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Thread: Stumbling '81 R100RS

  1. #1
    Registered User plehman's Avatar
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    Stumbling '81 R100RS

    Hi all,
    Thought I fixed it... as I get on the throttle smoothly up to about 2800 rpm it stumbles. It is an R100RS from 1981. When I got it a year ago this problem was really bad. So I rebuilt the carbs at an airhead tech day, well, just replaced the needle and needle jet, cleaned it all up and things were good. Problem solved.

    Just went out in 90 degree weather, the bike had been sitting for a few weeks... and the stumble is back. Does not have a problem at very low throttle or .5 to .75 throttle and up. I really thought it was carbs, but could it be timing? I have not yet checked or replaced that system.

    thanks

  2. #2
    Liaison 20774's Avatar
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    If you were at a tech day, I'm guessing that you were able to get help to synch the carbs. Just about any time you dive into carbs, you will need to ensure that all parts of the operation - mixture, idle speed, cable tension - are working together. If I had to guess, the stumble is in the range of transition from the idle circuit to the tapered jet needle. Do you remember what clip position the needle was set at? I'm wondering if you were go to the next clip position, ie make the mixture richer, it would run better. It would also affect your mileage since you're increasing fuel usage at the mid range setting.

    What about the pulse air system? Is that still on the bike? It's been suggested by many that should be removed/defeated. Snowbum has info on his site.

    The '81 was the first year of the poor metallurgy for the valve seats. As long as you're keeping a close eye on the clearances, that shouldn't be an issue. But something to be aware.
    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 plehman's Avatar
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    Right, all good issues.

    - I did sync the carbs. Mixture, idle, balance at idle and 3000 rpms
    - I did remove the pulse air system. Really good to pull that junk off. Looks much better, should run cooler
    - I did set the valves. They seem to tighten up, maybe should do this again.
    - I carefully measured the new needle jet (47mm for the 1000cc 40mm carbs. That's better than thinking you have the right notch. Again, something I learned at Tech day).

    So I don't know. I'll keep running it.

    thanks

  4. #4
    Liaison 20774's Avatar
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    What about float height? You could be running a bit low on the fuel height, making it harder to pull fuel into the carb throat at a critical time.

    Snowbum's take on synch is to get the mixture right for each cylinder, that is max RPM for each side independent of the other and then go 1/8 turn rich on the mixture screw. Certainly balance at idle, with each cylinder pulling the same RPM when only running on one cylinder. Finally, balancing the throttle cables he does that just off idle, say 1500 RPMs or so. The response to vacuum is more sensitive at lower throttle/butterfly openings...the higher the RPMs, slight fluctuations in vacuum differences is less noticeable.
    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

    Gas Tank Ventilation

    As many have suggested before check to make sure your gas tank vent is OK.

  6. #6
    How many miles between the carb rebuild, which solved the problem at the time, and the return of the problem? And is this the first time it's been out in hot weather since then? And how recently has it been running well? The odd thing is that it is a sudden problem; carb issues usually develop slowly. As such, I'd suspect fuel flow or ignition.
    '61 Clubman's Gold Star, '13 690 Duke, '13 Daytona 675R, '14 Street Triple R, '17 1290 GT (gone but not forgotten: '76 R75/6, '84 R100, '76 R90S)

  7. #7
    Registered User plehman's Avatar
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    - under 200 miles since it felt pretty good. Yes, first time in really warn weather.
    - Float hight was set when I had it apart at tech day. I think it's good.
    - Have been troubled by the weird gas cap. The ratchet doesn't work. Wonder if this is messing up the tank vent.
    - I did find the carb and throttle duel cables sticky. I cleaned up the carb cable, but need to pull the throttle duel cable off and clean it. Might be something as simple as that.

    Have not been through the timing. New to me, too many things to sort out.

    thanks!

  8. #8

    stumble

    Most carburetor problems are ignition. If it's electronic ignition, I'd put some heat sink paste under the unit under the tank.90 degree weather problem?

  9. #9
    Registered User lkchris's Avatar
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    Carb rebuilding included new diaphragms hopefully.

    Valves tightening up is a sign of valve recision, a problem with these bikes. If you can, pull the exhaust headers and inspect the sealing surfaces of the exhaust valves. If they are at a knife edge, they're about to break causing a dropped valve and big damage. You didn't state mileage on bike. Mine were in that condition by 50K miles.
    P1180005.jpg
    Kent Christensen
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  10. #10
    Registered User barryg's Avatar
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    Valve recession is/was a common problem. Some times the valves have recession and sometimes the seats have recession. And sometimes both have recession. And sometimes neither have much recession. You were lucky if your bike had properly matched valves and seats and you got the 100,000 life span on your top end.

  11. #11
    Registered User plehman's Avatar
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    The bike has about 54,000
    Only just removed the PUlse Air so I'm worried about the valves. Will pull the headers soon to inspect.

    I did work on the throttle split cables... and it's a bit more responsive and better. Not sure why I was getting that stumble so badly that day, but it seems back to where it was (ie, fairly smooth).

    And I'm still getting used to the very tall gearing. The R100RS is so different from my 1976 R90/6. The bigger carbs, the taller gears, the narrow bars all make it different and sort of difficult to ride. Did I change topics? Actually, I think some of the problem could be the rider struggling.

    More miles seems to be the only solution.

    thanks!

  12. #12
    Registered User Rinty's Avatar
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    Quote Originally Posted by plehman View Post
    ...The R100RS is so different from my 1976 R90/6. The bigger carbs, the taller gears, the narrow bars all make it different and sort of difficult to ride...
    I had an '82 RS for 15 years, and truth be told, I always felt a bit on edge, riding it through tight twisties. You just don't have a lot of handlebar leverage with this bike. By contrast, carving corners on my '05 ST is stress free; the newer technology makes a big difference.

    But it was a pretty good design, for its time.
    Last edited by Rinty; 10-14-2018 at 02:53 PM.
    Rinty

    "When you don't know where you're going, any road will get you there."

  13. #13
    Quote Originally Posted by lkchris View Post
    Carb rebuilding included new diaphragms hopefully.

    Valves tightening up is a sign of valve recision, a problem with these bikes. If you can, pull the exhaust headers and inspect the sealing surfaces of the exhaust valves. If they are at a knife edge, they're about to break causing a dropped valve and big damage. You didn't state mileage on bike. Mine were in that condition by 50K miles.
    P1180005.jpg
    It's easier to just pull a spark plug and use your handy Minnie Mag-Lite to inspect.

    Woodgrain

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