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Thread: R75/5

  1. #1
    Registered User
    Join Date
    May 2012
    east moriches, new york


    Greetings. So, I am a newbie and have just gone and purchased a 1973 R75/5. The bike is in very nice condition. 29000 miles.. We are getting to know each other.
    I like to tinker with stuff like this, so thanks for any one who can help me here.

    First issue I am having is with the engine running rough at higher rpm levels.
    As I get up to 3500 and apply power, the engine starts to sputter and will not accelerate. Max speed in 4th gear is 50 mph. It almost seems like at that point It isn't getting fuel. and bogs down. Logic tells me there is some blockage somewhere. . Any thoughts on this are appreciated.
    I am also a little confused on the proper location of the fuel petcocks. The manual says that when the cock is inline with the fuel line that this is the off position, but the label on the valve says down is on and up is reserve and the 3 o clock and 9 o clock postitions are off. The label thing can be moved, so maybe someone moved this around. Can you tell me what setting is off on this thing.


  2. #2
    Liaison 20774's Avatar
    Join Date
    Sep 2005
    San Antonio, TX
    Steve -

    Welcome to the forum. I'm going to move this to the Airhead forum which is located a little further down the forum. The Airhead forum is where more technical discussions are. The New Member area is more for general introductions re: you and the bike and the kind of riding you enjoy.

    As for the lack of power, the R75/5 should have CV or constant velocity carbs. The lack of power over 50 mph typically is torn diaphragms in the carbs. If they have a hole in them, no vacuum can be developed above them and thus the slide can't lift enough as you demand more throttle.

    As for the petcocks, not sure what manual you are reading, but that is wrong. The large fob on the handle pointing to the ground is the "main" tank. Fob pointing to the sky is the "reserve" tank and foward/backward is off.
    Kurt -- Forum Liaison ---> 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
    Sir Darby Darryl Cainey's Avatar
    Join Date
    Mar 2003
    Fenwick, Ontario Canada
    You may have a blocked gas cap vent.

    When riding, and it boggs down, crack open the cap.

    If it picks up, it's a blocked gas cap vent.

    A little air should do it!
    Ambassador BMW MOA Ontario Canada
    President Niagara BMW Riders #298
    Knights of the Roundel #333
    1977 R100RS, (Retired) 1993 R100GS (just getting started)

  4. #4
    #4869 DennisDarrow's Avatar
    Join Date
    Jan 2006
    Sugar Valley, Ga
    For sure, Kurt is probably 99.9% right. You get to it by removing the 4 large screws around the top of the is it 3 ??.......anyway that cap comes off and there is the piston with the rubber diaghram attached. Probably has not been renewed ever; but with ethanols one needs to replace it every few years.........While you have the top off go ahead and remove the fuel bowls and take out the main jet........the brass looking hex head thing looking up into the carb from the bottom..........Be careful taking it apart as it isnt complicated but it does matter to go back together correctly.......On it should be a couple of "O" rings that I bet are deteriorated............NOW.......blow out every orifice you see with like brake or carb cleaner.........

    Chapter 2 begins after you get it started again...........

    What is left is idle and syncing..............

    Be careful when removing those screws on the top of the carb.......the heads/slots strip out easily..........Worth your time to do some searching on here to find out several tricks to remove them..............God bless.......Dennis

  5. #5
    Registered User
    Join Date
    Sep 2008
    Planet Earth
    My first BMW was a 1973-1/2 R75/5 (Long Wheel Base). It was blue toaster tank. Wish I still owned it.

    Okay, here is what you should do (starting with the easy solutions and moving to the harder ones. Try 1-5 before doing #6.):

    1. Take the float bowl off and see what is in the bottom. If can sometimes find water, gel, rust, and inner gas tank paint chips. Dump the bowls and clean the bottom of the bowls.

    2. Disconnect the gas line at the petcock. Hook a clear plastic line (3-4' long) to it and place the other end of the plastic tube in a container. Turn on the petcock, and see if the gas flows out well. If it doesn't then you need to remove the petcock and clean it and the bottom of the gas tank. Check BOTH petcocks.

    3. Check the fuel lines to make sure they are not clogged.

    4. Check the vent on the gas cap.

    5. Check the throttle cables to make sure they are not frayed inside the cable. When you turn the throttle, they should move and return smoothly.

