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Thread: Help needed with '78 R100/7 - cutting out on one side only

  1. #1

    Help needed with '78 R100/7 - cutting out on one side only

    Help needed with '78 R100/7 - cutting out

    I have had this bike for some three years now, and have had it running perfectly - that is, until the other day!
    Got it out last weekend from winter storage, put tank back on (off for winter to clean and get all old gas out. Put in new fresh gasoline, and the bike started in probably less than a second. Took for 30 mile ride on Sunday and it ran perfectly!!

    However on Tuesday I rode it to work, and about half way there (after about 10 miles) it started cutting out (left side only) on acceleration or hard acceleration up a hill, and I get either total cutting out (on left side only) or partial cutting out.

    A couple of days later, I had a replacement carb (rebuilt by a reputable carb guy), so I simply replaced carbs. It ran seemingly better, until I had gone about 1 mile, and the cutting out started out, but not as severe.

    Usually this bike runs great and did on that Sunday. It has electronic ignition (Dyna III).

    Here are my ideas: could the slide and diaphragm on that side be bad? Why would it just start the problem rather than coming on gradually?

    Also, spark plugs - have any of you had such a situation where a plug (when under heavy load) went bad and quit firing? I have - on a Honda once.

    How about plug cap or plug wire?

    Finally, what about the coil on that side?

    Any ideas would be appreciated.

    Thanks,
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

  2. #2
    Administrator 20774's Avatar
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    I would try swapping electrical components side to side until you found the situation where the problem jumped to that side. Also, if you think it's wires/coils, do the cap-to-cap ohm check...should be 20K to 22K ohms.

    I've never experienced a plug failing, but I've heard others say they have. Vech at Bench Mark Works says that if you gas foul a plug, time to toss it and start fresh.

    But if you can get the problem to switch sides, you'd be onto something.
    Kurt -- Forum Administrator ---> 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 godfather's Avatar
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    Do you have Dyna III ignition Jimmy? If so maybe something is amiss with the left side contact on the ignition plate?

    Just grabbing at straws.
    Attitude is everything!

    08' V-Strom 650 great light weight tourer

  4. #4
    #4869 DennisDarrow's Avatar
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    Jimmy, my thinking is that when you took off or put back on the tank, you accidentally of course, knocked some wiring there on the backbone.........A very intermittent short of the electrical there at the coils/plug wires...........Just thinking.........God bless YA Bud.......Dennis

  5. #5
    Macrunch MCrenshaw's Avatar
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    While a lot of problems are electrical in nature I'd like to suggest you check the fuel taps and in line filters for good flow. The fact that you removed the tank and reinstalled it leads me to wonder about a blockage around the wire screens on the taps. Or possibly some condensation in the tank or float bowls.

  6. #6
    I'm going to deviate a little here and suggest something gas tank related. Have you considered vapor lock at all from inadequate tank venting? The problem you describe used to happen to me on my Aermacchi special (I adapted a non-stock gas tank to it) that didn't have a tank vent. Bike would run fine all day at a medium pace on back roads and level ground, but if I had to go hard acceleration for a sustained period or go over any bridges it'd start cutting out. In those situations I'd have to back off the throttle and coast for a little and it'd kick back to life. Installing a vent on the gas cap fixed all those problems (in my case I had to drill into the cap and install an overflow hose), but your BMW should have a vent built into the cap itself already. Maybe that's plugged after the winter storage somehow?

    Snowbum mentions that 78s and 79s were susceptible to this:
    http://bmwmotorcycletech.info/fuelmiscl.htm
    "NOTE: fuel flow problems such as stumbling after a bit of cruising....can be caused by fuel
    cap venting problems, not just clogged fuel screens and filters, etc. The fuel caps of ~ 1978
    into 1979 were particularly susceptible to vent clogging."
    // 1975 BMW R90/6 (cafe'd) // 1957 BMW R60 (in pieces) // 1967 Aermacchi/H-D Sprint 250 SS (custom special) // 1967 Moto Guzzi V700 (current restoration)

    http://symphonyofshrapnel.blogspot.com

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    Administrator 20774's Avatar
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    jimmylee indicates that the cutting out is left side only. In order for gas to be an issue, it isn't likely to be the tank only unless slower flow is exacerbated by plugged fuel lines to the left carb or a leaner float setting on the left side. If that's the case, then check fuel flow into a catch bottle...try it with different petcocks open, gas cap loose/tight, different levels of fuel in the tank. And check the amount of fuel in each bowl...for the /7 it should be 22-24mm measured in the center of the bowl.
    Kurt -- Forum Administrator ---> 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!

