Symptoms of bad carb diaphragm?

Discussion in 'Airheads' started by thebisch, Oct 11, 2021.

  1. thebisch

    thebisch Been here awhile

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    What are the symptoms of bad carb diaphragms?
    #1
  2. patrkbukly

    patrkbukly Life at 10,000 feet Supporter

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    On my PD (32mm) it would bog badly when I got up over 4K rpm's or so and then sort of catch and get going strong but sloppy again at 5k. Maybe some sloppy idle too but not much.

    I am guessing one carb side was not really opening presenting a massive carb "off balance" condition most prevelant under heavy load and speeds?

    It actually amazed me when I discovered the torn diaphram because while it did not run great it still ran for many miles like that for a few weeks till I finally had time to tear into it and diagnose.
    #2
  3. tennessee thumper

    tennessee thumper Long timer Supporter

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    When in doubt, swap it out.
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  4. Kiwiabbo

    Kiwiabbo Long timer

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    Pretty quick job to remove and check... at least you will know for sure rather than trying to guess.
    #4
  5. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    If you have over 30-40K miles since the last carb rebuild,,,it's time. Can completely change the engine - for the better. Torn diaphram is either due to time or a botched rebuild. Just don't forget to change the needle and maybe bump the needle just up a notch.
    #5
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  6. thebisch

    thebisch Been here awhile

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    So only one person is going to answer the question?

    FYI, I am not in doubt or needing diaphragms.
    #6
  7. PaulBarton

    PaulBarton Long timer Supporter

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    If you're conducting a poll post it at the top...if you want the answer to your question how many responses do you need?

    If you are just researching info to fill your cranium...Snowbum's website is an excellent resource: bmwmotorcycletech.info/ You can literally search your question on his site and get a very thorough explanation....
    #7
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  8. cide1

    cide1 Been here awhile

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    I was going to share my experience...but I guess it isn't needed.
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  9. TwoShoes

    TwoShoes Carbon-based bipedal life form

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    I wouldn't mind keeping this discussion going as a "New-to-me" RT is having symptoms of stuttering just off idle while under load. Idles perfectly, but between the idle and getting on the needle jet it stumbles, then works just fine up to wide open throttle. I've got new diaphragms as well as new needles, jets on order but they won't be here for a week or so. I was under the impression that the main symptoms of a bad diaphragm would be that the bike would not accelerate past a certain RPM, but I'm now wondering if a very small tear might just cause one of them to rise more slowly than the other but eventually pick up as more negative pressure ensued. I inspected them and they appeared OK, so I'm thinking it's probably the transfer ports (40 mm Bing) that will need cleaning once I have the carbs off, but will still swap out the diaphragms as well "just because"
    #9
  10. PaulBarton

    PaulBarton Long timer Supporter

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    I wouldn't really call this thread a "discussion"...

    Just so I understand what you're hoping to accomplish...you have replaced your diaphragms with new and visually inspected your new diaphragms to confirm they were not punctured, torn or otherwise rendered useless but you want to keep a discussion going about torn diaphragm symptoms?

    Why not start a thread about the specific issues you are having with your RT? That way you can describe as best as you can the symptoms that you are experiencing and the different steps you've taken to diagnose/resolve the issue. I would suspect that you will get more qualified responses to your specific issues and increase your knowledge base exponentially relative to this thread.

    Just remember to include the year and model of the bike as well as any aftermarket do-dads you have installed.
    #10
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  11. crazydrummerdude

    crazydrummerdude Wacky Bongo Boy

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    If we're still on topic of torn diaphragm symptoms; in my experience, the bike runs and functions fine below about 4k RPM and 55mph. Any more throttle just resulted in a louder bogging and no increase in speed.

    Around town, it was OK. On the back roads and highways, it was torture.
    #11
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  12. TwoShoes

    TwoShoes Carbon-based bipedal life form

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    Nah, have not replaced the diaphragms yet, they are on order. I visually inspected the ones that are on the bike now. Sorry if my reply was confusing to you. I'm not looking for help on my RT at this point, as I haven't even taken the carbs off to properly inspect them and clear out any ports that may be blocked. I was simply musing about symptoms of a torn diaphragm thinking that a small tear/puncture could potentially cause a small leak that might not present as a full-blown "won't accelerate beyond 4000 RPM" kind of scenario but might be subtler in symptomatology.
    #12
  13. patrkbukly

    patrkbukly Life at 10,000 feet Supporter

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    Be sure both cables are seated in the carbs with same freeplay in each.
    2mm off of one or the other can contribute to that classic stumble off of idle.
    #13
  14. Plaka

    Plaka Brevis illi vita est

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    while riding along, reach down and pull up on the throttle cable on one side. Then try the other side. If one side does not pick up, that side has a bad diaphragm. Do not do this in traffic.

    I only replace things that have proven to be bad. It has been a great number of years since I saw a pic of a bad needle. The clip had wallowed out the groove.
    #14
  15. motu

    motu Loose Pre Unit

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    I had diaphrams fail a couple of times on my Honda XLV750 - I could remove the air filter and look down into the carb throats to see the slides lifting...dangerous at the speed required. This will be difficult to do on an Airhead.
    #15
  16. Disston

    Disston ShadeTreeExpert Supporter

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    I was suspecting the diaphragms once about 15 years ago. Turned out to be the Grove in the carb body for the D to sit was too deep. This was the subject of a column by Oak in the Airmail. Following his advice I put the carb main body on a flat plate with some fine sand paper and gave them a whirl for a bit. Levels and flattens the mate surface. Can't be done on the carb top. I don't remember his figure for what the depth of the groove should be but the number is not important. Placing the diaphragm on the carb body should be explanatory.

    Another time on a trip back from York, Pa, to visit the Hardley factory one of my carbs stopped working. Rode the last 50 miles on one cylinder. The bad diaphragm had a tiny hole in it that could only be seen when stretched between the fingers. That was 40 years ago.

    So I check the diaphragms whenever I rebuild or just clean the carbs. We were getting fewer miles from them about 20 years ago but today's stock seems to last pretty well. My current pair have been in use for 8 or 9 years.

    I use BMW diaphragms. Always have. Some riders get good service with the cheaper substitutes but I have also heard otherwise from others. I think the bad reports out number the good reports but not something that's easily surveyed.
    #16
  17. Disston

    Disston ShadeTreeExpert Supporter

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    One other problem that is simuler . The metal disk in the carb top that seals the air pressure raising the slide. It is crimped in place and it can be loose due to several problems. When the pressure leaks here the slide doesn't raise far enough. Simuler to the problem of a bad diaphragm. The fix this time is epoxy on the carb top. Sometimes the leak is so small it can be sealed by painting the top, around the edge, which is the crimped seal.

    So I have given you 3 problems that may be mistaken for bad diaphragms and their fixes. What do I win?
    #17
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  18. Runaway Train

    Runaway Train The Left-Right Tourer

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    My undying gratitude and respect.....but you already have that.
    I always appreciate your posts disston
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  19. thebisch

    thebisch Been here awhile

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    That makes no sense. I am asking for experiences, so you decided not to share yours?
    #19
  20. thebisch

    thebisch Been here awhile

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    Your kidding right? A poll? WTF? I asked for symptoms. One person gave theirs. A bunch of others made remarks to just change them. I have read that mess of words on the yellow site. Doesn't mention what the actual symptoms are. Maybe its buried in there somewhere. Seriously, how do you think the other responses answer the question? And yes, I am trying to fill my cranium. You seem to have a real problem with that and my questions.
    #20