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Thread: 1981 fxs breaking down when hot

  1. 1981 fxs breaking down when hot

    Hi I have a 1981 lowrider last couple of times I have been out it has misted and come to a stop leave it 10min and it would fire up then do it again down the road .
    My first thought was the fuel cleaned fuel bowl and air filter still doing it next was condenser but this model does not have any .
    What would be my next step to go over .
    Thanks Glen

  2. #2
    Join Date
    Mar 2008
    Beautiful Northern NM

    Re: 1981 fxs breaking down when hot

    Possible coil failure, possible ignition module failure, possible gas cap venting issue, plus lots more. Need more info. And how about posting an intro so people know who they are talking to.
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  3. #3
    Join Date
    Feb 2013
    Sydney Australia

    Re: 1981 fxs breaking down when hot

    First of all welcome on your first post. ( intro would be nice)

    Your issue is possibly due to a non vented fuel cap causing a vapor lock and starving fuel.
    Next time it starts to miss, loosen the cap and see if it clears.
    Last edited by KELD; 12-31-2019 at 02:36 AM.

  4. Re: 1981 fxs breaking down when hot

    Hi all Glen here from Australia sorry for no intro concerning the fuel side I rode it and took off the cap it didnot have any pressure behind it and made no difference to the bike it still slowed down and stalled . I have checked the high tension leads they are o/k I have fitted new plugs but haven't had a chance to see if this is the fault yet . I was told it could be the sensor pick up or the module but I thought the Module either works or doesn't work is this correct

  5. Re: 1981 fxs breaking down when hot

    Sorry for no intro I have been on hear before Glen from Australia member 20 odd years . As to my problem I have ridden it and taken off the cap while riding there was no vacuum and it did not change the bike stalling.
    I have checked the leads they are good new orings on the manifold carby checked fuel cleaned air filter Have fitted new plugs but haven't tested it yet .
    I get around 40klm before it plays up and it was a cool morning .
    I have been told that the ignition module is a on /off as it works or doesn't . How would I check the sensor in the cone to see if it is my fault. and if the coil is the fault I would have to check it when it is hot for a flying short I believe .

  6. #6

    Re: 1981 fxs breaking down when hot

    How about the fuel strainer inside the tank? Perhaps a fuel delivery test to eliminate restriction?

    Best way to eliminate the coil would be swap in a known good one from another running bike. Pay to be sure the existing coil is the correct ohms type etc.

    I've seen pickups fail and falter with age. I recall one time when a BMW R80 would quit after it got hot and resume once it cooled down. I was able to get a temporary workaround by leaving the big ignition cover off and air cool the pickup. Finally I was able to solve the issue completely by fitting a replacement. Funnily enough the pickup was salvaged from a junkyard VW car in Austria with the help of local bikers. I continued my ride (c1995) doing several thousand more miles eventually selling the bike off in UK.
    Here again you may only be able to eliminate the possibility by substitution. Sadly not all of us has pieces on a shelf ready to go.

    Good luck, hope this helps. Hope you are safe from the big fires...!

  7. #7
    Join Date
    Aug 2010
    Twin Cities, MN

    Re: 1981 fxs breaking down when hot

    no experience with FX Harleys but just thinking out loud.....Coils are relatively inexpensive. Around $60. ? if you want to be a parts swapper you could always try that first.

    but if you want to be scientific and test it.....
    maybe test the coil by heating it up with a hair dryer? ohm it out? maybe ride it and when it fails check its temperature with one of those fancy laser thermometers? then see if you can duplicate it on the bench by heating it up? or take your ohm meter with you on your ride and then ohm it out and see if it passes or fails? Again just thinking out loud to eliminate the coil as the problem. then I would move onto the sensor and then the module which sounds expensive.....Im sure there is a way to test the module but Im low tech kind of guy.
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  8. #8

    Re: 1981 fxs breaking down when hot

    Pull your points cover and see if the potting mix is leaking out of the triggger module. A well known fault with these ignitions. This picture is of a later model, but the issue is the same.

    Sensor melting.jpg

  9. Re: 1981 fxs breaking down when hot

    Since this is an electronic ignition system you can load test the entire system with a electric engraver, older soldering gun, or tattoo gun. (actually any device that creates and collapses a magnetic field. With nose cone cover off, plugs out and grounded, rest of system connected, and ignition on. Turn engraver on while holding close to ignition pickup, the expanding and collapsing magnetic field will simulate the hall effect switch working and fire the plugs. Continue for several minutes or however long you want noting firing of plugs. If something is weak in ignition, problem will occur. Sadly, this will not tell you which component, but will confirm ignition system in general is good or bad.

  10. #10

    Re: 1981 fxs breaking down when hot

    A hair dryer is also a good way to heat up the system and possibly show any heat related issues.

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