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

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

  1. Cubdriver

    Cubdriver Stampede Swimmer

    Joined:
    Aug 3, 2007
    Oddometer:
    810
    Location:
    Palmer, Alaska
    I run filters also, but this was in a newly acquired bike so who knows. They’ve gotta help.
    #21
  2. Beemerguru

    Beemerguru Beemerguru...G/S guy

    Joined:
    Nov 7, 2006
    Oddometer:
    2,076
    Location:
    Redwood City, CA
    First thing is drain the tank and pull both petcocks. You'll see right then if the internal fuel strainer is holding a lot of flaky gas liner, if you even HAVE the stock fuel liners, and what condition.

    Does this every 2-3 years and you won't need external filters... unless you just like to filter filtered gas
    #22
    PaulBarton likes this.
  3. squish

    squish Out of the office.

    Joined:
    Dec 4, 2003
    Oddometer:
    9,553
    Location:
    Where the Ghetto meets the sea.
    STs only have one petcock. At least if they have a stock tank. The tanks are small I only get around 100mike to reserve on mine.
    My needles on my 24k mile Gs were worn out at 20k miles. Replacing them cleaned up some serious fueling issues.

    as for poor fuel economy
    Check carbs for proper function
    New plugs and go for a ride stop and quickly pull and inspect them.
    Check brakes
    Check tank and carbs for leaks
    Check compression and leak down.
    #23
  4. Cubdriver

    Cubdriver Stampede Swimmer

    Joined:
    Aug 3, 2007
    Oddometer:
    810
    Location:
    Palmer, Alaska
    Good advice, Greg. A month ago I smelled gas in the shop and found that one of the external filters, plastic, had come apart and was leaking. Could have been catastrophic when the fumes hit the water heater. I never trusted them before, and now REALLY don't want to have them on the bike.
    #24
  5. Big Bamboo

    Big Bamboo Aircooled & Sunbaked Supporter

    Joined:
    Sep 29, 2010
    Oddometer:
    17,209
    Location:
    Big Island of Hawaii
    If you are running the straight petcocks, there is an additional filter inside the bottom nipple. Turn the petcock off, and it can be removed and checked without draining the tank.
    #25
  6. Plaka

    Plaka Brevis illi vita est

    Joined:
    Jun 19, 2010
    Oddometer:
    7,103
    Location:
    .
    The butterfly is down stream of the throttke slide and needle. Look at the upwind edge of the throttle slide. That is the first major venturi. See wear? It is soft metal. Part of the needle is always exposed above the jet. According to your theory this would see the most wear from particulates in the air.

    I am talking about very fine abrasive silt in the gas. This gets sucked through the jets. I have found it in my float bowls. Fuel filters stopped it.

    I baselined my carbs 8 years ago and tuned them. I check them every 2 years and except for losing idle air screw o-rings they need nothing on tune up day. I don't have to turn a screw. Looking through these pages people are ever fascinated by monkeying with their carbs. Not doing this is the secret sauce. as always, YMMV.
    #26
  7. Beemerguru

    Beemerguru Beemerguru...G/S guy

    Joined:
    Nov 7, 2006
    Oddometer:
    2,076
    Location:
    Redwood City, CA
    Yeah...should know better than post anything at 3am. Tend to forget the other stuff..massive brain fart..and yes, you're correct
    #27
  8. MLCOakland

    MLCOakland Adventurer

    Joined:
    Feb 3, 2015
    Oddometer:
    35
    Not yet. This one came from out of state via SoCal. But I suspect I’ll bring it by at some point over the next couple of years. For the moment, if I can resolve the fuel consumption issue, I want to put some miles on it so I can get a good baseline on how it runs.
    #28
  9. MLCOakland

    MLCOakland Adventurer

    Joined:
    Feb 3, 2015
    Oddometer:
    35
    Thanks, all, for the helpful comments. I am going to check as much of these items as I can over the next couple of weeks, and a local inmate has kindly offered to help me, so I am hopeful we will be able to sort this out. In the meantime, keep those suggestions coming, and I will post my results as I work through the bike!

    Greg
    #29
  10. Plaka

    Plaka Brevis illi vita est

    Joined:
    Jun 19, 2010
    Oddometer:
    7,103
    Location:
    .
    If it is running poorly getting a baseline on 'poorly running' isn't particularly valuable.

