BMW R100 Power Loss - Fuel or Spark?

Discussion in 'Airheads' started by akpasta, Jun 10, 2013.

  1. akpasta

    akpasta Been here awhile

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    Hello!

    This is my first post on this forum. I bought a 1976 BMW R100 a couple weeks ago and have REALLY been enjoying it. It has 83k miles on it, the previous owner had it for 10 years, but hadn't ridden it much in the last 1.5 years and I dont' believe he ever took it far, fast, for very long in more time than that.

    I took the bike on a 200 mile round trip north and at certain times, at maybe around 70mph+ it would begin to cut out and fail to make power, especially at higher rpms, like on a hill. If I downshifted/slowed down and lowered rpms, it would run again. It felt more like electrical than fuel. It happened maybe about 5 times on my trip and is something I can't seem to replicate unless I really warm the engine up (WOT for instance had been fine most of the time, except when the engine was already really warmed up).

    Once the bike cools down, the problem seems to cease, which makes me think it could be the condenser failing. They tend to fail when they heat up, then work when they cool down, also I believe the condensers are mounted where the points are; this is an area that tends to get pretty hot.

    What is a suitable automotive/cheap condenser I can use, that I may find at a standard auto parts store? I was thinking of re-locating it to where the main coils are under the tank as well, to help keep it cooler.

    I also cleaned one of my carbs and found it to be fairly clean already, so I'm not sure it's a fuel issue. Also I figure if it were fuel, it wouldnt' matter if the bike were warmed up or not, it would simply NEVER make power at 70+mph, rather than make power intermittently, ya? However, the tank is a little dirty, but it has fuel filters on both petcocks. I would think if it's a dirty fuel issue it could've happened in any rpm range at any speed.

    I'm going on a 500 mile round trip at the end of the month, the longest trip I've ever taken and I REALLY wanna get it tuned and ready to go for this trip. should I just go with the condenser and see what happens?

    The only other symptom I noticed was after the 200 mile trip the spark plugs were REALLY hard to get out, in fact I almost lost a thread out of the left-side cylinder when I was trying to extract the old plug. The plugs were dry/dark-grey with some pitting (they were very old). I replaced them.

    Here's a photo from one of my latest adventures.

    And here's a picture of my beloved Superhawk, a bike I wish could be magically transformed to a 750 with the same styling

    Thank you,

    Andy
    #1
  2. kadesean

    kadesean eyesuck

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    That Honda is cool as hell.
    #2
  3. akpasta

    akpasta Been here awhile

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    Thanks! I love it but wish it were more powerful. It is reliable though, I've put almost 5000 miles on it in just a year and a half.
    #3
  4. More_Miles

    More_Miles über-n00b

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    My bike (83 R100RT) used to "drop" a cylinder, seriously down on power, and run rough. I only discovered it when I convinced my girlfriend to go for a ride with me. Under the load, going up a hill, and even on some serious roll on throttle, one side would start to starve for fuel. I never noticed it solo, but now that I recall, loaded for travelling I remember it not being all it should have been. I had also ended up convincing myself it *had* to be electrical. What do I know, I'm an idiot by times.

    Additionally, and especially if the bike sat unused for a spell, are the screens in the tank (if you have any) blocked, or any in-line filters? I'm not saying your problem isn't electrical, but don't be too narrow in your focus. I chased my tail for the better part of two weeks on my problem.

    Re-read your original post, if you didn't put the new in line filters on, I'd toss and replace them as a maintenance item. Also, it sounds a *lot* like fuel starvation. Electrical, I would anticipate cutting out completely, not just being down on power. I find electrics tend to be binary, the work when they work. When they don't they don't!
    #4
  5. akpasta

    akpasta Been here awhile

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    I'm not really sure how to narrow that down. The tank is a bit dirty, but has fuel filters and flows clean and aplenty. I cleaned the carbs and they were looking pretty good inside. Also, it ran for 60 miles, including quite a bit of highway mileage for a good while before the symptoms ever showed. I suppose some rust on the tank could have knocked loose and only really gets sucked in at really high rpm? Who knows.

    I suppose like condenser this might be one of those things where I just have to soak the tank and clean it out simply to narrow it down, but I do have a hunch-- based on how/when the symptoms began, how intermittent they are, etc, that it is condenser. Who knows. I could take it for a ride right now, and it probably would not happen for 30 miles. That doesn't sound like fuel to me, but who knows.
    #5
  6. c1skout

    c1skout Been here awhile

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    My r100 acted like that when the coils were going bad.
    #6
  7. disston

    disston ShadeTreeExpert

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    I think you need to do a complete service of this bike. Change all the fluids and the filters.

    A tune up as part of this service starts with adjusting the valve lash, then installing new points and condenser, adjusting dwell and timing, then balance the carbs.

    My experience with condensers is that a bad one stops the bike from running. Is not an intermittent problem.

    The twin six volts coils are very reliable but they can go bad. Check the resistances.

