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4 stroke carb tuning - big single cyl

Discussion in 'Technical and Troubleshooting Torque' started by SHEPPO, Jul 3, 2010.

  1. #1 SHEPPO, Jul 3, 2010
    Last edited by a moderator: Jul 13, 2015
    G'day all,

    After some pointers...

    My XR600R donk is so close to starting it's killing me. I have spark, i have fuel, and i have plenty of air. The motor is a twin carb unit, with a pair of 32mm mikuni flat slides on it. It also has a big bore kit (660?), aftermarket bottom end, hotcams stage 1 camshaft, aftermarket high energy coil, and aftermarket CDI. This engine has ran on these carbies (with current jetting) before. Only change since then is the camshaft.

    When i follow the usual "XR" starting sequence, and give it a decent kick, it will get some combustion, however it's only slight and wont bark into life and tick over. [URL="
    ]This vid[/URL], about 56-57seconds in is what i get... a little cough/splutter 2 or 3 times then nothing.

    I have the idle speed screws backed off fully, so only the smallest but of the slide cut away is visable. I think i need to increase this a little is my gut feeling.

    If not, any suggestions?

    I just want to get it to start, then I'll worry about tuning the rest of the RPM range.

  2. Given a great deal of care, I'd be inclined to try a tow start. In my experience, once an engine has run once, it gets easier to start from there on, even if nothing is apparently changed.
  3. well i got it to start. opened up the slides another 0.5-1mm and the thing barked into life! gut feeling was right, for once.
  4. Good to hear, sheppo. Sounds like this thing will be a bit of a demon...

    Keep us posted ;D

    - boingk
  5. ok, so it starts more times than it doesnt.... not bad for a Honda XR600R if you ask me. but now comes the tricky bit of tuning it from scratch.

    the bike will run at a fast idle with the choke on (only one of the 2 carbies has an active choke). when i drop the choke off, it dies pretty much instantly.

    would this suggest that the mixtures (with the choke off) are too lean?
  6. Yes, unless someone has connected the choke to the kill switch.:angel:
  7. the kill switch & CDI wiring is another (long) story, hahaha.