Mikuni flatside, external adjustment (idle)

4 replies to this topic
  • pjpoo

Posted March 06, 2007 - 03:38 PM


My previous threads on this 41mm Mikuni flatside carburetor for my '94 XR650L Supermoto here:

Hooking up the throttle cable turned out to be fairly easy. The brass colored fitting inner diameter was supposed to be large enough for the cable stop to pass through, so I had to drill it out.

Everything hooked up well after that without issue and the throttle cable also worked well. Here's two pictures of my setup:

Posted Image

Posted Image

The first start took a bit of cranking, one I'm assuming to get the fuel flowing (intake is lower than petcock, but the hose routing has an upwards bend). I pulled the choke up (left side of carb). It started immediately. Then idled hesistantly at lower rpms. I pushed the choke back down and it idled faster. Throttle response was crisp! At least when compared to the dying and bogging tendancies with the stock carb and setup.

However, it seemed to idle a bit too fast. I assumed the screw on the right side (just above the pink hose) was for the idle adjustment speed, since it's the only external adjustment available on the carburetor. Turning the screw far left or far right had no noticable effect on the idle.

So, what is this screw supposed to do and how do I adjust the idle speed of this flatside? Also, I remember choke idling faster than no choke. Did it idle slower because it's fairly warm outside at 70 degrees?

My next response will include the internal setup of this flatside as provided by XRs Only.

  • creeky

Posted March 06, 2007 - 04:07 PM


Check the throttle cable for sufficient slack. You're correct, the knurled knob above the pink hose is the idle speed adjuster.

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  • goblin127

Posted March 06, 2007 - 04:49 PM


I had the same problem,it was the throttle cable got hung up on the fuel tank mounts. The nurled brass knob is for the idle adjustment,and mine was way off also. Other than that the carb has been great. When you switch to reserve watch out,you don't run out of fuel quicker. Because of the way the fuel line is routed into the carb it has a tendency to run out well before it should. The best I could do was to keep the line as short as possible.

  • Srad600

Posted October 28, 2009 - 07:40 PM


Humm, thought I'd resurrect this thread rather than start a new one. I'm running a 42mm Mikuni pumper on an XRL650 (big bore, uni filter, etc) and I'm having the same issues as those above.

Idle speed only drops down to 1900RPM no less, no matter what I do with the idle speed adjuster? :bonk:

BTW I've checked the throttle cables and they don't seem to be hung up on anything, and have a good amount of slack?

Has anyone discovered a work around for this? I don't suppose it'll kill me to leave it as is, but the normal idle speed is around 1200RPM and i'd like to at least get it down to 1500 if I can. :bonk:

Edited by Srad600, October 28, 2009 - 07:41 PM.
Added information

  • bigredpig

Posted October 28, 2009 - 08:48 PM


the carb in the piture is a that a tm40? screw on the side is the idle. your tm carb is all about the needle so I hear.

srad come over and we can look at that hsr.

and here is a good read for you guys.

hope you work it out fast. I love my hsr42mm!

Edited by bigredpig, October 28, 2009 - 09:10 PM.

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