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Thread: Mikuni carbs

  1. #1

    Question Mikuni carbs

    I need some help in setting up Mikuni 38 flatslides for an R100 (had Bing 40's) with 8.2 compression ratio. I just cant seem to get the jetting right for 3/4 to wide open. Anyone know how to do this?

  2. #2
    Loose Cannon flash412's Avatar
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    Quote Originally Posted by felaw
    I need some help in setting up Mikuni 38 flatslides for an R100 (had Bing 40's) with 8.2 compression ratio. I just cant seem to get the jetting right for 3/4 to wide open. Anyone know how to do this?
    3/4 to WFO is running on the main jets.

    Find a long uphill stretch. Install BRAND NEW spark plugs. Ride the bike WFO up the hill. When you get to the top, hit the kill switch and coast to a stop by the side of the road. Remove the spark plugs and "read" them. (This is called doing a "plug chop.") To read a spark plug, you want to hold it so that the sun shines WAAAAY down inside so that you can see the very deepest part of the insulator. If it is very dark brown, install smaller main jets. If it is very white, install larger main jets. If it is tan, or wet, then maybe your problem is not actually jetting; perhaps it is spark plug caps. It is considered prudent to change jets by only one size at a time. Install brand new spark plugs and do another plug chop.

    Write EVERYTHING down each time you make a change. There is NFW you will remember it all.
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  3. #3
    Flash, thanks for the advice. I am going to have trouble with the chop, its 20 degrees with 6 inches of snow and the nearest long hill is 20 miles away. I was hoping to avoid some of the hit-and-miss by finding someone who has already jetted a set of tm 38s so that I can get an informed idea about where to start. Just to get the thing to run and idle properly I had to spend the better part of 2 days fiddling with the idle and needle jets, road testing and more fiddling. Frankly, I don't really know if the needles are in the right position, the airbleeds, needle/main jets and slides are anywhere near the right size, etc. I didn't think it would be this tough, maybe it's just me. Thanks again.

  4. #4
    Sir Darby Darryl Cainey's Avatar
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    Hey Felaw;
    Why don't you just put a Turbo on it!!
    Haw - Haw - Felaw
    Ambassador Emeritus BMW MOA Ontario Canada
    Niagara Riders
    Knights of the Roundel #333
    1977 R100RS, (Retired) 1993 R100GS

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