Fuel Tank Capacity is Weird

[K75Slark]

Hello everyone,

I just acquired a '92 K75 from my father (nortonbrian on motobrick and damn near everywhere else he talks about motorcycles online), the previous owner has slid her down the road and he did a little work to get the machine back in tip top shape. It has come to my realization that the low fuel light comes on after using 3 gallons (~150 miles) worth of fuel and it started surging today when the light came on.

I'm going to drain whatever is left in the tank into a can to see how much fuel is left in there, just for argument's sake.

My bike has the standard tank with the flap and a new fuel pump, for what it's worth. Is there something I'm missing here? My Bentley manual hasn't been a great help, either.

I'm so excited to finally have a proper motorcycle and I'd like to do some touring on her this summer, I just want to ensure this isn't a serious problem indicative of greater fuel system problems.

Thanks for sharing your Motorbrick knowledge.

[Grim]

It takes some work to get the tank full. Have to prop the flap open with the nozzle and trickle the last 1/2 gallon in. A lot of people remove the flap assembly to make it easier but you can over fill them and they will puke a little gas if parked in the sun. The bad thing is if you still have the evap line hooked up it will puke it right into the crank case.
 
Now on my K1100LT my light comes on around 150 with mixed driving and it will take 3.3-3.4 to fill it.  Thats averaging 40ish. Mostly hwy I can squeak 45 now that's on a 1100.

 By all accounts you should do a little better.

Its very easy to bend the fuel gauge float arm. Open your gas flap and remove the 4 screws and the cap assembly comes right off. There is plenty of room to reach into the tank to tweak the arm.

[roninvt]

I have a 1990 K75.  My light comes on at about 150 miles and it takes 3.5 more or less to fill.  I still have the little flapper.  I would like to extend my range, but after 150 miles on a stock seat, it is time to give my butt a break. 

[billday]

On my '85 K100 -- the old kind with TWO fuel warning lights, yellow for 7 liters and red for ?3 liters -- the yellow light often comes on as I pull out of the gas station. With a full tank of gas.

They are just flaky.

[motodude]

Not sure what the surging is all about.  I suspect it doesn't have anything to do with the fuel level though.

On highway/road-trips (~70 mph) my '95 K75RT fuel light comes around 150.  On occasions I've gone an additional ~50 miles and it takes about 4 gallons to fill it.  I've also filed it up right after the light comes on and it consistently takes about 3 gallons.

I've also had the light come on around 110 miles but that was in nasty head-winds. All things being equal, I have not seen better mileage at lower speeds (below 70).

And, I have removed the flapper.

Tom

[mystic red]

Not sure what the surging is all about.  I suspect it doesn't have anything to do with the fuel level though.

That was my thought also.............

[Scott_]

Bending the float rod isn't any easy task either.
I adjusted mine last week, and to avoid potentially damaging the switch, I had to get BOTH hands in the tank to bend it. That was a challenge.

I emptied the tank, added 1 gallon, then adjusted the float accordingly for the warning light to light.

[motodude]

At first I was not happy with the fact that my low fuel light came on after using just 3 gallons.  But, after running with it that way for a while, I find that I do like it.  You just have to learn to "relax" when the light comes on in the middle of nowhere.  :eek:

If the light comes on significantly less than 150 miles into a tank, I know that my mileage for that tank is not what I expect and leaves me with enough gas left in the tank to [hopefully] get to a station.  Head winds I understand but if it starts to regularly come on "early" I know there's something amiss with the engine (tire pressure, etc).  Kind of like a canary in a coal mine.

YMMV,
Tom

[rocket23]

 My light comes on right around 3 gallons spent......first indication is while on the kickstand.....then eventually it comes on steady......I figure there's an easy 60 miles before I would start to panic. I have a guage and the idiot light, I can't seem to register a ''full'' tank reading. But I'm old school anyways, I rely on the trip meter more than anything.........If I hit 200 miles :falldown: , I'm looking for a gas station.

[K75Slark]

Thanks everyone! I'll check out the float arm in the tank.

I assume that if the fuel level sending unit's float reaches the end of its throw that the fuel level sending unit turns off the fuel pump?

[Scott_]

Thanks everyone! I'll check out the float arm in the tank.

I assume that if the fuel level sending unit's float reaches the end of its throw that the fuel level sending unit turns off the fuel pump?

Nope. The level sending unit has no control function of the fuel pump.

[K75Slark]

Very strange. So it's only function on models with the low fuel light is to illuminate the low fuel light? How very German of them...

[Scott_]

Very strange. So it's only function on models with the low fuel light is to illuminate the low fuel light? How very German of them...

On the schematics I've seen, on the '75's, one output of the sending unit went to the inst pod for the low fuel light, the other output went to a "connection for special/optional equipment" plug.
On the early(can't define early, guessing '85 thru '87 or '88) '100's both sending unit outputs went to the inst pod. (though the pod breakout only shows 1 low fuel lamp)
On the late(mid '88 through '93) '100's, 1 sending unit output went to the inst pod, the 2nd output went to a "additional instruments" connector.
On the '93 1100's, 1 sending unit output went to the inst pod, 1 output to the "additional instruments" connector.
On the '94 and later 1100's, 1 sending unit output to the inst pod, 1 output to a level gauge.

[K75Slark]

On the schematics I've seen, on the '75's, one output of the sending unit went to the inst pod for the low fuel light, the other output went to a "connection for special/optional equipment" plug.

According to the Clymer, post-90 K75's have a yellow and blue wire that go from the "fuel gauge" to the "fuel pump," with all voltage coming from the fuel injection relay. That's why I assumed if the fuel gauge didn't sense any fuel that it would cut off the fuel pump.

[Scott_]

On the schematics I've seen, on the '75's, one output of the sending unit went to the inst pod for the low fuel light, the other output went to a "connection for special/optional equipment" plug.

According to the Clymer, post-90 K75's have a yellow and blue wire that go from the "fuel gauge" to the "fuel pump," with all voltage coming from the fuel injection relay. That's why I assumed if the fuel gauge didn't sense any fuel that it would cut off the fuel pump.

The sending unit "assembly" is the "gateway" where the electrical circuit conductors enter the tank. The ground conductor is common for the pump and the level sensor. The pump power conductor has no electrical connection to the float sending unit. The wire just enters the tank thru the mounting plate. Just like the pump/level unit in your car or truck.

FYI, some times the conductors can break inside the molded assy. Requiring replacement of the entire assy as it's not repairable.

[K75Slark]

Gotcha. Thanks Scott_! Going from British Iron to UJM's to a Brick requires a new way of thinking about the machine, doesn't it?

[rocket23]

 and riding, your going to have a lower center of gravity, allowing you too ''dive'' into the twisty's! It's all good unless you come across some sand or leaves ......... :falldown: