Rejetting to Mikuni VM's

cswanson · Full Member Joined: Mar 28, 2007 Posts: 39 Location: KS

I'm getting new 34mm VM's for my 81 xs650. What do i need to do to rejet the carb?

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

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grizld1 · Posted: April 20, 2007, 12:08 am Post subject: Re: Rejetting to Mikuni VM's

5twins is right on target. If you buy cheap you'll get the generic factory components with maybe a change of pilots and mains. Your bike can be made to run on that, but badly. You're better off paying more and getting application-specific components. If you buy from 650 Central you'll get follow-on tech support from one of the best tuners in the business.

Nobody can tell you how to jet carburetors in one brief post. Sudco's Mikuni Tuning Manual is a great starting point; you might want to order a copy with your carbs.

royfisk · Posted: April 21, 2007, 2:44 pm Post subject: Re: Rejetting to Mikuni VM's

cswanson every bike is different, what filters your useing , compression, manifolds, etc. I too would like to find a starting point for a basic carburator set-up. It seems like no one will tell what they are running in there bikes. before I give anyone 25.00 for a starting point I think I will run my bike down and get it dyno tunned. Thats 25 for a starting point and then more cash each time you change a jet. Heck it would be worth it just to stick it on a dyno with a tuner there and do actual runs to your exact bike and take ALL the guess work out of it. theres a reputal tuner near by here that said he would do it for me for $150.00. Once I get it done I will post the results and what needles , jets , filters etc. I used. FOR FREE, AND NO ONE WILL HAVE TO BUY A THING FROM ME.

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

grizld1 · Posted: April 21, 2007, 7:38 pm Post subject: Re: Rejetting to Mikuni VM's

Roy, the variations from bike to bike and for altitude tend to be in selection of mains and pilots. Once you get throttle valve (slide), needle and needle jet selection down, you have most of it. VM34 and VM36 carbs tend to work with remarkably similar components: TV: #2 or #2.5; NJ: 159-P5 or 159-P6; needle: 6F9. I used the 6DP1 for awhile, but the upper midrange would never quite clean up--either surging or breaking up a bit on the rolloffs from WOT to 3/4, regardless of what I did with clips or mains.The 6F9 is the most popular needle in use in VM-equipped XS650's for a reason--it works.

ost stockers with stock headpipes and some kind of muffler and efficient air filters will take a main in the neighborhood of 180 and a pilot in the range of 20 to 25. If you're riding at significant altitude, you might want to work from the lean side of the range.

y specific set-up is as follows. Altitude: approx. 400ft. Engine: 77.5 mm. pistons, approx. 9.5:1 compression, Uni pods, MAC 2-2 with restrictors and straight-through glasspack baffles, ported head, Shell #1 grind cam. Carbs: VM36, as follows: PJ: 22.5; TV: 2.5; NJ: 159-P5; JN: 6F9, clip in the middle; MJ: 200. Fuel consumption with hammer down: 48 mpg.

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