NOTE: The Hitachi Carbs article found on this website tells about removal, disassembly, tuning, and common problems. Much of this information applies to Virago Mikuni carburetors as well, and I suggest you read the Hitachi article before continuing with this article. For carb theory and function try the CV Carb article.
In 1988, Yamaha finally retired the Hitachi carbs that had been serving up fuel to all of the bigger Viragos since the beginning of the line in 1981. Mikunis represented an updated, more modern design and were used with only minor changes through 1999 when the last big Viragos were built.
Mikunis are the same size (40mm) and have the same mounting dimensions as Hitachis. They are CV carbs with diaphragms, but have a flat slide instead of a round one. Like the later Hitachis, they have built-in coasting enricher circuits. The linkage between the carbs is greatly improved. In one or two ways, however, it can be argued that Hitachis have the edge in terms of tuning and ease of assembly/disassembly.
More specifically:
* Removal and installation of these carbs follows the Hitachi method. These carbs also use the same carbholder stubs as the later (’83-’87) Hitachis.
* Disassembly is another matter. These carbs use a different linkage between them which must be removed to get to one of the float bowls. In short, complete disassembly is somewhat more of a hassle than with Hitachis.
* Mikunis have a pull-push throttle cable which is slightly more complex than the typical Hitachi single cables, but not really a problem.
* And as time went on, Mikunis got harder to work on. Early Mikunis has a screw plug in the float bowl under the main jets, so these jets could actually be changed without removing the float bowls. These plugs later disappeared.
* The pins holding the floats were now pressed into place in towers (unlike Hitachis, where these pins float and are trapped in place by the float bowl). To change float valves, these pins must be drifted out carefully with a small punch. Since the towers appear none too robust, this procedure always brings a bead or two of sweat to the brow, since a broken tower would trash the carb body. But with care, they always seem to come out O.K.
* The screws holding on the float bowls and carb tops, seemed to get tougher and tougher to remove as the years went by (maybe due to thread lock at the factory). In my experience vice grips are sometimes needed to brake them loose. Substitution of Allen head screws might be a good upgarde here.
* The pilot circuit adjustment screws come plugged as with Hitachis. But they are now located on the tops of the carb bodies near the frame. In order to tune the bike, the gas tank must be raised up a bit and out of the way (unlike Hitachis, where these screws come down from the bottom of the carbs and can be more readily accessed–except when air pipes are in the way).
* Float levels are measured at the rear of the carbs, rather than in the middle as it is with Hitachis.
* The linkage between the carbs is a much improved design, and does away with the slop and potential wear problems found in Hitachis. The synchronization adjustment screw is very accessible (unlike Hitachis) making the synching process much easier. But the left hand idle set screw on the Hitachis is gone, with the result that there is no independent provision for adjusting synch at idle and again at higher rpms. In my view the best technique for adjusting synch on Mikunis, is to do it at 3000-4000 rpm, and then take what you get at idle. Usually it is close. The idle adjustment thumbwheel screw on the right carb moves both butterfly valves together, and is only good for setting idle rpm. This is nitpicking a little. Synch is generally not a problem with these carbs.
* In general, Mikunis are trouble-free carbs. I’ve personally seen a couple of float valves go bad (one leaked, one stuck closed) but that’s about it. Presumably the rubber materials used will withstand modern fuel additives (ethanol, etc.) and shouldn’t deteriorate if stored properly for the winter and otherwise treated reasonably.
* Parts are available from Yamaha and will be for some time. “Rebuild kits” are available in the after market, but here, as with Hitachis, you must be clear that you are getting the parts your really need. Dynojet offers jetting kits for these carbs which work pretty well when installed according to the instructions. When installing aftermarket pipes, the pilot screws should be uncovered (see the Hitachi article) and reset to (most likely) richen up the mixture a bit. See article on Aftermarket Pipes for more information on this subject.
* Factory service manuals should be available for all bikes fitted with Mikunis, and you should grab one if you are going to seriously do your own work. Try www.yamahapubs.com. They are not cheap, but very much worth the money in my estimation.
* Mikunis will mount up to pre 1988 700, 750, 920, 1000, and 1100 Viragos, and from 1983 on will share the same carb holder stubs. Some rejetting may be required to get the engine to run at its best. Expect some fiddling with regard to rerouting the breather vent tubes on the tops of the carbs, as well as the throttle cable, and some other small stuff.
For possible rebuilders see end of Hitachi carbs article.
Updated August 10. 2004