    I would take Greg up on his offer.
    #30
  11. Plaka

    Plaka Brevis illi vita est

    Joined:
    Jun 19, 2010
    Oddometer:
    7,103
    Location:
    .
    When I first found very fine, reddish, abrasive silt in my float bowls I wondered where it was coming from. Fuel pump nozzles? Those things are like public toilet seats. You wonder about the last guy. But finally concluded it was from the air. Burn 6 gallons of gas and you draw 6 gallons of unfiltered air into the tank. It contains road dust. It sloshes around with the gas and now that dust is in the gas. From there it goes past the screens and to the carb bowls. Filters resolved this.

    But why reddish? Carb liner was a red herring. It flakes rather than making fine grit. I live in Colorado. The soils here are red. So is the dust.


    I never wash out the tank. Unlike snowbum (sometimes), I do not see shadows where there is nothing to cast them. I can inspect the petcock screens well without pulling the petcocks. Always a little crap around the bases. It isn't going anywhere and interferes with nothing. Internal patina. So I leave it alone. Less worrying and fussing, more riding.
    #31
    Jim K in PA likes this.
  12. MLCOakland

    MLCOakland Adventurer

    Joined:
    Feb 3, 2015
    Oddometer:
    35
    OK, here’s something. These plugs are brand-new, with maybe 10 miles on them. Running way too rich?

    IMG_0761.jpg
    IMG_0760.jpg
    #32
  13. Plaka

    Plaka Brevis illi vita est

    Joined:
    Jun 19, 2010
    Oddometer:
    7,103
    Location:
    .
    No. The white insulator is what you look at. But in 10 miles the plugs have barely warmed up. Not a fair test. If it were way too rich the plugs would be wet with gas and the bike would be misfiring hurting mileage more although it may not be felt from the saddle.

    Think in terms of 40-50 mile runs on the freeway at 60 mph. Go to Fremont and back on 680, or out to Livermore.


    Edit: If you use the enrichers to start the bike you have to run far enough to amortize the extra fuel those used before thinking about your mileage. 10 miles will not do that.
    #33
    spo123 likes this.
  14. noman

    noman Long timer

    Joined:
    Jun 4, 2007
    Oddometer:
    2,928
    Location:
    the sticks, southern vt
    when you say your "tank goes empty" at 55/60 miles, do you mean you switch to reserve at 55/60 miles? if the petcock is similar to my R75, maybe the main petcock tube is too long.

    or are you putting in 3 gal at 60 miles. just curious.
    #34
  15. mnfrank

    mnfrank Adventurer

    Joined:
    Apr 5, 2013
    Oddometer:
    72
    Location:
    St Paul
    Prius--and bring 3 friends. My former R80 g/s guzzled at about 35-50 mpg. The ringing of the "bell" inside the engine was addictive though. Miss that.
    #35
    ccmickelson likes this.
  16. Beemerguru

    Beemerguru Beemerguru...G/S guy

    Joined:
    Nov 7, 2006
    Oddometer:
    2,076
    Location:
    Redwood City, CA
    Around 65-70mph seems to be the tipping point for MPG. Under 65 I can get around 45-50ish MPG, over 65, it can drop to 40ish or less when it sits at 80mph for long distances.
    #36
    spo123 likes this.
  17. Plaka

    Plaka Brevis illi vita est

    Joined:
    Jun 19, 2010
    Oddometer:
    7,103
    Location:
    .
    On my open 75/5 with home made spam cans I had to drop to 55 mph on a long run to stretch the gas running into prevailing winds. On the RS that has better aerodynamics and taller gearing it's about what Greg mentions.
    #37
  18. MLCOakland

    MLCOakland Adventurer

    Joined:
    Feb 3, 2015
    Oddometer:
    35
    I am putting in 3+ gallons at 60 miles. Don’t like running out of gas so I’ve taken to filling her up once I’m at 60 just in case.
    #38
  19. Plaka

    Plaka Brevis illi vita est

    Joined:
    Jun 19, 2010
    Oddometer:
    7,103
    Location:
    .
    are you on reserve at 60? Given the reserve on the petcock(s) it hard to run these things out of gas unless you are in BFE.
    #39
  20. noman

    noman Long timer

    Joined:
    Jun 4, 2007
    Oddometer:
    2,928
    Location:
    the sticks, southern vt
    odometer accurate? clutching at straws, sorry. : )
    #40
    bmwrench likes this.