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Thread: 1987 R80 rough running above idle

  1. #1
    Registered User harryhendo's Avatar
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    1987 R80 rough running above idle

    Hi Folks,

    My 1987 R80 (which I bought new in 1987) was sitting in a barn for 10 years, and I've been getting it back in shape. That meant rebuilding the Bing carburetors and also clearing out the mice damage to the airbox. There were also some frayed/nibbled wires, which I replaced.

    The bike starts and idles just fine, but when I increase RPMs to around the 3000-5000 rpm range, it runs roughly, surging and skipping.

    I tested the battery's output (which is 12.5 volts, normal), and the voltage delivered to the ignition coil's #15 terminal (11.8 volts), which seems fine. I then tested the voltage between pins 2 and 4 of the ignition amplifier (the black module mounted on a heat sink with 6 inputs) with the ignition ON, and found the voltage to only be 3 volts with the ignition amplifier connected to the wiring harness and also when I tested between the connectors on the disconnected wiring harness. That voltage is very low.

    Any idea where I should look to find the issue? The wiring diagram in my Haynes manual is not very helpful.

  2. #2
    Liaison 20774's Avatar
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    You didn't mention doing a carb synch after rebuilding the carbs. If that wasn't completed, could be that the carbs are not pulling evenly at elevated RPMs.
    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 harryhendo's Avatar
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    Quote Originally Posted by 20774 View Post
    You didn't mention doing a carb synch after rebuilding the carbs. If that wasn't completed, could be that the carbs are not pulling evenly at elevated RPMs.
    Yes, sorry for the omission. I did do a carb sync with a TwinMax sync tool, and the carbs are pulling evenly at 3000 rpm, where I tested, as well as at idle (1100 rpm).

  4. #4
    Day Dreaming ... happy wanderer's Avatar
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    Start by measuring the resistance (Ohms) by removing both plug leads and measuring the resistance between them. You should see around 21K ohms (from memory..) If you don't then test both plug wires one at a time. Normally they are 5K ohms because the caps have 5K resistors in them. If all these things check out and you know the plugs are new and OK then your coils, wires and plug caps are OK. This means the ICU (Ignition Control Unit) which I think is what you are describing may be flaky. It is mounted on the backbone under the tank on a heat sink and it might be the problem. If not serviced the heat sink paste dries out and the unit will overheat causing it to become flaky at first and finally it will fail completely. The other thing in this circuit is the Bean Can which has a hall effect sensor in it and they are known to fail with heat and time eventually. Testing them requires some equipment most don't have and usually a test jig so swapping with a known good used one is my approach here.

    Never change more than ONE thing at a time when you do this sort of testing and then do your baseline test (run the bike at 3K RPM) and see if anything has changed for every part change you make.

    You should also be checking everything you touch for corrosion. Corrosion in the coil output is common on these bikes and can cause high RPM missing. Loose connections or corroded connections also. Make sure your battery ground is perfect as well.
    Last edited by happy wanderer; 01-14-2019 at 09:08 PM. Reason: spelling...
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
    '81 R80G/S, '82 R100RS, '00 R1100RT

  5. #5
    Registered User harryhendo's Avatar
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    Excellent! Thanks Happy Wanderer! I have some homework to do tonight...

  6. #6
    Registered User Guenther's Avatar
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    Did this happen BEFORE you rebuild the carbs?

    One guess would be the diaphragm is not seated correctly in its notch. Take the intake elbow off and check that the piston in the carb correctly aligns within the tunnel. A mistake I once made with nearly the same symptom.

    /Guenther
    2017 F700GS

  7. #7
    Registered User harryhendo's Avatar
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    Before I rebuilt the carbs, the bike wouldn't even run due to the buildup of mice turds and other cruft inside the carbs. But 10 years ago, it ran very smoothly. So this is a very good clue. I hear a slight sucking sound from the right side carb on startup... might be another clue to the same problem. Thanks very much!

    Can I simply remove the domed top silver cover of the carbs (remove two phillips head screws and pull up) to check the diaphragm? Or will this screw things up?

  8. #8
    Liaison 20774's Avatar
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    You should be able to just remove the top. The issue is going to be that you will change the throttle cable settings which might require the carbs to be resynched. I seem to recall doing this myself in the past, but made sure of the exact movements/turns I made in order to get the throttle cables off. I just then reversed the process and went from there. Synching the cables is really only a minor step if needed.
    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!