  8. #8
    Registered User tanker4me's Avatar
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    I've had a resistor cap fail twice. The first time on a R90/6, there was an intermittent miss on one cylinder, the second time was on a R100, a cylinder quit firing while stopped at a traffic light.
    Bill
    P.S. Once I had a lumpy at idle and poor running on acceleration caused by a small bit of the float bowl gasket stuck in the jet.
    Last edited by tanker4me; 03-27-2016 at 11:06 PM.

  9. #9
    Registered User lmo1131's Avatar
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    I've had a resistor cap fail twice.
    Bing badda bump. I think we may have a winner!

    I've also had experienced a spark plug failure with another boxer; our '68 Karmann Ghia. Freshly tuned, oil changed, valves adjusted, and new, properly gapped set of BOSCH plugs the engine would run like a scalded dog on the flat, but subjected to any kind of a grade it would start to miss out and loose power. Took me hours to figure out what the heck was going on. Never did. Finally, in desperation, I replaced the new spark plugs I had previously installed with another set. Problem gone. One of the porcelain insulators was cracked; I later discovered an almost invisible hair-line crack.

    I got rid of the BERUs on my /5 for another reason. Moisture had migrated into the space between the HT terminal and the Bakelite housing. Not "period" correct, but I installed a set of NGK caps. 5 years and no problems.

    http://forums.bmwmoa.org/showthread....-be-a-long-day



    edit. I just recalled another incident when the engine started to miss on one bank. In the end it was that the main jet had ingested a tiny bit of grit. Pulled the carb, pulled that main jet and blew it out. Ran like a champ.
    "It is what you discover, after you know it all, that counts." _ John Wooden
    Lew Morris
    1973 R75/5 - original owner
    1963 Dnepr

  10. #10
    Thanks to all of you for suggestions!

    Here is what I have concluded so far (answering some of above suggestions):

    Can't be moisture in tank as I took tank off for winter, and put new oil into it to keep it from rusting or getting moisture in it. When installed back on, tank was completely washed out with fresh gas (as were carbs) and I am sure no vapor lock either - lots of flow from tank to both carbs.

    When I got home after work day (nursed bike home) I changed the left carb with a new one (used but completely rebuilt). Ran better, but still had cutting out on left side under hard acceleration, and on a pretty steep upgrade at approx. 15 - 20 MPH, where I had to turn throttle on quite a bit (similar to heavy load). If I went to a low gear and got rpm's up and got it up the hill. Some cutting out, but not seemingly at high speed unless accelerating rapidly.

    I recently put in new plugs (NGK BP6ES) but haven't tried it out yet. I did, in a Honda, have this problem and the problem turned out to be the plugs.

    I found out this past weekend, that nearby, in Greenville, SC, there is a shop where the two owners are Airhead experts. I talked to them and they suggested what was suggested above - start swapping out electrical things (left to right and vice-versa) and see what happens. It is also nice to know that they do work on older BMW's (which is their main business) and each had many years working at a BMW dealer, and one, at least, still races an older /6 airhead, and rides regularly and daily an old /2. So, I do have a back up plan, if I can't figure this out: take it to them and bite the bullet and "pay the price"!

    Thanks again to all of you. This forum is the best!!

    If I do find out the problem, I will certainly let you guys know and maybe my situation could help someone else out. Having ridden this bike now for some 15 to 18,000 miles, this is the first time that I had it do anything like this. Only other "on the road" issue was a broken clutch cable, and my son, on his K100 rode to a dealer about 60 miles away and I was up and going again in about 2 hours!

