Super Low MPG

[jimhickcox]

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?

[mattd1205]

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.

[jimhickcox]

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!

[Brendan W]

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.

[Gorminrider]

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.

[posplayr]

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.

[Cipher]

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?

[Gorminrider]

maybe even the odometer itself?

[Nessism]

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.

[jimhickcox]

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.

[jimhickcox]

Oh man y'all're the best and I want to respond to everyone but don't want to do individual quote responses.
For my highway number - that's off one trip but it was all highway. One full tank. The 20-ish number is off a thousand miles of riding or so, and it's hampered a little bit by my living in Austin, TX, where you end up at stoplights as often as you're running, so in-town numbers tend to be a few mpg lower than they should.

Gorminrider - I'm definitely not so good that I don't make dumb mistakes. I'd say I do that more often than doing stuff correctly. I just cleaned the carbs a couple of times (with spray, no dipping yet) because of sticky needles, and they seem fine now, but I can check my levels. I also, speaking of dumb mistakes, tried to synch my carbs but assumed like a dummy that #2 was my reference carb and adjusted accordingly, which made the bike very unhappy. So I brought it to my local carb guru, who's also a GS guy, and he re-synched and checked them and said everything was running perfectly. I want to trust him on that? But who knows.

As far as math - are y'all talking about the mpg math or some carburetor math? I have a spreadsheet that does all my mpg calculations - I just put in how much I bought and my current mileage. I do that for all my vehicles because I'm a weirdo. And to track their health over time. I don't know. It doesn't seem likely that it would be off because it's the same formula on all my spreadsheets. I guess it could be the odometer? I don't have huge wheels or anything. I'll have to go on a long ride with a gps and check that sometime.

Cipher - when I'm riding around town sometimes I'll rev up to 8k before shifting, but usually around 6. I spend a lot of time in second or third. I don't drive SUPER gently, but I don't rip around like a madman either.

Nessism - the bike only has 14k miles on it, so I've been replacing things that wear out with time and haven't really taken a dive into replacing things that wear out with mileage yet. I'm assuming it has the stock jets and such, but I don't rightly know. Maybe I just oughta do a full rebuild on the carbs and see what that changes.

[mattd1205]

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!

1 turn is probably gonna be too fat. Mine ran great at this setting but it drank fuel worse than my Volvo station wagon. I remember reading the fuel screws were not set to any particular base setting from the factory but they were tuned individually at the factory to comply with emissions. The air mix screws did have a base line setting though. These are the marks next to the air screw. I ended up buying a set of carbs that were undisturbed from the factory and still had the yellow tamper evident paint on them and none of the fuel screws were more than half a turn out. They were still somewhat lean after I cleaned them and put the air and fuel mix screws back to where they were when they were new.. Brand new they were jetted lean because the regulations were only concerned with HC and CO emissions at idle until 1998 when they added regulations to curb NO emissions (what causes visible smog). To remedy this I turned the air screws a little further in than the stock base line. I also used a color tune to get the air mix spot on. If you don’t have one, I’d highly recommend it as it gets your air mix very close and fast. From there you can drop your idle speed and tune for the highest idle.

[Big T]

Has it always had the low MPG, or is that a change?
What maintenance have you done since you bought it?

A few notes:
You can't say you've cleaned the carbs if you didn't follow the tutorial
Have you reviewed the Top 10 Newbie mistakes?
There is no reference carb on VMs

[Steve]

... when I'm riding around town sometimes I'll rev up to 8k before shifting, but usually around 6. I spend a lot of time in second or third. I don't drive SUPER gently, but I don't rip around like a madman either. ...

I know there is a lot to be said about keeping the engine in a range where it will respond if you need to accelerate quickly, but I think that's taking it a bit far. Try cutting those numbers in half to see if it helps a bit.

.

[jimhickcox]

mattd1205 - Yeah I'm gonna spin them back out a little. I think maybe I was mislead from the previous owner having richened it up to compensate for a few little air leaks? We'll see if that helps. I've never heard of a colortune but they're cheap enough. I'll pick one up and see what that does.

Big T - Is there something else I should call cleaning them with carb cleaner? Spray-cleaning? I recognize that it's not a full clean like a strip n' dip. I have read everything a few times. It's had this kind of MPG since I bought it about half a year ago. I've swapped the ignition to a dyna, replaced the regulator/rectifier (but not the stator), swapped the air filter, and worked on the carbs (replaced the boots and o-rings, synched them, had a professional synch them, sprayed them with cleaner). I've also replaced the headlight a couple of time and the socket once because it caught on fire.

Steve - I'm shifting around 6k so I'm cruising around 4k mostly. Is that high? Where do you live in the rev range?