  9. #9
    #4869 DennisDarrow's Avatar
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    What happens when you are cruising along at 4K to 4.5K and then roll on hard???.....If it stumbles quite a bit, picks up and goes like it should, then this is your diaghrams ...pardon spelling......Anyway, should have been part of a rebuild after this long.....Yes, and then do a sync.....Otherwise, yes, it is ignition, and try another module......Good luck......Dennis

  10. #10
    Registered User harryhendo's Avatar
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    If the bike is at 4-4.5K RPM, and I roll on hard, there will be a stumble through the acceleration, then when the RPMs stabilize, the engine will stumble at the new steady higher RPM.

    I did not replace the diaphragms when I rebuilt the carbs, as they looked to be in perfect condition, but now I'll have to look a lot closer. They are not costly, and as you say, should be replaced at this mileage/age as a matter of course.

  11. #11
    Liaison 20774's Avatar
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    My sense of having a torn diaphragm is that due a leak of pressure through the rubber, the slide in the carb won't be raised as high as it should be, meaning that the carb would be running rich. Now I suppose that this could cause a bike to stumble, but I would think that the issue is that you just can't increase speed beyond a certain point...the power output is stunted due to the over-rich situation.

    But worth an inspection...let us know.
    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!

  12. #12
    Quote Originally Posted by 20774 View Post
    My sense of having a torn diaphragm is that due a leak of pressure through the rubber, the slide in the carb won't be raised as high as it should be, meaning that the carb would be running rich. Now I suppose that this could cause a bike to stumble, but I would think that the issue is that you just can't increase speed beyond a certain point...the power output is stunted due to the over-rich situation.

    But worth an inspection...let us know.
    Another possibility was mentioned - that a diaphragm was rotated slightly and the little tank was not engaged in the notch in the carb body. This would rotate the slide, also leaving reduced air flow. It would also reduce the venturi effect so it becomes a guess whether it would be a rich or lean stumble. Too many variables for me to know for sure.
    Paul Glaves - "Big Bend", Texas U.S.A
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  13. #13
    B Reams brook.reams's Avatar
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    Quote Originally Posted by PGlaves View Post
    Another possibility was mentioned - that a diaphragm was rotated slightly and the little tank was not engaged in the notch in the carb body. This would rotate the slide, also leaving reduced air flow. It would also reduce the venturi effect so it becomes a guess whether it would be a rich or lean stumble. Too many variables for me to know for sure.
    I've also seen the slide stick. The steel shaft on the top of the slide fits into a sleeve in the carburetor top. This can get grit in it and stick the slide. I've even seen wear in the top sleeve (which is made of soft pot metal) cause the steel shaft to bind inside the sleeve. You can verify smooth movement of the slide by pulling the air tubes and with your finger push the slide up and let go. It should fall all the way to the bottom quickly and smoothly. If there is resistance to pushing it up or it falls slowly, something in the sleeve is obstructing movement of the slide.

    You can remove the top and not have to remove the throttle cables from the carb top or change the length adjustment. Instead, remove the return spring, rotate the lever and remove the cable from the hole. You can do the same on the choke. Then remove the screws from the top and pull the top off. Assemble is the reverse of disassembly.

    I've also seen jet needles end up in different positions after a rebuild and that creates a stumble.

    That said, diaphragms that are that old need to be replaced. Even if there is no hole, they get flabby and that affects the slide movement.

    Hope that helps.

    Best.
    Brook Reams.
    Brook Reams - Arvada, CO
    Endeavor to Persevere
    Various Two-wheeled Vices, All BMW || Website: Airhead Rebuild Projects
    2004 R1150-RS||2002 F650-GS||1983 R100RS||1977 R100RS||1973 R75/5

  14. #14
    Registered User harryhendo's Avatar
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    Thanks Folks! I removed the throttle and choke cable, the choke return spring, and the two Phillips head screws from the right side carburetor, and pulled the top off gently. The diaphragm appears to be in fairly flabby shape, and the diaphragm tab that fits into a cutout in the top of the carb body is not very stout (looks like it was pinched). So I ordered two new diaphragms and they should be here tomorrow. I'll let you know how it turns out. Thanks again for all the insight!

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