    I really suspect (if plugs don't help) that the issue is in the plug cap or wire, or connection to coils or something like that. Again, only happens under load.
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

  11. #11
    Registered User
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    As others have suggested the way to pin down one side only problems is swapping parts from side to side - be systematic and logical

    First confirm;
    both fuel taps on in the lever up(reserve) position with gas in tank
    both choke enrichener levers are fully down to hard stop pin on the side of carbs (look at the lever position on the carbs - not on the other end of the cable)
    valves are adjusted near to spec
    a reasonable and visible points gap exists (all kinds of wierd stuff starts to happen when gap closes up to just barely opening and sometimes affects only one side)
    and problem still exists then:

    1) Swap plugs side to side and confirm problem did not change sides
    2) Swap plug wires side to side leaving undisturbed at top or coil ends and just swap the plug ends and confirm problem did not change sides
    3) Swap carbs side to side - looks funny but works ok don't bother to hook up choke cables just see that enrichener levers are down to hard stop and confirm problem did not change sides


    If problem changes sides on:
    1) above - confirms problem is bad plug - stop change plug you are done
    2) above - confirms problem is coil or wire or plug cap for that side - stop and report back here for wires, caps and coils advice
    3) above - confirms problem is a carburettor and identifies which carb is wrong - report back here for carb advice

    report back here before making more wild guesses

  12. #12
    Did the plug swap already and nothing changed. I then put in fresh NGK BP6ES on both sides, still no change.

    Rode it approx. 10 miles, and in the beginning the problem seemed to be near the lower end of engine revs down to idle. As it got hotter, it became more frequent and at any speed as long as the throttle position was in the lower range of travel. Downshift to lower gear at same speed and of course the throttle had to be raised to compensate and that helped for the most part. Pretty good at 5,000 RPM's as long as I kept in lower gears (fourth and below).

    I am going to do the spark cable swap when I can get a chance.

    I do have a question: When a diaphragm becomes bad (either pinhole or simply worn out and too flimsy) what are the operational characteristics of the engine running?

    Thanks!
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

  13. #13
    Administrator 20774's Avatar
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    Quote Originally Posted by jimmylee View Post
    I do have a question: When a diaphragm becomes bad (either pinhole or simply worn out and too flimsy) what are the operational characteristics of the engine running?
    I've always heard that it affected the higher speeds...the slide can't be raised high enough for the higher demand for air. Lower speeds it might be OK. Of course, it depends up the size of the tear.
    Kurt -- Forum Administrator ---> 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!

  14. #14
    #4869 DennisDarrow's Avatar
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    Let's say you are cruising along in 5th at 4500 or so RPM and decide to do a hard roll on or even a medium if the load is gonna be uphill.......The engine will be slow to respond, perhaps backfire; but typically just be kinda dead and then respond in a sluggish manner up to where you want to be. In addition, the top speed in 3rd or 4th gear will be diminished at 5K or so......Other than that, for me, it idled and was just not bad. This went on for a couple of thousand miles while I tried to figure it out without doing what I should've done........Tear into the carb top.......That is always a tricky proposition for me, as in the past have had a couple of those durn phillip head screw slots strip out for me and I had to end up drilling them out..........Good luck......Dennis

  15. #15
    Registered User
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    "Put in new fresh gasoline, and the bike started in probably less than a second. Took for 30 mile ride on Sunday and it ran perfectly!!

    However on Tuesday I rode it to work, and about half way there (after about 10 miles) it started cutting out (left side only) on acceleration or hard acceleration up a hill, and I get either total cutting out (on left side only) or partial cutting out.

    A couple of days later, I had a replacement carb (rebuilt by a reputable carb guy), so I simply replaced carbs. It ran seemingly better, until I had gone about 1 mile, and the cutting out started out, but not as severe."

    =================

    Above from initial post indicate classic symptoms of tight valve on left side - ie - starts and runs ok when cold or cooled down and goes ragged when warmed up

    When cold the valve has slight clearance and closes completely - When warmed up the engine metals expand and there is no clearance and the valve does not close completely and engine runs ragged from poor compression on that side

    Can you confirm valves are adjusted near to spec? or forgive me if I missed it somewhere in posts

    Please confirm the following basics to start with:

    __both choke enrichener levers are fully down to hard stop pin on the side of carbs (look at the lever position on the carbs - not on the other end of the cable)
    __valves are adjusted near to spec
    __a reasonable and visible points gap exists (all kinds of wierd stuff starts to happen when gap closes up to just barely opening and sometimes affects only one side)

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