firm little cluster of rice. Hokkigai surf clam is meticulously scored, lending the otherwise chewy meat a ribbed texture that has a gentle snap, a savory wash of ocean sweetness.

More complicated rolls like the volcano, with crab, shrimp and smoked salmon wrapped inside a rice tube of deep-fried seaweed, convey contrasting myriad flavors and even temperatures without losing harmony. Wibisono (or, rather, his sushi-bar assistant) is occasionally guilty of going a tad heavy on the spicy mayo, as was the case with the tuna pizza and baked scallop American dream roll - which was, nevertheless, still yummy.

Zento's signature sashimi, meanwhile, adds only the slightest, but effective, flourishes to pristine fish: the pop of roe to striped bass, the crunch of shredded daikon to alabaster yellowtail, a salty-sweet miso glaze to lightly seared salmon.

Zento produces some worthwhile hot food, as well, but it's less consistent than the sushi, and in the process of being downsized by a newly arrived kitchen chef to focus more on small-plate starters than larger entrees.

Among the highlights were some excellent tempura - greaseless billows of pale crust encasing snappy vegetables and sweet shrimp - that I loved dunked into ramen soup. This wasn't the 50-cent dorm room ramen you boiled from a bag in college. Zento's long egg noodles come nested inside a rich dashi seafood broth warmed by sesame oil and a lip-numbing shake of addictive togarashi spice.

The more soft-spoken seafood yosenabe, meanwhile, served in a lidded ceramic hot pot, was a pristine bowl of Tokyo comfort. It harbored a medley of surf clam, scallops, shrimp and king crab legs that had simmered alongside vegetables and clear rice noodles in a light broth that married flavors like a most delicate pot au feu.

Zento had other winners, like a crisply seared teriyaki scallop at lunch. But the dinner version was rubbery, as was a slice of overcooked arctic char that came beneath a light teriyaki that didn't carry much depth. The deep-fried and stuffed "drunken shrimp" was heavy and strange.

I understand that Zento's spicy miso kobe beef entree is a delight. But the restaurant never had it available during my visits - a symptom, no doubt, of a small new eatery still trying to master an unpredictable flow of diners. The same might be said for the service, which was pleasant, but always seemed shorthanded and slightly confused.

If Wibisono gets his wish to expand into the second floor, he'll need to refine the dining room staff for the future. But once those Old City crowds stop to really taste this sushi, Zento's future might be now.

Next Sunday, Craig LaBan reviews Blackfish in Conshohocken.

Contact restaurant critic Craig LaBan at 215-854-2682 or claban@phillynews.com. Read his recent work at http://go.philly.com/craiglaban.