    Much more likely to have a bad plug wire or plug cap. On an Airhead the wire is low resistance the plug cap has 1000 Ohms. Get some new spark plugs.

    New fuel line. In line filters if you like but they are not really needed. I would flush the tank (get over it ) and clean the petcocks. Use some grease when working on those rubber seals. At this age you may need new rubber parts in a lot of places.

    Such as the rubber boot on the speedo cable where it goes into the transmission. If this is cracked at all and showing it's age replace this boot. It is the number one source of water getting into the transmission.

    This is just a beginning. This bike needs some attention not one or two little things.
    #7
  8. Plaka

    Plaka Brevis illi vita est

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    Sounds electrical. I Used a big Mallory in my /5. Stock location, new clamp (the thing was BIG). Never had to change it.

    Do a full tune up. Inspect the points and replace if worn. Make sure they are super clean.

    At that speed/load you are running mostly on the main jets. Remove the main/needle jet stack and make sure it's clean. You do this on the bike. If the main jet stays in the carrier, fine, you want to take the carrier out anyway. Wouldn't hurt to put fresh o-rings on the jets. Do this carb-on-bike. Just get a rebuild kit and only use the o-rings. Make sure the carb top screws are free (replace with the stainles ones is nice) and carry one spare diaphragm on the trip. Easy to change by the side of the road if you have to.

    check for intake leaks between the carb and head. Spray some starting fluid at idle on the joints. If it speeds up...
    These will also hurt you at speed but should be more consistent.

    Always disconnect battery ground when removing front cover. You can run with the cover off but if the points are exposed, avoid dust and water and don't lose the compartment strip seal. Points in a can are protected.


    Carry a can of chiller spray (radio shark of similar). Diagnoses thermal faults in a hurry. (like, spray the coils).

    Check resistances on spark plug leads. inspect inside of coil towers. Use a bit of silicone grease inside the boots, both ends.

    The thing should cheerfully cruise at 90-100 all day long--at high altitude.
    #8
  9. Stan_R80/7

    Stan_R80/7 Beastly Gnarly

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    Yes. You have a fuel or spark problem.

    Did you have both fuel petcocks open when on the highway? Did you try unscrewing the gas cap to see if the vent was partially clogged? Have you tested the fuel flow rate (w/container and timer) from the petcocks? Can you make these symptoms occur riding tomorrow?

    A stock '76 R100 comes with points ignition. The condenser in a points ignition system operates to protect the points. When the condenser fails (which is not common) the points burn and pit quickly - shortly before the engine dies.

    I always test and check the simple to fix items first. I also agree that, at a minimum, all fluids should be changed. Good luck!
    #9
  10. Plaka

    Plaka Brevis illi vita est

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    One other thing you can do is carry a nice spark plug wrench in the tank bag. Go for a ride and when it does it count to 25 and hit the kill switch. Coast to the shoulder. Pull the right plug and have a look. This may reveal the problem by looking at the plug. (called doing a plug chop). If the plug is wet with fuel, the cylinder wasn't lighting up. Spark problem. if it's burned white, too lean. Fuel problem or intake air leak. If it's black and sooty, too rich, fuel problem. Diaphragm not raising the needle (bad diaphragm, bad vent, sticking slide), choke screwing up.

    The circuit to throttle position diagram in the Bing Book will tell you what circuit you were running on depending on the throttle position at the bar. So make note of that when it screws up.

    It's also possible to have an intermittent short or open in the ignition wiring. Usually these result in totally dead and then it comes back.
    #10
  11. akpasta

    akpasta Been here awhile

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    Thanks for the advice, there's a lot here to go on. Looks like the points are fairly new, they look very good. The gap was a bit small so I re-gapped but when I attempted to check timing with my strobe the F-mark was in the hole at 2k rpms, not 2.5k like my book says; also I could see no 'S' mark at all at idle. Do they all have S marks? I took it for a ride and although it ran great at first with a nice slow, consistent idle, it started sputtering at idle (left cylinder I think) after that. I'll have to re-check my timing again tomorrow. What's up with the timing marks? Would it be better to do a static timing of it?
    #11
  12. Plaka

    Plaka Brevis illi vita est

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    I always static timed points. Still have my little light built into a plastic kodak 35mm cartridge can. Use your timing light to check that it is advancing properly. Faces of points must be completely clean. Draw a slip of white paper wetter with alcohol (shellac thinner, not scotch) through them to clean before running. Dirty points (like the slightest trace of oil) burn quickly.

    Do check the dwell with your tach/dwell meter to ensure points gap is doing what you think.

    They all have the marks on the flywheel. Turn it around with rear wheel in 4th or 5th and find+clean each one (wire brush used for 1/2" copper pipe or brass gun cleaning brush) then add white paint. Let dry and go around again scrubbing off the paint so it is only in the bottom of the marks. (Scotch brite taped to tongue depressor)

    Or something.

