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Thread: Gradual decline in performance, R75/6

  1. #1
    Airmarshal-IL James.A's Avatar
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    Gradual decline in performance, R75/6

    Hello all, one of our IL members is experiencing a loss of performance with his R75/6. We are looking for ideas. Original owner with an extensive list of experience with his bike. All stock except for a Dyna III ignition. Earlier this year, the owner noted a decline in fuel economy (MPG). Now the bike has trouble achieving a highway speed much over 60 mph, however, the motor revs to 4000 RPM easily in neutral on the center stand. If anyone has experienced this, or has any ideas about it, please let me/us know.
    1973 R75/5

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    ABC,AMA(LIFE),MOA,RA,IBMW MANICMECHANIC's Avatar
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    Greetings, Jim! Simple stuff first, clutch adjusted properly, compression check, valves adjusted properly. When was the last time the carbs were cleaned/rebuilt? Clean air filter? Timing set properly? Something in there should direct one to the fix.
    F.O.G.Rider, Rounder #6,
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  3. #3
    Liaison 20774's Avatar
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    Not being able to get to or hold 60 mph suggests torn diaphragms in the carbs. Or possibly a restricted air filter - dirty or a house for local varmints.
    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!

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    Registered User barryg's Avatar
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    How many miles on the bike? Do you have the history of minor and major services performed on the bike?

  5. #5
    Registered User lkchris's Avatar
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    Diaphragms.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

  6. #6
    Ditto on above. Check low hanging fruit as well: 1) what was the last thing you did to the bike; 2) make sure the carb to head connections are tight.
    '61 Clubman's Gold Star, '13 690 Duke, '13 Daytona 675R, '17 1290 GT, '18 Street Triple RS (gone but not forgotten: '76 R75/6, '84 R100, '76 R90S)

  7. #7
    Bluenoser
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    First thing I would check is fuel flow. Check fuel flow coming out of the gas tank, and at the float, which would require the removal of the bowls, and put something to catch the fuel when you open the petcock at the gas tank.

    The symptoms described are classic lack of fuel.
    1995 R100Rt with Kenna Sidecar, 1998 VT1100T

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    Check the advance mechanism.

  9. #9
    Quote Originally Posted by cycleman2 View Post
    First thing I would check is fuel flow. Check fuel flow coming out of the gas tank, and at the float, which would require the removal of the bowls, and put something to catch the fuel when you open the petcock at the gas tank.

    The symptoms described are classic lack of fuel.
    Also, make sure that the cap vent is open and not creating a vacuum situation not allowing enough fuel to flow.
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

  10. #10
    Airmarshal-IL James.A's Avatar
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    Quote Originally Posted by cycleman2 View Post
    First thing I would check is fuel flow. Check fuel flow coming out of the gas tank, and at the float, which would require the removal of the bowls, and put something to catch the fuel when you open the petcock at the gas tank.

    The symptoms described are classic lack of fuel.
    What I don't know is if Howard W., the owner, rides with one petcock open or 2, I will suggest to him to try using both taps to see if that makes a difference. That doesn't cost anything to try. Howard is a pretty good owner/mechanic and has assured me that the diaphragms are O.K. This is why he and I are somewhat stymied by this.
    1973 R75/5

  11. #11
    Registered User lmo1131's Avatar
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    Extreme, but possible; might be a crank bearing(s). I had a VW that went that route. It would idle fine, rev up okay (sitting still) but a slow decline in performance (at speed) until it finally soze up one afternoon.

    But I'm a strong proponent of "all-at-once" tune ups ... not doing the valves one weekend, the points the next, changing the oil the following weekend, etc. Too many "fine points" can get lost in the shuffle.

    Air filter
    carbs (pull/inspect diaphragms, clean, pull jets, blow out, etc)
    valves
    plugs (pull check gap as required)
    ignition (pull advance (check springs & lube) gap (I still use points (NORIS), condenser, and time )
    test ride
    sync carburetors
    "It is what you discover, after you know it all, that counts." _ John Wooden
    Lew Morris
    1973 R75/5 - original owner
    1963 Dnepr

  12. #12
    Quote Originally Posted by lmo1131 View Post
    But I'm a strong proponent of "all-at-once" tune ups ... not doing the valves one weekend, the points the next, changing the oil the following weekend, etc. Too many "fine points" can get lost in the shuffle.
    Doubt if the problem would be crank bearings as I would think this would be accompanied by some sort of noise.

    However, my real comment is about above comment. I am not a proponent of an all-at-once method of doing much of anything unless one item is "one the way" when doing something else. The Reason? I think doing only a small thing at a time is better, as it allows for better diagnosis if there would be a change. At least you can narrow any changes to just what you did. If you do a whole ton of things at once, then troubleshooting becomes more problematic trying to determine which thing you did caused the problem.
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

  13. #13
    Bluenoser
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    Quote Originally Posted by SCBuckeye View Post
    Doubt if the problem would be crank bearings as I would think this would be accompanied by some sort of noise.

    However, my real comment is about above comment. I am not a proponent of an all-at-once method of doing much of anything unless one item is "one the way" when doing something else. The Reason? I think doing only a small thing at a time is better, as it allows for better diagnosis if there would be a change. At least you can narrow any changes to just what you did. If you do a whole ton of things at once, then troubleshooting becomes more problematic trying to determine which thing you did caused the problem.
    I agree, do one thing, see if it changes anything then move onto the next. Standard problem solving.
    1995 R100Rt with Kenna Sidecar, 1998 VT1100T

  14. #14
    Airmarshal-IL James.A's Avatar
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    Well, I have mailed a set of known to be good coils. We are grasping at straws here, I don't hold out much hope for the coils to reverse the decline.
    The bike is a 1975 R75/6 with 177,000 miles on it (original coils). Howard is the original owner and a competent mechanic who does most all of his own work. If he says the diaphragms are O.K., I have no reason to not believe him. If or when I get a chance to travel, I will meet up with him and we will go over the obvious items. This would not be the first time I have done this with one of our "Airheads". I will report any results here for the benefit of all. I suppose the answer will be revealed "in the bye and bye".
    1973 R75/5

  15. #15
    Registered User barryg's Avatar
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    Just guessing, the bike has been well maintained; but. The top end needs a rebuild/rework. The valves and seats are worn out. The piston and rings and cylinder are probably worn out. Carbs probably need a rebuild. 166,000 miles is a lot for an airhead. This is more than a standard minor or major service. Good luck with your project.

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