The carburetor on the 1982 CV80J is not the same as the model installed in subsequent versions of Yamaha's 80cc scooter. My CV80 was equipped with a Mikuni VM13SS with the ID mark 5G3-01. The latter is a Yamaha OEM part number that is based on the IBM number of the scooter for which the carburetor was built—5G3 suggests the 1981 CV80E—and a revision number. Yet according to Yamaha's parts catalogues, the carburetor that shipped with the 1983 CV80K and 1984 CV80L was also a Mikuni VM13SS. But it had a different ID mark, 13T-00. There were other significant changes—so many, in fact, that in retrospect either Yamaha or Mikuni should have changed the carburetor's model number to avoid confusion. The jets on the CV80K and CV80L were different, the cutaway on the throttle valve was deeper, the starter plunger was positioned on the top instead of on one side, a different float was used, and the throttle cable connection was new.

Yamaha compounded the uncertainty about carburetors for its CV80s when it advertised models for the European market, where product literature indicated the 80cc Beluga shipped with a Mikuni VM16. I'd like to see one. But that's another story.

All available evidence suggests that Mikuni Japan probably built the carburetor specifically for Yamaha's scooter applications, and then forgot all about it. According to one expert I consulted, Mikuni "only starts talking about carburetors at 18mm," which means you won't learn anything useful concerning the VM13 (or VM16) by asking Mikuni or its overseas distributors. I know—I've tried. Parts for the CV80 carburetors had to be ordered directly from Yamaha, which is notorious for its poor support of older products and its scooters in particular. So owners of a CV80 are left in limbo. Neither Mikuni nor Yamaha will acknowledge any inquiry about the VM13SS.

The news is not all bad. I can order gaskets and a float valve for the Mikuni on my scoot, but other vital components are hard to find. For example, the rubber starter plunger diaphragm that upon inspection invariably appears to have been reduced to a gooey glob of black tar (which when exposed to cool air hardens to a deformed circle only vaguely resembling the original part) is unobtainable. So much, it would seem, for a part that is essential to the automatic starting circuit on your CV80. Be careful then with that diaphragm: if it's in reasonable condition, keep it that way or consider its potential as a prized listing on eBay the next time you're hardpressed for cash.

Other OEM parts that are difficult to replace include the main jet, pilot jet, needle jet, jet needle, starter jet, and throttle valve assembly.

So what is one supposed to do to overhaul or repair an original type 5G3-01 Mikuni VM13SS? You can convert to Buddhism for starters, which is not a bad idea for anyone and for aspiring scooter mechanics in particular. But you might be served just as well by remembering Robert Pirsig's advice in Zen and the Art of Motorcycle Maintenance: "Assembly of Japanese bicycle require great peace of mind." At the very least you should start by getting to know your carburetor inside and out.

THE BITS AND PIECES
I will skip detailed instructions or advice about how to remove the carburetor from your scoot. Just use a sensible amount of care and prudent safety precautions while detaching it from the reed valve assembly and air filter and then getting it to your workbench. Depending on whether or not you've run your scoot recently, you might find petrol still in the float bowl.

Most parts on the carburetor can be removed easily (I will post pictures soon). Unscrew the float bowl and remove the gasket. Then carefully remove the pin holding the float and detach the float itself while using some care to avoid damaging the wire that holds the valve dangling from the float arm. Set the float and other pieces aside in a safe place. Next, remove the starter diaphragm cover. As suggested earlier, you will probably find that the starter plunger assembly inside is damaged (if it isn't, you've phenomenal great luck: go out and buy a lottery ticket now). Carefully peel the diaphragm out of the carburetor body. The starter jet might be stuck, but that's good. It is simply jammed into the carburetor body with a friction fit. Put a shop towel and some pliers on it and tug gently until it comes out. To remove the carburetor from your scooter you have to disconnect the throttle cable from the slide, which means you have probably disassembled the cable housing already. But now you can remove the gasket inside and the throttle slide itself. The air and throttle set screws should come out easily as will the main jet, which can be removed with a slot-head screwdriver.

You could stop there, as I did the first time I cleaned up my carburetor. But later I decided to tackle other parts so I could be more certain of their condition.

LIKELY PROBLEMS
One of the first difficulties I encountered was getting the pilot jet out of the main body. I tried soaking it with carburetor cleaner and other solvents, but it wouldn't move. I took the entire carburetor with me when I first visited a helpful chap I'd met online via a local motorcycle newsgroup, and he coaxed the jet out by carefully applying some heat using a plumber's torch. Another essential tool was a jeweller's slot-head screwdriver that fit snugly into the shaft that holds the jet. But even the heat trick is not always a reliable remedy. I tried it with another carburetor with a stuck pilot jet and for my efforts was rewarded with a new plumber's torch (which I needed anyway) and a jet that still wouldn't move. Eventually after consulting with another scooter guru I knew only by email, I drilled out the jet on that carburetor using an extractor bit, a variable speed drill on a low speed setting, and a great deal of care. The jet was ruined in the process, but at least it was out of the carburetor body.

My next foray into forensic "carbology" was to remove the needle jet. The same guru (whom I have affectionately dubbed "Cyrano de Beluga" in recognition of his friendly and insightful correspondence) sent me a picture of where to insert a nail punch on the venturi side. Initially I used one that fit over the top of the jet and then a smaller one, followed by other items I won't mention (to mechanical purists I have already said enough to sound like fingers on blackboard) to tap it out. I was glad I removed it. On one of the VM13SS carburetors I own, the needle jet is badly pitted. I discovered, too, that a number stamped on the side does not match the one given in Yamaha's factory service manual. Instead of D-8 as I expected, I found "3306" (or possibly "3308"—it's hard to read even with a magnifying glass).

The fuel inlet to the starter jet was clogged on my carburetor. There's a small brass fitting at the bottom of the float bowl. I cleaned it out until finally I could see a pin-hole of light. I don't know how the scooter started with that inlet blocked.

Finally, I used a bit extractor to get the float bowl drain screw out. On both VM13s I own, the head was stripped to the point that no screwdriver could be used. I discovered that the drain screw is identical to the air screw, complete with spring.

NOW WHAT?
Some carburetor cleaner, fine steel wool and a small brush will help you to restore most of the carburetor parts (some pro shops use an ultrasonic jewelry cleaner with a heated cleaning solution, but that's beyond me right now). If the carburetor as a whole is salvageable you will want to replace the gaskets. Inspect the float valve for wear. Make sure the rubber tip is intact and that the spring-loaded end moves freely. The narrow wire hanger should attach fully to the valve just above the spring-loaded part.

Gaskets and the float valve can be replaced. Depending on its condition, you might be able to repair the starter plunger diaphragm (see the article about the automatic starting circuit for additional comments on this subject). If you want or need to replace other parts (so far my list includes a new pilot jet, needle jet, jet needle, and throttle slide), you have a lot of detective work ahead of you. I certainly haven't had any success with carburetor dealers in North America (I might try Japan next), but at least now I know a bit more about the heart of my scooter.

Another option is to install an aftermarket replacement carburetor. For that you need to consider the diameter of the openings where the carburetor attaches to the air filter and reed valve assembly, the number of air hoses the CV80's system uses, where the oil and fuel lines attach, access to the air and throttle set screws, and the space in which a carburetor must fit. You will probably want to consider also the impact of putting a larger carburetor on your scoot, given that it's unlikely you will find anything (new, at least) with a venturi size under 18mm. I'll explore this subject later.