R90/6 hop up questions

Discussion in 'Airheads' started by arbalest, Jan 27, 2021.

  1. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    Did you also replace the needle and atomizer
    #21
  2. arbalest

    arbalest Long timer

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    I have not replaced anything yet. So, I should replace the needle and atomizer? Bike has about 32k miles.
    #22
  3. bmwrench

    bmwrench Long timer Supporter

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    I'd look at ignition timing. Boyers often have slow or short advance, and this can really affect mileage and power. However, one should expect poor mileage with a sidecar rig.
    #23
  4. arbalest

    arbalest Long timer

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    Mileage was lousy before the Boyer was installed. I do expect low fuel mileage from a rig, but it was low before the rig was installed.
    #24
  5. mattcfish

    mattcfish R90X Supporter

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    The R90 heads work great, but you need to either widen the squish band (best solution) or buy the Siebenrock kit with stepped pistons. Bigger valves actually rob power at most of the rpm’s you deal with on a street bike because the velocity of the mixture is reduced and the valves are heavier and have less piston clearance. This increases their tendency to float and do damage. Especially true with a cam.
    #25
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  6. arbalest

    arbalest Long timer

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    Update. Replaced the jets and jet needles in the carbs. The rig now gets 36 mpg with a 37/11 final drive. Now I am told by my son, who was riding behind me, that the bike is smoking blue smoke. Looks like a top end job is in the works. That said, how deep should I go?
    #26
  7. bmwrench

    bmwrench Long timer Supporter

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    Here's your chance/excuse to go to a bigger bore.
    #27
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  8. Kiwiabbo

    Kiwiabbo Long timer

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    On my chair bike I run 100 cylinders with r80 heads on it. No modifications and I get very good low down power. Runs out of puff a little on the top but you are not doing that on a chair bike. Still have the 33/11 final drive and it sits very nicely at 70mph.
    #28
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  9. arbalest

    arbalest Long timer

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    With a 37/11 final drive, I cruise at 70 at 5500 rpm. I assume that with the R80 heads and R100 cylinders I would still be able to do that? Did you rejet the carbs? To 32mm R100 jets?
    #29
  10. Kiwiabbo

    Kiwiabbo Long timer

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    135 main. I’m cruising at about 4000k
    #30
  11. arbalest

    arbalest Long timer

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    I guess that's the difference between 37/11 (3.36:1) and 33/11 (3.0:1). What are you running for wheels/tires on the tug?
    #31
  12. Kiwiabbo

    Kiwiabbo Long timer

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    Standard wheels and tyres at the moment but may look at the metzler sidecar tyre for the back when it needs changing
    #32
  13. arbalest

    arbalest Long timer

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    Changed the needles and needle jets. I am quite happy with the mileage I'm getting. With the lower gearing and sidecar attached, I'm getting the same mileage I got without the sidecar and taller gearing.
    #33