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Thread: New Airhead Owner - Needs Help with Mikuni Carbs

  1. #1
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    New Airhead Owner - Needs Help with Mikuni Carbs

    Fellow airheads, I am a new proud owner of a 1982 R65. This is my first motorcycle so I am not entirely sure what I am doing. I live in the Manassas, VA area. I got an aerospace engineering degree and have no idea how engines work, so here I am. Besides this project, my free time is focused towards skateboarding and rock climbing. Im interested in meeting other airheads so let me know!

    Here is a picture of the bike:


    A quick description (from seller):
    16000 Miles. Features upgraded Marzocchi shocks, beefy K100 Forks, 34mm Vikuni VM Carbs. Bike has been serviced including: top end refresh, valve adjustment, final drive service, carb sync, etc. (but I am assuming none of these have been done).

    I got this bike from a little shop in DC. Owner said it has been sitting for a long time.

    Current things I have done:
    Valve clearances - these were completely closed on the right side
    Check spark plug arcs
    Empty and fill fuel system - there was leftover fuel
    Carb clean
    Here is where I'm at right now:


    The bike is able to start with Enrichment: ON.
    Idle backed out: 1.5
    Air mixture screw out: 1.5

    However, I need to give it some throttle and rev to about 2-3k rpms to keep it alive.

    If you notice in the video, the left side is popping and there is light coming from the intake side. What do you think is causing this?



    You can see the light coming from the intake here. Is this the arc from the spark plug (misfire) or actual combustion?

    Attached Images Attached Images

  2. #2
    Liaison 20774's Avatar
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    Welcome to the forum! Congrats on the new bike! Now the learning begins. Where are you located? Filling in your profile will help people see where you are and find some local assistance.

    You said carbs clean... what does that mean? My first thought is that the carbs need to be overhauled and jets, settings verified.

    What are the blue things? Semi clear tubes? Seems like you're getting some upstream ignition...not normal. Maybe it's so rich, it's lighting off. Could be spark plug cap arcing.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Registered User Anyname's Avatar
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    You may want to do a Google search on Mikuni conversions for BMW R65. If you can find a vendor who sells kits, they may have information on jetting and starting points for adjustments. I took a look and noticed references of Mikuni 34MM conversions for R100s. It's possible that 34mm is a bit large for a R65.
    BMW R bike rider, horizontally opposed to everything...

  4. #4
    Liaison 20774's Avatar
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    You might also consider becoming a part of the R65 online community:

    https://www.bmwr65.org/

    If you get some info on what works/doesn't, please share with both forums!!
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  5. #5
    Liaison 20774's Avatar
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    I'll throw out this toss up! Is it possible that the left cylinder is pre-igniting and the flames are shooting out the open intake valve. Discuss!
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  6. #6
    Registered User 6322's Avatar
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    If valves are adjusted correctly, it looks like your timing is off. It may be too far advanced.
    Gary Phillips - #6322
    Wildland Firefighter, Retired, Riggins, ID
    Heartland Moto Locos BMW Riders
    '77 R100/7 Dirt Hack, '83 R80ST, '85 K100RS w/EML, '00 R1100RS

  7. #7
    I would go for low hanging fruit first. Once you have a manual and can identify the pilot jets, pull and clean both. You can usually get at them by loosening the clamps that hold the carbs in place and pivoting them a bit so you can see the jets once you remove the float bowls. Most manuals say never poke a wire through the pilot jet hole. After soaking them in carb cleaner, I'd poke a wire through the hole; just make sure it's small enough and copper and don't force it.

    I put a Mikuni carb on a DR350 and often had a similar problem.
    '61 Clubman's Gold Star, '13 690 Duke, '13 Daytona 675R, '17 1290 GT, '18 Street Triple RS (gone but not forgotten: '76 R75/6, '84 R100, '76 R90S)

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