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Mikuni conversion for Honda XR500
Turning ponies into horses
There is nothing wrong with the Keihin 36mm carburetor. It does its job very well for a very long time. It just needs a new air cut-off valve every 30 years or so...
However, if you want more oomph out of your XR500, fitting a Mikuni carburetor is how you get will the most bang for your bank!
Once you have started playing with less restrictive exhausts, then you really have to fit a Mikuni anyway to see the bike's behavior get transformed.
Do not get into hot cams, high comp pistons and head porting before you have fitted a Mikuni carb, this would be putting the cart before the horse.
I like to tweak my XRs with period parts, so I stay away from the more performant and modern set-ups such as flat sides and accelerator pumps. A good Mikuni VM 36 or 38mm round slide keeps it all the way it would have been back in the day.
Now the best source for a Mikuni VM is one that come from a 2 strokes bike. BUT, the settings of a 2 strokes are VERY different from what a 4 strokes needs.
Genuises and magicians have spent years testing and refining the settings to make a XR500 roar. I took a simple route to get my settings right.
Mikuni 38mm: Our Products
Mikuni 38mm: Image
I bought a XR500 tuned by Don Vesco that runs like a bullet train, took the carb apart, documented all the references, bought all new Mikuni innards from SUDCO and fitted them all to the other Mikuni VM carbs I had lying around (38 and 36mm).
So, here are, for your delight (and for free) the Mikuni VM38 settings that I KNOW work very well on my XR500's.
Mikuni 38mm: Image
Check out what happens when you fit the Mikuni in a XR500 engine... :)
By the way, to squeeze the Mikuni inside the frame of a 1981 XL500S with a 1982 XR500R engine, , I had to shorten the choke lever.
Note that the VM36 is plenty for the stock XR500, and the VM38 will be better suited for engines that have been worked on for racing.
Finally, in my case, these settings are combined with a very open air intake (foam Uni filter and no airbox), a very "free" Supertrapp exhaust and operation at sea level. On a stock set-up, the settings above will run too rich.
Mikuni 38mm: Products
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