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Thread: VM/Mikuni carb...only starts after adjusting the air screw

  1. #1

    VM/Mikuni carb...only starts after adjusting the air screw

    Hey guys, I have an 82 SR500 that has a VM carb. Had it for a few years. Its always been a bit hard to start (harder than I think it should at least). This past spring it got to where it would not start at all. I found that if I turn the screw (air screw?) in all the way (the one on the choke/left side) that it starts right up...but idles crazy high...I can then immediately turn that screw out down to idle speeds and its fine, and also will restart easily after all of this. But its back to the same thing once its cold.

    I have pulled the carb and cleaned/cleared the main and slow jets, and everything looked cleaned, so I think the internals on the carb is OK.

    Is there a stock adjustment for these screws? There is one on the right side also. If there is not 'stock' adjustment, what is a good starting point to work from? Obviously, I'd like to find the right setting to where it will start easily...but not rev itself off the center stand when it does.

    Anything else I should be looking at besides the settings on these screws?

  2. #2
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    If I understand you correctly you are raising the slide by turning the idle speed adjustment screw all the way in to get the bike running. Then you are adjusting the slide again after it is running to get your proper idle speed and then it is good until your next "cold" start.
    One thing to look at before you get into carb adjustment is to verify that you have the correct rocker arm to valve clearance.A too tight valve gap will reduce the vacuum effect through the carb at kick over speed making kick starting difficult.Do a compression check as well for the same reason.
    Since there are so many air box/filter and exhaust combinations it is difficult to give anything but ball park jetting advice,but Stew Ross uses 38mm VM and has posted his successful specs on this site.
    Last edited by bmonk; 09-05-2017 at 04:02 PM.

  3. #3
    Senior Member MadMax's Avatar
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    Have a look through this vmmanual.pdf then come back and tell us which screw you are adjusting, if you need to adjust the slide to get it to start then it sounds way rich?

  4. #4
    Thanks for the pdf MadMax. So yeah, it is the Idle Adjusting Screw (#32) that I am fiddling with. So is adjusting this the same as cracking the throttle a bit?

  5. #5
    Senior Member MadMax's Avatar
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    Assuming if you're having to lift the slide then you're not using any choke for cold starting? Very unusual, having to lean it off for cold starting? How does it run once under way?

    What size VM have you got? (there's some dimensions of the various sizes in that PDF)

    Max

  6. #6
    Don't know if this will be of help, but I had a situation where mine just wouldnt start.
    I push-started it--when it was running, right away, before it got warm, adjusted the idle to about 1250.
    Then adjusted the mixture screw the usual way. Turn until revs pick up then leave it. I have to just
    live with the fact it doesnt have a perfect idle in some situations if I want to start easily, 1 or 2 kicks.

  7. #7
    I am using the choke to start, and no, I am not cracking the throttle to start...I always read not to touch it during starting so I don't. I was only asking about cracking the throttle because that screw is called "Idel Adjusting" screw...guess that is not the same as "throttle adjusting". It runs great once it starts...great power through the enitre rev range.

    I honestly don't know what size the carb is...I'll look for some identifying mark(s) or measure the bore and report back.

  8. #8
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    Make yourself a simple idle up lever from a piece cut from a nylon bread board.
    With the right size hole (size is 7.5mm) it will push straight on and stay on.
    For a hot start just move it up 1/4 turn and for a cold start with choke (primer kick) open it about half turn.

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    Adding power makes you faster in the straights.... adding lightness makes you faster everywhere: Colin Chapman

    There is a quintessential elegance to riding a single cylinder machine: OCR

  9. #9
    Or, you could just bring me the bike or just the carb and we can give it a once over. If you're interested, I'll PM you details.

  10. #10
    Senior Member MadMax's Avatar
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    There's an offer you can't refuse, be sure to post up details of what was wrong & what you changed.

    Max

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