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Thread: '83 R100RS - runs poorly but doesn't lose power?

  1. #1
    Ran Bush
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    '83 R100RS - runs poorly but doesn't lose power?

    To sum up: I recently bought an '83 R100RS with about 50K miles on it, from the original owner. The bike has good service records from a local BMW dealer. I rode the bike for about a month, it seemed strong and in good tune, but something changed while I was on a 100 mile ride a few weeks ago.

    I've tried troubleshooting a few things, but the bike is still missing noticeably in 5th gear at around 4K RPM. The bike doesn't lose power, it just feels like the choke is on and it's running too rich. Both spark plugs still look clean and white, no cracks in porcelain, gapped okay. I don't feel it as much in lower gears, at lower RPMs. It still feels "off", just not as much at lower speeds around town.

    I've swapped carbs with a set from another bike, no improvement. Checked choke and throttle cables, they seem okay.

    I've checked spark plug leads for failure and correct resistance, swapped out the ignition coils, checked to see if the bean can was loose, no improvement. Only thing I haven't tried is to adjust timing, but it seems to me that if the timing was off, the bike would run poorly in all gears, at all RPMs.

    Thanks for any additional suggestions and thoughts.
    Ran Bush in NorCal
    '68 /2 '83 R100RS '92 R100GS

  2. #2
    Sir Darby Darryl Cainey's Avatar
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    There is no conection between the running of the engine and the transmission.

    If it rums poorly in 4th it should run the same in 1st, 2nd, 3rd & 5th at the same rpm.

    Does it hold idle around 1000 rpm?

    Valve adjustment?

    Gascap vent clear?
    Ambassador BMW MOA Ontario Canada
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    1977 R100RS, (Retired) 1993 R100GS (just getting started)

  3. #3
    Registered User
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    All I can think of is that not enough fuel may be getting to the engine based on your description. Have you measured the fuel flow? When was the last time the air filter was replaced? Other than fuel delivery, any 'missing' from the engine should occur at rest and/or in any gear. Good luck!
    Stan

    AH# 13238

  4. #4
    Administrator 20774's Avatar
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    Try cracking the gas cap at speed...does it improve? That would mean the cap is not venting.

    Possibly the diaphragms are torn in the carbs. A tear or hole will not let the slides rise to the required level and the bike will be starved. It will typically run in lower gears/speeds OK.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
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  5. #5
    Registered User lkchris's Avatar
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    Check compression.
    Kent Christensen
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    '12 R1200RT, '02 R1100S, '84 R80G/S

  6. #6
    Ran Bush
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    Thanks guys;

    I should've said it is _most_ noticeable in _5th_ gear at 4K-4.5K RPM and highway speeds. The symptom is still there at lower speeds, just not as bad.

    And, I replaced the carbs so that should eliminate the torn diaphragm (unless there was a torn diaphragm in one of the swapped-in carbs. Uh oh ...) I'm rebuilding the carbs with the kit from Bing, so that should eliminate the carb aspect when I get them back on there.

    I'll check compression, try cracking the gas cap at speed, try another air filter, and check the fuel screens in the petcocks. There are older Karcoma filters in each fuel line, the kind that you can open up and clean, they didn't look clogged.

    Thanks again for your thoughts.
    Ran Bush in NorCal
    '68 /2 '83 R100RS '92 R100GS

  7. #7
    Beemerphan Radar41's Avatar
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    Question One More thing

    Quote Originally Posted by ranbush View Post
    Thanks guys;

    I should've said it is _most_ noticeable in _5th_ gear at 4K-4.5K RPM and highway speeds. The symptom is still there at lower speeds, just not as bad.

    And, I replaced the carbs so that should eliminate the torn diaphragm (unless there was a torn diaphragm in one of the swapped-in carbs. Uh oh ...) I'm rebuilding the carbs with the kit from Bing, so that should eliminate the carb aspect when I get them back on there.

    I'll check compression, try cracking the gas cap at speed, try another air filter, and check the fuel screens in the petcocks. There are older Karcoma filters in each fuel line, the kind that you can open up and clean, they didn't look clogged.

