'76 R75/7 restoration

Discussion in 'Airheads' started by Steril, Mar 26, 2021.

  1. korinthias

    korinthias slightly behind the curve…

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    @Steril – that bike is indeed a nice unmolested example, despite the tank swap. If it is indeed a ‘one year only’ bike, it would be good to keep it that way, more-or-less. Post some more pics, when you have time…
    #21
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  2. Steril

    Steril Another Millenial

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    @korinthias I'm keeping it stock, no hipster or performance mods are planned. Might upgrade the suspension since it's well past its heyday but will try to find shocks that are as close to the originals as possible.
    Here are some pics from the original classified I've bought it off.
    R75_7 chomutov.jpg R75_7 chomutov_2.jpg R75_7 chomutov_3.jpg R75_7 chomutov_4.jpg R75_7 chomutov_5.jpg R75_7 chomutov_6.jpg R75_7 chomutov_7.jpg R75_7 chomutov_8.jpg R75_7 chomutov_10.jpg R75_7 chomutov_9.jpg
    #22
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  3. Disston

    Disston ShadeTreeExpert Supporter

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    I'm constantly surprised at the parts Motobins comes up with. I think they have them made tho, not OEM. Still my experience with them has been good and like all their substitute parts except the Venhill cables. may work for others but I prefer BMW cables for fit and function.

    They have the spring. great. What I did not see was the lever, the one you rotate, or the two washers, one with holes and one with pimples. These three parts are what makes the click positive. All three need to work and they wear. The levers loose the indexing nub. The washers wear away. Petcocks will work with these parts worn but the positive click feature of them is a desirable feature. Maybe Motobins will do them eventually. They have made or sourced the spring which nobody else has.
    #23
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  4. Steril

    Steril Another Millenial

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    The two washers you are talking about are worn away in my case, no clicks at all. My fuel taps worked okay otherwise, the refurb was just to be able to trust them in the future, so changing the whole original taps to get indexing back doesn't make sense to me. Would totally buy the separate replacement parts if they become available eventually though.
    #24
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  5. Steril

    Steril Another Millenial

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    Some weird electrical issues came up, I was afraid of that, electrics is absolutely foreign to me so this is going to be tough.

    1/ The starter switch intermittently doesn't do anything at all, I can't find any variables that decide whether it does or doesn't.

    2/ The started engine cuts out when I move the light switch to the low position. The headlight works as it should.


    3/ The other is blinking dash lights and weird noises when I turn the key in the ignition. Intermittent again. Seems to depend on the light switch position.


    Any expert advice, please? I hope this is not some edge case no one will be able to advise on.
    #25
  6. Beemeup

    Beemeup 1978 R100/7

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    Bad connections to put it simply. There's a relay that engages the solenoid that engages the gears to turn the starter. The solenoid contact can get rough, I've smoothed mine out. You can chase electrical gremlins as they occur but it could happen on the road. For a bike that old I would go through everything. Clean up, replace connectors, check with a meter, etc. And don't forget the grounds.
    #26
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  7. R27warrior

    R27warrior Adventurer

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    #27
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  8. Steril

    Steril Another Millenial

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    @Beemeup Thanks, I'll do it this weekend and update on the result. I've researched the replacement relays and voltage regulator, the electrical components are surprisingly relatively cheap. Even the whole starter can be had for as low as €45. Not that the bargain basement option is something I'd want to use, but the range is nice.

    @R27warrior Thank you, found out today that one of my rebuilt fuel taps leaks, so I'll consider ordering one.
    #28
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  9. sprouty115

    sprouty115 Long timer Supporter

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    I'm in...:lurk
    #29
  10. Beemeup

    Beemeup 1978 R100/7

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    I've never bought a BMW relay or voltage regulator. The $5 relays work fine and the regulators (I've gone through two) were about $15 from the same. Used on VWs I believe. The plugs are even the same.

    One other thing to mention is looks and even a meter may not tell the whole story on connections. I found my main starter lead at the starter to be pretty well corroded under the insulation. I was having trouble cranking and the battery was good so I tore into it.

    I cut about a 1/2" off and resoldered the big lug back on. It was at about 35 years old by then.
    #30
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  11. Steril

    Steril Another Millenial

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    I had some time today, disconnected the ground from the battery, and started to look for gremlins.
    There was a loose wire on the left coil, black/green leading to the cut-out switch, and the contacts were disgusting, so I cleaned them and tightened everything.
    coil_contact.jpg
    Kept going around the bike, cleaning all the contacts I could lay my hands on.
    The headlight spaghetti looked surprisingly clean and tidy.
    IMG_20210428_200458.jpg
    The voltage regulator and its connections looked okay but it seemed prudent to change it for new parts. From all the options, Hella 5DR 004 243-111 won my heart by looking like the original and costing €11.
    Moving on, the starter relay didn't look that bad, I ordered a replacement anyway (€3 Hella 4RA 933 791-161), but the connections to it were beat. One of them even broke when I took the bracket off (black cable, leading to the starter motor). Everything in there was super corroded, so I'll hope this was the cause of my electrical problems.
    Won't find out till the new connectors arrive next week - we are just coming out of lockdown and the shops are still closed.

