Appalling Fuel Consumption

[jasonisme]

I only get about 60-70 miles from a full tank on a '95 GS500E, and I'm trying to work out if there's a problem and what it is.  There's no obvious fuel leaks, no puddles, drips, petrol smell, dampness around fuel lines or connections.  I use it almost entirely for city riding, I rarely ride it more than 30mins at a time and most of that will be stop and go in 1st or 2nd gear below 40mph, so I'm not sure if that'll lead to more fuel use.  I had the valves checked about 4 or 5 months ago (1500 miles or so), the carbs were cleaned a few weeks back, everythings stock as far as I know.  It starts fine, idles fine, seems to run fine though it can start to sound a little rough after a long ride.  I've no idea where I should be looking or what might be wrong, it was doing about 150 miles to the tank over long distances six months ago, and the only thing thats changed since then are having the carbs balanced and replacing some electrics. I don't suppose anyone has any ideas?

[Kerry]

Can you be more specific about what "from a full tank" means?  Something like:

"I fill my tank to the bottom of the filler neck [with the bike vertical / on the side stand].  After riding 60 or 70 miles [and having to switch to REServe?] I fill the tank to the bottom of the filler neck again, and it takes about X.X gallons/liters."

[jasonisme]

I don't use reserve due to petcock problems, i keep it on prime, 60-70 miles is from just below the neck on the side stand until I unexpectedly run out of fuel on my way home.  So filled to the neck on the sidestand, 70miles till empty, then filled with just short of 15 litres.  I filled it today after 40 miles and it took 8.5 litres up to the neck.

[Kasumi]

So i don't know whether your in the UK or the US but anywhos i might be way wrong with my calculations - someone can correct me but your getting roughly...

UK gallon: 21mpg
US gallon: 18mpg


Either way if im working it out right based on some assumptions that.

1.) The tank was full before you drove 40miles.
2.) You filled the tank in the same way and to the same point as it was before the 40miles.

Then you've got something wrong.

First thing your only filling up like 1/2-3/4's of your tank by filling it on the sidestand as you can't fill it up properly due to it been leant over so you need to sit on the bike, or put the bike on the centrestand while you fill up at teh pumpy, this will get you a full tank (full as in the literal meaning). However if im right with my working out it ain't guna matter how much you fill it if your getting poor mpg.

You should be aiming from 45-60mpg on the GS so your half or less. One problem could be your running the bike on prime. This won't be a problem while the bike is running but do you switch the bike to the ON or REServe position when you park it for your night or anytime your not riding it? With the fuel petcock on prime fuel can flow freely through the carbs even without the engine off as you have bypassed the vacuum which regulators fuel flow into the engine. If you have it on prime when your not running the bike the fuel will drain through and sit in the engine or exhaust and get burned off when you ride. Normally you would notice a bike flooded with fuel (but not definatly) when starting but if your bike is relatively new or has been re jetted and has good electrics then the engine will start just as easily and burn more fuel to start with.

The readings you are using for long distance are misleading also, if your running the bike in prime but riding continuously for many miles (i.e. running the tank dry then filling up again) your not getting many of the after affects of running the bike in prime as the fuel is always been burned and not draining away and evaporating. Plus based on rough calculations using US gallon you would only be getting 37mpg over those long distances and you should be getting more.

Seeing as you have had your carbs balanced and valves checked not long ago i would suggest you sort the obvious problem which is the petcock- its the other componant directly involved in the fueling of the bike that you know has a problem, you need to fix that first so you can run the bike properly using the ON/REServe functions so the engine is only drawing as much fuel as is needed and when the bike is off fuel isn't draining away.

Im pretty sure its a petcock problem letting too much fuel into the engine. You can do a simple check if you have a spark plug tool... Run the bike for a few minutes till warm, then switch it off. Take each spark plug out and look at the tip of the plug, A normal plug should be light chocolate brown, im guessing if you look at your spark plugs they are very very black and very sooty, this means they are too rich i.e. too much fuel. If they are whiteish they don't have enough fuel (lean) - this is highly unlikely as your going through fuel like no end. If they are black wet and oily they are oil fouled.

I suspect you will find them to be black and sooty, if you know your carbs have been balanced and valves checked i would suggest doing the simple thing first, fix the petcock, get a new one or fix the problem  you have with the original. If thats not the case then you need to be looking in the carbs, possibly for wrong jets etc...

[ohgood]

Let's simplify things.

1) Fill the bike one the center stand, or with youself balancing the bike, just until it touches the filler neck.

2) Ride 50 miles / kilometers whichever you can read on the odometer.

3) Refill the bike in the exact same pump you used the last time, same way, same touch on the filler neck.

4) Calculate your MPG or MPL, and keep in mind we use different gallons across the pond.

If you're actually around 40-48 MPG (thats US miles / gallons) you're getting close to average.

Riding style and jetting can effect things alot. Check your plugs as pointed out above. Is you air filter dirty ? Do you rev the motor to 10,000 rpm on every shift ? Letting it warm up for 10 minutes can effect your mileage also.


Remember, anything from 40-55 MPG is pretty normal. Riding style can influence it just as much as your jetting setup. Alot of people can achieve 60 MPG in city riding, ie not fighting the wind.

I average 40-50 MPG depending on my mood. 

Hope we've helped.

[resistor]

Perhaps it coulld be a issue of running the bike on prime, have you checked your plugs to see how they look?

[The Buddha]

Are you spewing out fuel into the airbox How about while parked.
Cool.
Srinath.

[sledge]

Your riding conditions are not ideal for economy but the figures quoted are very low even for the GS. First thing I would check is the choke and make sure its not staying part-on. If there is no signs of leakage the prime or reserve setting shouldnt matter, the fuel is still going into the carbs. Do an acurate calculation for mpg then you have a benchmark, know were you are starting from and can tell if things are improving. I would replace the airfilter without hesitation, a dirty filter dramatically increases fuel consumption and can look clean even when its clogged. Plug chop-tests at 50% 75% and 100% throttle are also a good idea and will indicate any issues with upper range jetting.

[davipu]

ok to paraphrase what you siad into american terms,  it was fine and i messed with the valves and the carbs, and now i get 1/3 the milliage that i should.   valves won't affect milliage, they will cause the bike to run rough if they are worn and the shims are too thick, they will allow a false low compression reading when the motor is hot. due to the valves staying open too long. 

so the only thing left is the carbs, check the airbox for fuel leaking past the needle and seat.

[Kerry]

valves won't affect milliage

Wanna bet?    The '96 I sold to my neighbor went through a period where it was under-powered and getting rotten mileage.  We found that one of the exhaust valves was too tight by, like, 4 shim sizes.  The other was a little tight as well, maybe only one size off.  Anyway, putting in the right shims worked wonders for that bike ... until the oil drain plug fell out on the freeway. 

[sledge]

I agree entirely, think about it. If the clearances are significantly out it will effect valve timing, too tight and they open earlier and for longer, too large and they open later and for a shorter period.....whichever way, the engine wont be running as efficiently as it could do.

[davipu]

well i've been known to be wrong before...