    Thanks again for your thoughts.
    Remove the tops on the carbs to inspect the diaphragm's AND make sure the four screws which hold the diaphragm in place are not loose!! If those screws are loose it will cause EXACTLY the same problem you are having.
    Don "Radar" Wreyford
    00 K1200LT, 98 R1100GS AE, 84 R100RS, 76 R75/6 (rebuild complete)
    MOA # 91738, RA #27032 , ABC #7915 - "Insanity is doing the same thing over and over again, but expecting different results" - Albert Einstein

  8. #8
    Themason 42906's Avatar
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    Touring Australia on an R-80G/S some years ago I had a similar problem. The bike would not go any faster than about 80 mph and the faster I tried to go the weaker the engine felt. I field stripped the carbs to no avail, double and triple checked valve clearances with no effect. I checked every connection and fastener I could think of.
    In frustration I had to double back and find a shop in Mount Isa to see what the problem was. I shake my head to admit the whole problem was a tank bag strap blocking the intake snorkels, and the faster I tried to go the tighter they sealed the intake horns. Full speed, such as it is on a stock geared R-80G/S, was now available. Cars on the Stewart Highway were still passing me 20 -30 mph faster than that G/S could muster, but that is another story. Only the road trains were slower.
    Your problem could be something as simple, and as obscure. Just saying .....
    Experience is what you get when you don't get what you want.

  9. #9
    1960R100S 46211's Avatar
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    It sound like ign system. Did you check the IGNITION CONTROL UNIT under the tank. BMW had a problem with them around that year, you were to remove and put die-elect grease under them. You did say that you check the ign can out. Check the timing. Does the bike have two coils or the one peace two out let coil.

  10. #10
    Registered User m_stock10506's Avatar
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    Heat Sink Paste

    Quote Originally Posted by 46211 View Post
    Did you check the IGNITION CONTROL UNIT under the tank. BMW had a problem with them around that year, you were to remove and put die-elect grease under them.
    Sorry, not quite ... not di-electric grease - that's used on your electrical connections. Go to Radio Shack and buy a tiny tube of HEAT-SINK PASTE. Remove the ICU and clean under it and then put a layer of fresh heat sink paste there and screw the ICU back on. Even if that's not the problem, it should be done on the bike anyway.

    Sounds more like torn diaphragms or fuel starvation.
    Michael Stock, Trinity, NC
    R1100RT, R100, R60/6

  11. #11
    Ran Bush
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    Just to follow up on this thread, last month I took the bike to a competent local wrench, he synched the carbs and he pronounced the bike done and problem solved.

    When I left his shop, the bike ran great for about 10 miles, then the "roughness" came back for a few miles, but not as badly. Then it smoothed out again and ran fine the rest of the way home. The bike has been running fine on short rides around town. I went out this past weekend for about 100 miles, and it felt great.

    I also went through the service records from the P.O., and there are work orders dating back to 2003 on similar issues; e.g. "customer reports bike cutting out on acceleration", "bike sputters", "losing one cylinder" etc. Over the years the same local BMW shop cleaned and replaced various carb jets, replaced one carb (?), and replaced fouled plugs. There is a note on a work order in 2006 that says, "advance unit not operating perfectly but bike runs ok as is". The BMW shop put in a new Ignition Control Unit in 2009 and that was the last reference to "rough running" and etc. Only service was for oil and filter changes since then, and I bought the bike in April 2012.

    Because the problem first surfaced for me on a ride in July, and then it was fairly warm last month when I brought the bike home after the carb synch, I'm thinking maybe a heat-related electrical issue like the Bean Can getting too hot. The weather on this last weekend ride was a lot cooler, and the problem didn't appear.

    Thanks for all previous thoughts and any additional suggestions. I'm going to keep riding it and see if the symptoms happen again.
    Ran Bush in NorCal
    '68 /2 '83 R100RS '92 R100GS

  12. #12
    Registered User mt4jim's Avatar
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    I had the same problem. After riding for a while, the engine behaved like it was running waaaay too rich or was starving for fuel. Or had a venting problem. Or the float levels were set wrong.

    Took the bike to well known dealer. Lots attempts to solve the problem, but no luck . I eventually gave up and used another bike.

    Retired and had more time on my hands. Removed the tank and flushed it again (had been done a couple of times before). After a very thorough flush I found threads of Kreem sealer were blocking the fuel petcocks. Happend often but not every time.

    Recommend carefully checking the inside of the tank if it is resealed . Or for some other kind of obstruction. If you can find one of those camera probes that will tell you if you have this kind of problem.

    I've been told that Acetone will melt the Kreem sealer so you can clean and reseal the tank properly after thorough cleaning.
    Last edited by mt4jim; 11-27-2012 at 05:31 PM. Reason: Left something out..

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