Terrible Gas Mileage (<20 mpg) - Possible Causes?

Discussion in 'Airheads' started by MLCOakland, Nov 6, 2020.

  1. MLCOakland

    MLCOakland Adventurer

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    Hi all,

    I have a new-to-me R80ST, with about 35K on the clock. I don't have a history on it, but the fellow who sold it to me claimed to have gone through everything to bring it back into shape. I think he did a pretty good job, although I am finding some minor issues here and there (leaking brake reservoir, missing o-rings in the shifter). It runs pretty well: starts easy enough, engine sounds good, decent power, and everything seems to be on par with my previous airhead (R65).

    The biggest problem is that the tank funs empty at ~55-60 miles, which means I'm getting somewhere under 20 mpg. I do expect to get lower mpg because I mainly ride around town, short distances, and it's hilly here. But <20 is ridiculous.

    I've changed the air filter, spark plugs, oil and filter, new rear tire, and checked valve clearances. Now I think it's time to take the carbs apart and clean them.

    But I wanted to see if there's something glaring that I'm missing. (The only thing I could think of was the brakes dragging on the wheels, which they don't.)

    FYI, I have the manuals, some tools, and am generally handy, but not a ton of experience working on motorcycles.

    thoughts and advice?

    Greg
    #1
  2. boxermoose

    boxermoose Regressive airhead Supporter

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    :jackbut the fellow who sold it to me claimed to have gone through everything to bring it back into shape:jack

    So 4 US gallons of fuel are used between fill-ups? - your tank should be bigger than that - empty and pull/check petcock screens and make sure they are not reversed

    I'd suspect a whole bunch of little things are adding up
    #2
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  3. dave0

    dave0 Been here awhile

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    Worn out jets/needles.
    Timing.
    #3
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  4. Warin

    Warin Retired

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    Carbs
    ingntion - timing
    compression

    The last 2 can be tested by easy measurement. Do it and put them out of the picture.

    Then do the carbs - note check parts have not been swapped from left to right!
    #4
  5. noman

    noman Long timer

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    what did the plugs look like?
    #5
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  6. Plaka

    Plaka Brevis illi vita est

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    the brakes were a good guess. you ruled them out.

    an overly enthusiastic right wrist will do in mileage. I could get 50mpg out of my RS on bay area freeways if I was restrained. This dropped to 40 if I wicked it up. So clean up your act on that and yes, we know what you've been doing.

    petcocks don't clog and if one did you would be feeding less gas, running lean, over heating and getting great mileage.

    Gas can get burned, can leak away, or both. I gather no leaks have shown up. They look like puddles and smell like gassy boots.

    Needles and needle jets do not wear out in 35k.

    If the motor is perky the compression is good enough.

    If it starts well the timing is good enough.

    Carbs can get dirty but there are very few small passages to be affected and when they are distinctive symptoms appear, all having to do with running lean, that is, burning less gas.

    Three carb issues will burn more gas; sunk or maladjusted floats, an enricher not shutting off and incorrect jetting.


    On a new to me bike I base line the carbs: never touch the butterfly shaft, never poke the diaphragms, do make sure all screws free, fresh o-rings on the jet stacks, spray through passages, fresh cork gasket, fresh enricher o-ring and gasket (glued in the right place), all jet numbers recorded in the log. I live at altitude now so I will be re-jetting. Stock jetting and the corrected jetting are in the Bing book and elsewhere. Weigh the floats. Ignore the float needle until proven bad.
    #6
  7. Solo Lobo

    Solo Lobo airhead or nothing Supporter

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    Are you shutting the petcocks every time you turn off the bike?

    does the oil appear to be over filled and smell gassy?
    #7
  8. Square1

    Square1 Been here awhile Supporter

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    Changing the carburators needle and needle jet is where I would start.
    Check the parts fiche with your VIN number , there was a change of jetting midway through production.

    When I had that problem on my ST it might have been a weak Ignition coil.
    It was running well but high fuel use, could not fix it until the coil stopped working and a new one with a stronger spark fixed everything.
    #8
  9. Plaka

    Plaka Brevis illi vita est

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    tire pressure
    #9
  10. Dreossi

    Dreossi Been here awhile

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    I’d start by adjusting the hight of the needles, it’s easy to get the clips in the wrong grove. See if you find them at the factory setting, second notch I believe.

    Maybe confirm factory jetting as well.
    #10
  11. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    Or wander down to Redwood City and put her on a lift and see what might be a little off.
    #11
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  12. MLCOakland

    MLCOakland Adventurer

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    They were black, but not super sooty. They just looked like an old set of plugs but I didn’t think they showed anything unusual...
    #12
  13. MLCOakland

    MLCOakland Adventurer

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    I am shutting the petcock (there’s only one) every time. I will have to check the oil.
    #13
  14. tlub

    tlub Long timer Supporter

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    Chokes in correct orientation and the arms not on backwards? That will do it every time. There are about 4 possible combinations of orientation and arms. Only one is correct.
    #14
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  15. MLCOakland

    MLCOakland Adventurer

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    @tlub - do you mean how the choke cables are attached at the carbs?
    #15
  16. tlub

    tlub Long timer Supporter

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    Not just that.
    Two things:
    The internal parts can be assembled wrong backwards- the dot needs to face forward.
    The arms can be upside down, in which case the choke never goes fully off. I friend of mine had the left one upside down and his mileage was in the high 20s. This is more of a problem with the later models without the rigid Bowden cables, but with the springs and the flexible cables. Later means those with the choke on the handlebar instead of the air filter housing.
    #16
  17. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    I work on a ton of G/S, GS, and STs in the Bay area..wonder if yours is one? Does it have 10mm flange nuts holding the valve covers instead of regular nut and wave washer?. I do that to all the bikes that come through.
    #17
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  18. Cubdriver

    Cubdriver Stampede Swimmer

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    I don’t agree with Plaka that the needle and needle jets won’t wear out in 36,000 miles. Mine were shot at 28,000 and I was getting 28-30 mpg. They’re easy enough to check. A simple swap got me up to 40mpg.
    #18
  19. Plaka

    Plaka Brevis illi vita est

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    I run fuel filters so I am not sucking grit through the jets and steel needles, not aluminum. I get better than 90,000 miles. YMMV.
    #19
  20. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    The needles don't wear from anything in the fuel...when the air goes through the air filter, it captures MOST of the dust, bugs, stuff but some micro particles get through..and head for the carb. When they get to the butterfly, everything accelerates (venturi effect) and those little particles hit the needle at speed.

    Early Bings where the needle is installed by pushing it in a numner of clicks means the needle isn't solid in the slide..it wiggles..so the slide going up and down with a wiggling needle under pressure from a fast air flow means is constantly rubs on the jet...and ovals it out over time. Late models held in place with the screw also wear, just not as fast because of litle wiggleing.

    Over time..around 30-40K miles, the little dents in the needle starts affecting the air flow around the needle..creates turbulance....like a golf ball dimples. And that means changes in air fuel mix and gas mileage. Not good.

    Bottom line is that over time..a long time, there is a gradual reduction in efficiency that you really don't notice...like rear shocks..

    So one day you try to adjust the carbs but when you turn the air/fuel mixture screw, nothing happens..so O ring replacement time. Unless you replace the needles and jets,,you miss most of the benefit of the rebuild.

    Difference of night and day. Better mileage, smoothe idle, better acceleration..all the good stuff from a few bucks and a great learning experience afternoon.
    #20
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