    If you have a double image in the timing hole this is noteworthy.
    #12
  13. Stan_R80/7

    Stan_R80/7 Beastly Gnarly

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    Getting the 'F" mark at 2600 RPM is more important than the 'S' at idle using a timing light for the bike to run best. There is no way to check full advance using static timing - but static timing will get the bike running. Airheads will run with the timing off considerably and carburetors that need work - it's part of their charm.

    In some ways that's great (to get you home), but for those unfamiliar the bike will run but never be 'quite right'. There is an investment of the proper tools and knowledge to tune up these old bikes. But, they don't have to be in great tune to run. Good luck!
    #13
  14. Mikepotter86

    Mikepotter86 Been here awhile

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    If this is happening only at high speed and consistently high revs, I would suspect your floats need adjustment. I adjusted mine roadside a few years ago after a carb rebuild when my bike exhibited the same issue.
    #14
  15. akpasta

    akpasta Been here awhile

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    Thanks again for the replies folks, you've got a pretty helpful community here.

    I'm not able to find any marks on the "flywheel." Is the flywheel the giant rotor in the center of the points area? Is the flywheel the thing that spins inside the timing hole with the non-visible S and visible F marks?

    Timing at advance feels really good, but after I ran for about 5 miles the left cylinder seemed to drop a bit at low rpms, I'll recheck my points and set the float on the left side- I set it on the right. Maybe some of the grease I put on the advance cam slipped onto the points, who knows.

    I also need to do a general re-setting of carbs, per manual.

    The fact that I can't see the S-Marks in the window is weird though. But I suppose with only one set of points, if your gap is right, they ought to open at the right time anyways, right? Maybe not, I dunno. If I want to static time for idle, what marking would I look at with my little points timing bulb?

    Adjusting the points is kind of a bitch with that advance mechanism in the way. Anyone know where I can get one of these nifty tools that fits over the points cam so you can get full gap without the advancer in place? I suppose you could make one out of a worn-out advancer but I don't have one of those yet :D
    #15
  16. More_Miles

    More_Miles über-n00b

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    Flywheel = AKA clutch carrier on "newer" airheads. I think yours is considered "newer" as is my '83.

    It's the big honking rotating thingie behind the timing hole.
    #16
  17. trabbart

    trabbart hellrider

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    when a car looses power alway change the rotor and cap from the distributor,
    loosing power is ignition,,,,weak spark. weak explosion,,weak power......

    my bike had the 27 year old coils, when i changed them for new it was way faster,,i also replaced the coil wires,the points looked like new and i lett them in there ,,,, remove the old ignition stuff before you get stranded. its junk after 25 years,,,maby some will ride with it,,but they dont know how weak there spark is,,,if it was not the problem you have won a few horses and can exclude the coils and wires and win some horses somewhere else.

    start the bike cold, feel at both exhaustpipes ,,you might feel differances in heat.
    #17
  18. chasbmw

    chasbmw Long timer

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    Static timing on the points will be at the S mark, but your problems will most likely to be due to the float level being set too low. Very easy to do? And you do need to do a full tune up including all fluids and greases.
    #18
  19. akpasta

    akpasta Been here awhile

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    I did a little research and found where to look for the marks at static. I'll start there with a static light, then confirm advance with strobe.

    Took apart right side carb to examine things, not the left side yet cuz the right looked so good. Setting float level is incredibly easy i'll do it on the left. Replaced engine oil already of course, but haven't done front disc brake drain or the rear drive shaft fluid, will get to those next.

    I also really wanna check valve clearance and rocker arm clearance but I don't have a torque wrench, is it absolutely necessary to re-torque the valve cover bolt with a torque wrench?

    Also going to check compression on each side as the left side seems to burn some oil and smoke a bit. How much compression is decent on these guys?

    Is there a standard setting for the mixture screws on each carb?

    Thanks everyone for all the advice, huge help.
    #19
  20. Plaka

    Plaka Brevis illi vita est

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    I agree---partially. Spark plug wires take a beating in the wind. OEM BMW wires are very good, but still wear in time and are worth checking. If the resistances are good, they're good. Coils last a very long time except for the crack-o-matics. That was a defect issue. The points mechanism lasts forever. The point faces and rubbing block are the wear items. Theoretically the advance springs will weaken but it's undetectable.

    The bottom line is: just test the sparks. If they are good all is well there.

    On a long distance move w/my Toyota truck I was getting power loss on the hills, heavy load and pulling a trailer. I hit an auto parts store in a small town and replaced the coil, cap&rotor, plugs and I think the wires. World of difference. But it had 250,000 on it. More recently I smoked the ignition module hooking up a battery backwards. Not wanting to spend $300 on a new module I put a $40 chevy unit in. Turns out the modules are only good for 300,000 anyway. I opened up the old one. Talk about a brick shit house! The electronics were potted in a clear gel that was still gooey. BMW should build something like that. I carry a spare for the Chevy unit in the glove compartment. I give it a couple of years maybe. I carry a spare for the BMW unit on longer trips. When I ran Boyers and Dynas, I carried my points plate setup on longer trips.

    Better running truck:

    [​IMG]
    #20