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  1. #1

    Join Date
    May 2018
    Posts
    4

    Default Mikuni HSR42 dies out when going from 1/8 to 1/2 throttle

    Hey all!

    I did a few searches but didn't find anything specific to this issue. I've been running a Mikuni HSR42 for about two years with no issues on my '92 1200. Recently, the bike died when riding it, and I traced it to a bad coil and replaced that (That may not have been the only issue). Bike ran fine for about a week, when it started to die out when pulling on the throttle.

    I've pulled the carb apart, cleaned the main, pilot, and needle jets. I've adjusted the accelerator pump and stop screw, as well as adjusted the air-fuel mix screw.

    It still dies out when going from about 1/8 to 1/2 throttle.

    Here's a video of it running (idles decent, but not as good as when it was running right): https://i.imgur.com/MIY0cob.mp4

    I've got a jet kit on the way from JP Cycles, and my next step is to play around with different sizes, however it doesn't explain the sudden onset of the issue.

    Any advice or info is appreciated. Thanks!
    Last edited by Mandirt; 09-01-2020 at 5:55 PM.

  2. #2
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    Join Date
    Dec 2018
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    I think you still have an ignition problem.

    Jim

  3. #3

    Join Date
    Jan 2013
    Posts
    138

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    I haven't run a mikuni, but I like carburetors and tuning them. When you say it's been running good for two years I would say it's probably not in need of rejetting. Did you change to a different air cleaner or change anything else about fuel delivery? I would be inclined to clean all passages again, and check for crap in the needle and seat. Sometimes if you pull a rubber hose off a barb fitting it can tear the inside of the hose and when you put it back together you get rubber bits in the float needle seat.
    Another clue is does it backfire through the carb when the problem occurs or does it just get weak and blubbery? Backfire through carb is usually a lean condition, blubbery is usually too rich.

  4. #4
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    Timing advance maybe?

  5. #5

    Join Date
    Oct 2018
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    342

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    If it was running right for two years then I wouldn't bother with rejetting the carb, your issue wouldn't be a sudden need to readjust the air/fuel mix. What ignition are you running and does it use a mechanical advance? Does your carb normally have a mounting plate that pushes it onto the intake manifold boot and hold it there? If not have you checked for vacuum leaks at the intake? Have you checked the fit of the slide and whether it's worn? Flat slides can hang up or stick within the carb body as they age, especially on motors that vibrate quite a bit.
    Last edited by Skjoll; 09-08-2020 at 5:38 AM.

  6. #6
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    I replace intake seals when troubleshooting as a matter of course (they're cheap and dry out when they age), but sudden problems with bikes that had one ignition problem can often be caused by related parts. 1992 is ancient history (I still had hair and could see my toes without a mirror) so I'd suspect the ignition module, pickup or both.

    When I get a new-to-me HD that elderly it gets a replacement ignition if it's still on the original parts so I get a fresh start instead of riding until failure. If your bad coil drew too much current before it died it could have damaged the ignition. Pickups can gradually fail too and thermal failures may not show up in testing.

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