Filling the gas tank

[Altevogt]

This may seem trivial, and probably is, but I recently bought a 1991 K100 LT and  absolutely love it. However, whenever I fill the tank the gas gauge doesn't go to Full.  I stand it up and fill the tank to where the little spring door is and it still registers shy a load.  What do I have to do to this thing to get it to fill the tank?

[Laitch]

Welcome, Altevogt.
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[Altevogt]

Thank you. I asked the Guy I bought it from whether he had had that problem and he said that he hadn't, but couldn't enlighten me as to what I might be doing wrong. He was very straightforward in letting me know everything else that might be a problem so I tend to believe him.  AT this point I'm still in the mode of blaming operator error.

[Chaos]

Many K owners remove the spring door to get a little more gas into the tank.  I doubt that would move the needle to full, it probably needs adjusting as in one of the links Laitch provided.  I wouldn't worry if the gauge is a little off when the tank is full,  I'd be more concerned what it reads when the tank is near empty.

[Altevogt]

Good point. What I've been shooting for is a consistent marker so I can get a good feel for the mileage.  But obviously, knowing where zero is would be the most important information to have.   In my case, thus far I fill it at one of the few stations that has real gas with no ethanol and then keep it filled regularly. I also have been doing that to run out any gas that might have been in it for awhile before I got it.

[Laitch]

It's good practice to remove the cap and baffle assembly from the tank, siphon or drain the fuel and take a look at the condition of the fuel pump vibration damper and the tank. If there are no particles, sediment or deterioration of components visible, you're good to go, especially if the fuel filter isn't as ancient as the bike.

How many miles on the bike; how many have you put on it?

[White Dog]

Do what I am doing to measure how far to ride before worrying about buying gas.  Fill a one gallon container with gas, put it in one of your side bags, fill the gas tank and run it until it quits.  Note your mileage and put the extra gallon into the tank and go fill up both again.  I suspect the accuracy of my low gas warning light so this is what I am doing to get a feel for my K75 mileage.

[gazman]

Personally I don't like to let the fuel level get to low if I can avoid it. my thinking is the fuel acts to keep fuel pump cool. No pump No go. :riding:

[Motorhobo]

Personally I don't like to let the fuel level get to low if I can avoid it. my thinking is the fuel acts to keep fuel pump cool. No pump No go. :riding:

This was my argument at the end of this recent thread on more or less the same topic:

http://www.motobrick.com/index.php/topic,9045.25.html

The universal consensus appears to be that running a tank consistently low isn't advisable for a number of reasons. This is true for a car or a bike, but the issues are exacerbated with a moto because of the low fluid volume. 

http://mechanics.stackexchange.com/questions/1876/does-the-amount-of-fuel-in-the-tank-matter

I think the BMW engineers designed the light to come on with +1 gallon of fuel remaining for a reason -- as a reminder not to run the tank dry because it's bad for the tank, the fuel, and the pump and therefore bad for the overall health of the motorcycle and rider.


BTW at the recommendation of a good buddy old school BMW wrench back in the early 2000s I removed the baffle completely and ran the bike without it for 60k miles. He said it was there for environmental reasons, perhaps because of that big rubber boot they used to have on fueling station nozzles in the past -- with that big boot on the nozzle you needed the baffle to get the fueling pump to pump fuel into the tank.  Haven't seen one of those annoying boots in ages -- not even in New Jersey...and New Jersey would be the last state to do away with useless environmental regulations. So -- I'm not sure what that baffle really does anymore. Can anyone explain what it's purpose might be and what reason there is for not removing it? I have one on my current K75 but it's an impediment and if there's no good reason for it to be there, I'll put stick it in a Baggie and put it on the old parts heap.

[kennybobby]

Removing the baffle is the first thing i've done with all my bikes.   The only bikes that i've seen with it still there have been low mileage garage queens.