    I opened the front engine cover, the alternator doesn't look all that fresh, so I'll consider changing it for good measure.
    alternator.jpg

    There seems to be a recurring problem with this bike - the right side and the parts there are in a much better shape than on the left. My hypothesis is that it must have been parked outside, on a side stand, for at least one season. That would explain the super rusted seat pan and the need to switch the gas tank. Yeah and the horn is on the left side, so it obviously doesn't work, the contact is corroded so I will try to clean it but will order the "Horn kit" from motobins if it doesn't work.

    All in all, working on the wiring of an old motorbike isn't as terrible as I thought, thank god I don't have to dig into the electrics of my 2010 BMW.
    #31
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  12. Jim K in PA

    Jim K in PA Long timer Supporter

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    Someone has spent some time in that headlight bucket. I see a bunch of splices, and a partially wired extra relay.

    That alternator actually looks cleaner than most that I have had to service.
    #32
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  13. Disston

    Disston ShadeTreeExpert Supporter

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    Agreed. That alternator doesn't look as crusty as the one in my bike which works. Problems in addition to crusty wiring include the brushes being too short to make proper contact. Has nothing to do with looks tho.
    #33
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  14. Beemeup

    Beemeup 1978 R100/7

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    I gained significant charging improvement when I took that three wire harness totally off the alternator and diode board and meticulously cleaned all the contact points. The red charging light doesn't even come on at a proper idle anymore. I also use dielectric grease on all electrical contacts.
    #34
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  15. Steril

    Steril Another Millenial

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    Thanks for the hints guys, I'll be happy to keep the alternator and just change the brushes and clean the contacts.
    @Jim K in PA Glad you wrote about that extra relay, I had troubles reconciling it with the wiring diagram!
    #35
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  16. arbalest

    arbalest Long timer

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    Motobins?
    #36
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  17. Steril

    Steril Another Millenial

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    Thanks, PaulBarton and korinthias have kindly directed me to Motobins and a package is already on its way.
    I've done most of my parts purchases either on Siebenrock and bmwbayer.de so far. There's also a German general classifieds website https://www.ebay-kleinanzeigen.de/ with a lot of vintage BMW stuff, where I found the seat.
    #37
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  18. Steril

    Steril Another Millenial

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    In another twist of events, I came to the motorbike today and there was a puddle under it. The source seems to be in pushrod seals on the right side. The liquid doesn't seem to be 100% oil, it's not thick enough.
    My tank is off now but it's been on the motorbike for a few days last week and the fuel tap on that side drips quite a bit even when it's closed. I've checked the spark plug on that side and it's very wet with gas.
    Does this mean what I think, that my oil got contaminated with gas? If so, would changing the oil and filter+cleaning the oil pan on a cold engine be a bad idea? Do I need to take the engine apart to change the pushrod seals now?
    #38
  19. Disston

    Disston ShadeTreeExpert Supporter

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    Sounds like the float needles in the carbs letting gas flow. If the petcocks don't turn off the gas then the float bowls fill and gas will both drip and enter the combustion chamber. If this is happening the drip is the least of your problems. Remove spark plugs, ground spark plug leads and spin the engine with the starter to shoot accumulated gas out the plug holes. This is both remedial and a test to see if there is a problem. If gas exits the plug hole/s take care of this problem first.

    #1 always turn off the petcocks when the bike is parked for more than 5 minutes. To test if the petcocks work disconnect the hose at the carb and leave a clean paper under it over night to see if an gas leaks out. If so rebuild or replace petcocks. Faulty petcocks or lack of petcock discipline are a serious problem with our old bikes and lead to the destruction of several bikes every year.

    If the petcocks leak or were left on the carb floats and float needles should stop the gas. The floats don't float given too much age. They can be tested. They may be fine but need adjustment.

    The float needles are also subject to issues. The Bing's system of a metal needle tip in a metal seal will wear out. Adjustment of the float can keep the system working but eventually the needle and/or the seat need replacing. Replacing the needle is the first step. If that doesn't stop the needles from leaking it is time to replace the float needle seat which is a bit of a surgery procedure we'll have to tackle if you need it.

    By all means change the oil if gasoline is suspected. You have to ascertain where the problem is before going much further. The PRT seals may have a little oil on them but be fine. Zero oil leaks is too often the aim when zero oil leaks is the enemy of proper function. Is it an oil leak, a drip or an ooze?

    I've lost my train of thought at the moment and have to stop. Find out what the problem is. Gasoline leaking is a bigger issue than a little puddle of oil. In the old days we would say a puddle of oil was a sign of a healthy engine because it had oil.
    #39
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  20. Steril

    Steril Another Millenial

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    Thanks, Disston, Id' call it an ooze, cleaned it to see how much it changes after a day of standing. I was hesitant to change the cold oil but it's clearly better than doing the warmup ride with contaminated oil.
    #40