R90/6 hop up questions

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

  1. arbalest

    arbalest Long timer

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    I would like more HP for my sidecar rig. What suggestions do you folks have for upgrading the power of my R90/6? I am looking for reliable, street horsepower. I do not have unlimited funds, so I would prefer sensible modifications.
    #1
  2. Bigger Al

    Bigger Al Still a stupid tire guy Supporter

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    A Siebenrock 1070 kit is probably the best bet for a quick gain in HP. It'll set you back about $1,200, give or take.
    #2
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  3. Disston

    Disston ShadeTreeExpert Supporter

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    What final drive are you running? Side cars usually use bigger rear and so get more rpms putting more power to the rubber.
    #3
  4. tennessee thumper

    tennessee thumper Long timer Supporter

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    Good question, Disston.

    Is top speed the end goal or better useable HP on the existing platform.

    I'd be inclined to investigate/experiment with FD ratios and options before a full top end overhaul.

    This assumes the engine and other components, in their current state, are operating optimally and in a full state of tune.

    How a Mikuni or Dellorto carb change would help achieve the goal? Maybe an option as well but probably not useful for sidecar application?

    In the end the final drive seems to be a key component for achieving better power delivery for a sidecar rig.
    #4
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  5. arbalest

    arbalest Long timer

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    Disston and Tennessee, I am running a 37/11 final drive. 70 mph is about the fastest it likes to cruise at. I am OK with that. I am looking for better useable HP. The R90/6 never got better than about 35 - 37 mpg as a solo with 32/10 gearing. With the sidecar attached, I get about 25 mpg. Everything I've read about Mikuni's is that throttle response and starting is much better, but that fuel mileage suffers. I don't think I can afford worse fuel mileage.
    #5
  6. Achtung

    Achtung Wicked Pisssah Adventurer Supporter

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    What's your budget? As stated before a big bore kit is probably your best bet. You could also upgrade the cam and jetting for less money but more work but in the end I doubt you'll get much more performance out of that bike. I had a R90/6 with a big bore kit, dual plugged heads, electronic ignition and over sized valves, I always felt the performance increase was marginal, so now I just leave my airheads stock.
    #6
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  7. Jim K in PA

    Jim K in PA Long timer Supporter

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    Wonder what the difference is between the R90/6 engine and the R90S engine in terms of mechanical configuration, as well as power.

    Edit - other than the Dels, of course.
    #7
  8. robtg

    robtg Long timer

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    9.5 cr pistons. Intake stubs are different.
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  9. Disston

    Disston ShadeTreeExpert Supporter

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    Bigger valves are so often a disappointment because they only work better at high rpm. The standard sized valves are good. Big valves go racing run at peak rpm.
    #9
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  10. bmwrench

    bmwrench Long timer Supporter

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    What year is the bike?
    #10
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  11. thetubespoke

    thetubespoke Long timer

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    There's no way switching to Mikunis would improve things. Sorted Bings seem to work pretty well in my experience.

    I think you're looking at punching out the motor to get more power. 70mph cruising with a sidecar sounds reasonable off the top of my head.

    If you punch out the motor and then make your gearing taller, it *might* improve your mileage.
    #11
  12. Carbyhead

    Carbyhead Adventurer

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    If funds allow it I'd put a Siebenrock 1000cc kit in it (I'm not sure if the 1070 kit fits the 900cc crank case), the added mid range torque will make it feel like it's not working nearly as hard and with correct jetting and will possibly return better milage than the 900 getting it's neck wrung all the time.
    #12
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  13. airheadPete

    airheadPete Wherever they send me. Supporter

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    Large or small case cylinder bores? ‘74 or ‘75/76?

    (I was just planning to put later (‘81+) R100 cylinders and heads on my ‘76, but had to use those spares for a different project with damaged cylinder walls. :(: )
    #13
  14. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    The 1070 kit comes with a new cam, rods, and pistons/cylinders. The parts you want to put on the table and just stare at them. beautiful machining.

    I'd start with the rear drive first since you have options you can bolt-on and try as a comparison. For the engine, dual plugging with digital ignition. That will give you more grunt 1-3 gears, not so much 4-5. Experiment with the main jets and needle positions. Can also make a big difference. Even with a rig, you should get in the 30s for mpg IF you keep it under around 65. Over that, the brick effect comes at you and mpg will drop 10mpg.
    #14
  15. mattcfish

    mattcfish R90X Supporter

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    A 324 Motoren Israel cam or a 320/324 Asymmetric Siebenrock cam will give you more low end grunt and high end performance. A moderate compression increase to at least 9.5:1 is best.
    #15
  16. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    Depending on your budget, I have a newly built r100, big valve, lightened top end, digital ignition, tested engine you can drop in. Around 80 hp. Everything new.
    #16
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  17. arbalest

    arbalest Long timer

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    That sounds like fun, but I'm pretty sure is well beyond my price range.
    #17
  18. arbalest

    arbalest Long timer

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    R100 cylinders and pistons are available used for reasonable money. Would I gain anything with R100 cylinders and R90 heads? Bike is 76, so 99mm spigots.
    #18
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  19. tlub

    tlub Long timer Supporter

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    Wow. Our R90/6 with the 3.09 gets 43mpg on a really bad day. Headwinds, 2-up, luggage, 70mph plus. Around 48-50 is more normal, running about 65 mph. Those are US gallons. If you were getting 35-37 solo, I'd look for what is wrong. When you fix that, power may improve too.
    #19
  20. arbalest

    arbalest Long timer

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    So what do I look for? Bike starts easily, runs well. No flat spots, no surging, no sputtering, just lousy mileage. I have pulled the carbs for rebuild this winter. Besides diaphragms, gaskets, and o-rings, what else should I replace? What else do I look for, power wise? Bike has Boyer ignition.
    #20