Now at the gas station you can't just shove the fuel nozzle into the tank, pull the lever wide open, set the clip and walk away--you will need to monitor and slow down on the last gallon to completely fill it to the brim and not make a mess on your tank paint job.  And you can't fill it up and then park it in the hot sun while on the side stand.  But to me it's worth it for the extra miles between stops, ~250 on a good running k75.

[White Dog]

How full is "full" when talking about the K75 gas tank?  While sitting on mine with the cycle being most upright, I push on the flap and it touches gas.  Is that "full" or would there be room for more gas--short of it running out when I park it on the extreme lean of the sidestand?  I recently ran 153 miles and still had about 1" of gas left in the tank on the sides.  Not sure if the fuel pump was covered or not.

[kennybobby]

There is room for more but you can't add it with the flapper in there--the pump turns off the nozzle when it senses the back pressure.

The flapper is ~3/4" (19mm) thick, so there is a plenty of room for more fuel.

The cap lid extends ~3/8 (10mm) down into the tank below the thick flange, and there is about 1" of air space still open above that even with it filled to the 'brim', when the fuel is touching the bottom of the cap lid where the flapper device was mounted.

25 years ago i might have put 5 gallons in one once or twice, but i've got a bunch of recent receipts for 4.7 gallon fillups at the pump--how does that compare with fillups with the flapper?

The Haynes k75 manual indicates 5.55 gallon capacity--i would guess that includes the 1" air space between the 'brim and the inside top where the vent plenum (small metal puck) resides.

The fuel pump runs half or more of its life uncovered above the fuel level in the tank--it's not the fuel on the outside that cools it, it's the passage of fuel thru the interior of the pump that provides the cooling.  If it is pumping fuel then it is being cooled.

i run em down to squeeze every mile out of that pig and i've never run out of fuel or had to push it--but now i probably jinxed my smart-ass self and will have to suffer shame, fear and loathing when i get caught out ridin'.

[Motorhobo]

The fuel pump runs half or more of its life uncovered above the fuel level in the tank--it's not the fuel on the outside that cools it, it's the passage of fuel thru the interior of the pump that provides the cooling.  If it is pumping fuel then it is

i run em down to squeeze every mile out of that pig and i've never run out of fuel or had to push it--but now i probably jinxed my smart-ass self and will have to suffer shame, fear and loathing when i get caught out ridin'.

I'm not sure everyone would agree with you about whether immersion provides additional cooling. If the liquid inside the tank is cooler than the air inside the tank, then the liquid is cooling the pump housing relative to whatever part is exposed to the warmer air, can't really argue with that. You could argue whether than has any mechanical relevance for the integrity of the pump, though...but I think you might be in the minority on that count.

Then there's the additional bonus of having the pump at least partially immersed that the pump doesn't whine as loud but that's a purely aesthetic thing I guess.

So kennybobby, you run the tank dry as a rule -- have you had to replace the pump or are you still running the original OEM pump that shipped with the bike? Not that it's relevant -- my OEM pump failed at about 85K miles and I replaced it with a Ford one...just wondering what the lifespan of your pump has been with your fueling habits.

[Chaos]

minor factoid, the flapper was originally there to prevent fueling up with leaded gas.  Unleaded gas pumps had a narrower nozzle, the leaded pumps bigger nozzle would not fit through the flapper.  Another point to consider, the inch or so of air space give the tank some "compress-abiltiy" in the event of an accident.  The airspace will compress if the tank is dented, preventing it from rupturing since a completely full tank would be non-compressible.  That would only matter right after a fill up.  Also, topping all the way up and leaving it in the sun, the gas expands and leaks out the vent. I usually leave a little room even without the flapper.  Removing it makes it easier to look in the tank, too.  And the official tank capacities are wildly optimistic.  I put in 4.9 gallons after running it bone dry.  Usually fill up with about 4 gallons, and I still have the original pump, 30 years and 185000 miles.