    6. Remove the carb and remove the 4 steel screws on the top of the carb. Sometimes the screws can be difficult to remove, so don't strip them. You can carefully use an impact screwdriver if the screw is seized. Once the screws are out, CAREFULLY remove the top. When you remove the top it will be attached to a fragile rubber diaphragm and the slide. Put your finger in the throat of the carb and help push the slide up when SLOWLY removing the top. If the diaphragm is ripped, you will need to replace it. I like to replace both diaphragms at the same time if one of them is ripped.

    Most likely your problem is one of the above issues. Here are some other notes:

    1. Varnish from the gas can build up in the slides and make them stick inside the carb. If this happens, you will need to take apart the carb and soak them in carb cleaning fluid bath for 10 minutes. Then hose them off and dry them completely. Remove the jets and blow them out with air.

    2. You need to balance both carbs once you are done. There is a forum post on how to make your own carb vacuum balancer for a couple of bucks. Both carbs must be balanced perfect for the bike to run well. Not hard, but it must be done.

    The other issues is that the points or condenser might be bad. Not hard to replace and reset. There are plenty of places that will teach you how to do it. But, I think your problem is a gas issue. Also check the air filter to make sure that a mouse didn't make a nest in the air filter box.

    This bike is real easy to work on.

  6. #6
    Registered User
    Join Date
    May 2012
    The only universal piece of advice I have for someone who likes to tinker and recently purchased an old motorcycle having expensive parts is to buy some kroil:

    With a good dose of patience, this works minor miracles. Use sparingly and only where needed - kroil can loosen parts that were never intended to loosen such as parts with loctite and retaining compound. Liquid wrench is a similar consumer product - kroil is the industrial version.

    AH# 13238

  7. #7
    Any FLAPS will have PB Blaster. Almost as good as Kroil for penetration. Better than both, and cheaper, is half acetone and half ATF.

  8. #8
    Registered User JB1982RT's Avatar
    Join Date
    Jul 2003
    South Carolina

    I have been riding, and working on an R75/5 for about 20 yrs now and you will not find a bike that is more fun to ride and maintain.

    I don't think I can count the number of people that have come up to me at convenience stores, gas stations or restaurants that comment on the bike.

    Get the Haynes and Clymers manual. If you want to pay close to $100 you can find an actual BMW shop manual for the bike. I did and have found it useful as a cross reference.

    I purchased a parts book in seven languages many years ago that shows the fiche by part number.

    Now, you can see that same thing on Max BMW site, or most other Dealers' site. They are very good in helping you put things back together.

    Bing puts out a good book on the carbs. I think they are in Kansas City.

    Do not be afraid to ask questions on this site. These guys have got me out of more than one fix.

    You have the classic bike. Be proud, be patient, and do not take shortcuts.

    You will discover a riding experience that is truly special.

    Good Luck and Safe travels.

  9. #9
    Registered User
    Join Date
    May 2012
    east moriches, new york
    Thanks for all the help. I did take the bowls off the carbs. They are clean. Actually previous owner tells me the were completely disassembled and bolied and reassembled. I tend to believe it as this machimnne has been restored and is really quite nice.
    I did notice, I think< that there is no gasket on the one float bowl I removed. If that is the case, maybe air is getting sucked in when I put the power to it?

    It think the biggest problem with this bike is that has been sitting unused for too long. The previous owner told me he bought it out of a collection and only drove it 10 miles in a year.

    Anyway. This begins a new journey for me so thanks for the tips. I look forward to more.

  10. #10
    Registered User
    Join Date
    Sep 2008
    Planet Earth
    If you are sure that the problem is not with the carb, then move on to the spark.

    1. Check the spark plugs to make sure they are not cracked or bad.

    2. Hook a timing light up and check the timing (remove rubber plug on the left side of the engine. Check timing at idle and full advance at higher RPM's

    3. Take the front cover off the front of the engine.
    (a) Check the points.
    (b) Replace the advance springs if there is a problem with the timing advance.
    (they are cheap so you might want to replace them anyway)
    (c) Check the condenser

    4. Test the coils and spark plug wires.

    5. If 1-4 is okay then go back to the carbs.

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