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Thread: Super Low MPG

  1. #1
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    Default Super Low MPG

    I picked up a 1978 GS750E in incredible shape a little under a year ago and I've been using it as my daily on and off, as I've had a bunch of little issues related to it having spent the better part of 40 years sitting. It was well-tended and went out once or twice a year, but it's had some rubber degradation and whatnot.

    Right now it's running really well. I recently cleaned the carbs and swapped out the rubber boots on both sides (and the o-rings) as there were a couple of minor leaks. I put in a K+N filter and got it all tuned up and running well. But I always track my mileage and fuel because I'm obsessive, and it's getting around 20mpg in-town and 28 on the highway, which seems absurd. My 550t (with pods from a previous owner) gets WAY more than that, and it's a much more abused little bike.

    Are there any things I should be looking at that anyone's aware of?

  2. #2
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    I had the same issue with my ‘78 gs750 when I cleaned my carbs. Many people suggest between 3/4ths of a turn to 7/8ths of a turn on the fuel screws for a stock bike. In my experience, this was way too rich and my fuel mileage was very similar to yours. I’ve been running my fuel screws between a quarter and half a turn with no issues and low 30s mpg. Might be worth checking what your plugs look like as well. You probably want to make sure your brakes aren’t dragging, your points gap/timing is spot on, carb sync is perfect, tire pressure is adequate but I’d start with doing some plug chops.
    1978 Suzuki GS750

    Past bikes owned:
    1979 Suzuki GS750, 1980 Suzuki GS850, 1977 Suzuki GS550, 1969 Honda CB350, 1976 Harley Davidson SS175, 1979 Motobecane 50V, 1978 Puch Maxi, 1977 Puch Newport, 1980 Tomos Bullet, 1978 Motobecane 50VLA, 1978 AMF Roadmaster

  3. #3
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    I've looked at the brakes, swapped out the points for a dyna, and I check my tire pressure fairly regularly, but I'm probably due to. A plug chop is probably smart. My fuel screws are at a full turn right now, because I read somewhere that that was factory? But clearly that's way off. I'll try running them at half for a while and see if it still seems happy. Thank you!

  4. #4
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    Leak?
    If this was down to mixture the plugs would be black and it probably wouldn't run very well.
    Get them out and see what colour they are.
    80 GS850GT
    79 Z400B

  5. #5
    Gorminrider is offline Forum Sage Past Site Supporter
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    20mpg in-town and 28 on the highway
    Your mileage does seem low. It could be lots of things, but first, need say being aggressive on your throttle is the easiest way to reduce mileage. otherwise,

    ...beyond tweaking the idle mix, (that are maybe more pertinent to idling and coasting), perhaps also check high speed operation.. the throttle itself- CV carbs? these may have worn or maladjusted needle and/or jets allowing too much fuel...
    And:
    -check that the float-needles are holding fuel back when the tap is turned to "prime". Check the fuel-level in the bowls, in other words.
    -Check that the choke/enricher is functioning and fully off when the bike is warmed up.

    Lots of soot at the mufflers is another indication of richness, not to mention dark smoke, that's more visible on startup....

    Finally, and perhaps above all, recheck your math. I'm not saying it's wrong, but stranger things have happened to smart people.
    Last edited by Gorminrider; 11-26-2019 at 11:43 AM.

  6. #6
    posplayr's Avatar
    posplayr is offline Forum LongTimer GSResource Superstar
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    Iirc I used to get about 38-40 mpg with my fat jetting on an 1166 bb. Below 30 mpg for a 750 seems definitely something wrong.
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  7. #7
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    Does it accelerate and drive well? Does it rev well or is it boggy and miserable?
    I too would redo the math and when you say highway is it all highway or mostly? What gears are you staying in and for how long? How high do you rev before shifting?
    1983 GS 550 LD
    2009 BMW K1300s

  8. #8
    Gorminrider is offline Forum Sage Past Site Supporter
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    maybe even the odometer itself?

  9. #9
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    Nessism is offline Forum LongTimer GSResource Superstar
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    Did you check all the jet numbers to be sure they are stock and are you using real Mikuni jets? Maybe the jets are worn? I've read that worn jet needles and needle jets can reduce MPG as well.

  10. #10
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    Quote Originally Posted by Brendan W View Post
    Leak?
    If this was down to mixture the plugs would be black and it probably wouldn't run very well.
    Get them out and see what colour they are.
    I figured if it were super off it wouldn't run well, but it seems pretty peppy? It's been like this since I got it and my only point of reference is my GS550t, so maybe I don't know exactly how zippy it's meant to be and it's not doing great, but it pulls well and doesn't gurgle or sputter or anything.

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