[kennybobby]

i don't see anything in the pump hardware that couldn't handle 105C temperatures, which is pretty standard for good electric motors--now that cheesy radiator fan is another story.

i've got 2 on the road, 1 in the shop, 1 in a box.  The 1 in the shop has original pump and 135k miles, never used ethanol, pump looks okay and was running when it went on the lift.  The 2 runners have unknown past history, 25k and 35k miles. 

The submerged pump theory sounds all good and stuff, like some Popular Mechanics story all scienced up--i could probably get some CSI evidence for my theory also, but i'm happy to sit on the minority bench.  i've never had a problem with a hot load running thru a pump.



But it seems to me the design was more about fuel vaporization, aka vapor lock, when the temperature and pressure of the fuel are such that the fuel 'boils', rather than whether a mostly metal pump will have it's service life shortened by it's outer shell environment...

[The Mighty Gryphon]

I'm with KennyBobby's school of thought on fuel level.  I have to believe the designers figured on the pumped fuel cooling the pump.  It would be foolish otherwise.  I typically run my bricks to 200-210 miles between fillups(which usually take about 4.5-4.8 gallons), and haven't had any issues yet.

Most electric motors are designed to run continuously at temperatures up to 105C which is hot enough to cause 2nd degree burns.  At that temperature the fuel, even at the kind of temperatures seen in K bike tanks, will look pretty cool and remove quite a lot of heat. 

Regarding the low fuel light, it comes on at about 180 miles on all three bikes when commuting or riding around town.  When doing the slab at 80+mph it will come on around 150 miles.  My rule of thumb is that I have a solid 25 miles to find a place to fill up after the low fuel light comes on.  I can recall one occasion where I had to go about 230 miles and put in just over 5.1 U.S. gallons.  That was a scary one on the interstate at night when the exit I had planned on did not have gas available.

The bike I use for long haul traveling(12 hrs/700miles per day) has a fuel cooler which helps keep the tank and my thighs cooler as well providing cooler fuel for the pump to handle. 

I have found the hard way that when you remove the flapper you should never park the bike on it's sidestand immediately after filling the tank to the top.  If you do, you will have a permanent gas stain on the tank from the dribble that will leak out.

[Martin]

On hot days 30 C+ my black tanked brick suffers from vaporization in slow traffic if I don't have a near full tank. I have just fitted double sided reflective foam insulation in order to alleviate the problem. It hasn't fixed the problem, but it may allow me to run with less fuel in the tank than previously. The cooling system handles the heat fine the fuel system does not as soon as I enter heavy traffic I switch my cooling fan on. The only thing I can do is not let the fuel get down to that level or look at Gryphs fuel cooler.
Regards Martin.

[Motorhobo]

i've got 2 on the road, 1 in the shop, 1 in a box.  The 1 in the shop has original pump and 135k miles, never used ethanol, pump looks okay and was running when it went on the lift.  The 2 runners have unknown past history, 25k and 35k miles. 

So where do you get no-ethanol fuel in the US? People talk about it here a lot but I don't see it sold anywhere.

[kennybobby]

There is a map at  http://www.pure-gas.org/

[Altevogt]

There is a map at  http://www.pure-gas.org/

There are two stations by where I live. One has 91 octane and another one has 87 octane.

[The Mighty Gryphon]

When I travel cross country it never fails to pique my sense of irony that in the states where the people who foisted ethanol on us make the crap it is so much easier to find non ethanol gasoline.

[Chaos]

Just think, they could quit putting it in gas and sell all that as 195 proof attitude adjustment beverage.

[kennybobby]

i'll bet it's only 80 proof, mostly water and run thru a still once--there's no time for multiple runs.  Ever notice how it never gets revealed what proof they actually add to the gasoline...

[stokester]

Dealing with automotive systems where it is accepted that a restricted fuel filter will result in a failed fuel pump because of the extended duty cycle to keep pressure, does this same thought apply to our K-bikes?