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  1. #1

    Default Issues with Idle - Revs high when pull in clutch

    Picked up a mutt with an 80” rev tech and a mikunti carb.

    Backstory: Ran great when I first got it, but still did some TLC and cleaned the carb. Put it back together exactly how I disassembled it (or so I thought). Bike was running super high at idle and not with its original cadence, so I disassembled the carb again to check for any missed debris - was spotless. So I started messing around with the air/fuel mixture screws and ended up flooding the engine (was running super rich, learned after the fact). Couldn’t even get it to start the next day, checked the spark plugs (they were black) and swapped them out.

    Yesterday I put the fuel/air mixture screws back to factory setting (or as close as I could). Then tried starting it again - the engine was turning for me, just didn’t want to catch. Then I turned off the fuel valve and opened the throttle when starting it and I get her to run (FINALLY) but had to continually give it gas - it would stall otherwise.

    I then tightened the throttle cable until the bike was able to sustain itself on idle without me giving it gas. Zero slack in the throttle cable FYI. This at least let me ride it

    The issue: So here we are now - when I’m in neutral and rev the engine, it seems like it’s “catching” - the engine revs high and then slowly (over the course of say 5-6 seconds) comes back down to normal idle. It’s not a quick return to normal revs like it was. Additionally, the bike revs higher when I pull in the clutch. There is no more engine breaking, I’ll give it no gas without touching the clutch and I’m able to ride as if those things are engaged (in 1st gear).

    I have no idea what to do, it’s driving me nuts. Any suggestions on what I did wrong and how to fix this idle issue?

  2. #2
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    Wow. Where to begin?

    1) Using the throttle cable adjustment to set idle speed is very bad practice.
    2) You referred to idle mixture screws. There is one mixture screw and one idle speed screw (slide stop) only. If it's an HSR, there are also screw(s) to adjust the accelerator pump, but leave that aside for now. Use the slide stop (idle speed) screw to set the carb so the motor can stay running, and you can do the mixture adjustment.
    3) A throttle cable that drags or binds will keep the motor from returning to idle.Inspect the cable and the throttle control carefully to see that they are in working order.
    4) A vacuum leak will keep you from setting a consistent idle, and will keep the motor from returning to idle, and you may have a problem there.
    5) In the carb, a correct float level setting, and a good float, and a good needle and seat are needed to control the fuel level in the bowl. Without that consistent fuel level, all carb adjustments are an exercise in futility.
    6) There are very small passages in a Mikuni, including the pilot (intermediate) jet, and the air bleeds. Are you sure those are clean?
    7) The position and size of the slide needle governs the jetting over most of the rpm range in a Mikuni. Get that wrong, and you will definitely have tuning problems.
    8) Going back to #2, you can observe the slide, and the slide stop screw is the one that will raise and lower the slide from its lowest position.
    9) Again referencing #2, the idle mixture screw controls air in an HSR, if I remember correctly, and if the pilot jet is close to the right size, the mixture screw should be 1 1/2 to 2 1/2 turns out from seated, to give a good idle mixture. Turning it in makes the mixture richer, and turning it out makes it leaner. Outside of that range, a different pilot jet is needed.
    10) If the motor has been running good, and you clean and reassemble the carb properly, no jet changes should be needed to get the motor running good again.

    Jim

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    excellent ^^^(as always)...do you know which Mikuni you have?

  4. #4

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    It’s a TM42-6

  5. #5

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    Appreciate the detailed post Jim. So how do you recommend I go about fixing this?

    I’m thinking I return the throttle cable to its normal state (more slack). Then just toy with the mixture screw until I get it right...? (Although this is what I did last time that got me in trouble)

    You mentioned the float could be the issue and I also thought of that - how would I go about checking to make sure it’s working properly?

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    Quote Originally Posted by TheRealSpongebob View Post
    It’s a TM42-6
    tuning manual;

    http://www.nightrider.com/biketech/d...kuni_carbs.pdf

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    Yes, start with the throttle cable slack and use the slide stop screw to set idle speed. Then adjust the idle mix and see if the idle mix screw ends up 1 1/2 to 2 1/2 turns out. If not, you have a vacuum leak or a problem with your assembly of the carb.

    Sudco has a Mikuni repair and tuning manual that you need to get if you are going to work on this carb. Educate yourself, the carb is very straightforward in operation.

    Jim

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    Quote Originally Posted by TriNortchopz View Post
    ^^^ Ah, perfect. ^^^

    Jim

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