Just picked up my first motorcycle; Honda carb tuning?

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  • gearheadE30
    No R3VLimiter
    • Jun 2007
    • 3734

    #1

    Just picked up my first motorcycle; Honda carb tuning?

    So I've been having this itch to pick up an older Honda motorcycle for some time now, but I never have, partially due to cost and safety, and partially out of respect for my parents' request for me not to buy a bike. Well, I found a good deal and my parents are as okay as any parent could be about it, so this past saturday, I loaded up the Tahoe with a horse trailer and drove to Ohio to pick up two Honda CM400s.

    Basically, this is the exact style of bike I wanted, and I wanted something smaller, so this was about perfect. According to the all-knowing intarwebz, they also make good starter bikes. Easy to work on, too. Just one issue-carb tuning. It runs well up to about 3k revs and then is an absolute riot from 6k up (redline at 10k), but it can hardly make it through the rev range between... I know its an e30 forum, but worth a shot. Any ideas?

    BTW the black bike is a 1980 CM400t, 9000 miles on it. The reddish one is a rather rare 1981 CM400 Custom, with air-adjustable (and blown) forks, reverse comstars, and much bigger 2-piston brakes. It, however, is a parts bike due to a lack of title. the T will be the rider, and I'm probably going to use the best of the parts to put it more or less back to stock, plus the big brakes and cooler comstars. the Custom has a good engine and trans, and the carbs are good but gummed (and not on the bike).

    So yeah. I'm excited. just gotta get insurance and turn signals and brake lights on the black bike and I'll be good to ride!







    Project M42 Turbo
  • LJ851
    R3V OG
    • Nov 2010
    • 7918

    #2
    Ive been a professional motorcycle mechanic since the lat 90s. I would start with making sure the carbs are clean (jets, etc) . This will prob sort your issue.
    Lorin


    Originally posted by slammin.e28
    The M30 is God's engine.

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    • gearheadE30
      No R3VLimiter
      • Jun 2007
      • 3734

      #3
      Okay, will do. Apparently these carbs have the normal pilot and main circuits, but have an additional set of jets for the midrange... looks like that's what's jammed. The left cylinder seems to do okay in that range, so it might even just be the right carb, although I'll end up tearing them both down to be sure. A lot different than the other lawnmower-ish carbs I've rebuilt, hah.

      Project M42 Turbo

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      • lennon
        E30 Fanatic
        • Jun 2008
        • 1416

        #4
        Yeah, clean the carbs WELL. I use a guitar string to run through the jets. Set floats, etc. All normal stuff.

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        • Restoman
          E30 Fanatic
          • Jun 2006
          • 1311

          #5
          Break the carbs down and dip. Run some fresh gas through them and you should be good. Don't take me too seriously, but I want to say Honda's ran a bit lean in the early 80's due to emissions.
          i'lldoitforacaravan

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          • Dozyproductions
            R3V Elite
            • Jan 2007
            • 4682

            #6
            I smell cafe racer!

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            • gearheadE30
              No R3VLimiter
              • Jun 2007
              • 3734

              #7
              Restoman-right on the money. They do run pretty lean. For now I just want to get it running like stock, and then I may change out the mains for a size or two larger. Apparently the carbs are a bit complicated on these bikes, and I don't really want to get them too out of wack.

              And eventually, I probably will end up going cafe racer, as I love the style, but for now I just want to bring it back to stock, oem+ kind of deal and ride it around for awhile as a comfortable road bike. It'll probably be my main transportation to work in the summer. It's really a very comfortable bike to ride, even with the basically unpadded flat seat on there now.

              EDIT: oh, and the top pic is the most recent pic I have, with the two below it being basically right after I unloaded them.

              Project M42 Turbo

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              • gearheadE30
                No R3VLimiter
                • Jun 2007
                • 3734

                #8
                Two more for a good measure:



                Project M42 Turbo

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