# Thread: methodology for measuring reserve (r100gs)

1. ## methodology for measuring reserve (r100gs)

hi kurt, all,

i'm prepping for a west texas trip and i am changing out petcocks and would like to measure my reserves. i had a pretty good thread that explained how to do this but it left out some steps and since i don't exactly understand how these work in conjunction with the petcock "straws" i am hoping someone will help me out.

i'm guessing here but since i don't really understand this i'm hoping someone has a link or a better explanation:

1. change out to new petcocks since these (height of straws?) can affect amount of reserve.
2. fill tank with fuel to a point at which one or both petcocks turned to ON position and has fuel flowing.
3. turn right petcock to RESERVE and drain out. measure amount.
4. turn left petcock to RESERVE and drain out. measure amount.

does this sound like it is sufficient?

-

longer question (since i'm interested in knowing how this works with the straws):

anyone understand exactly how the straws on the petcocks work in the actual tank?

as i understand it my tank has a "dividing line" down the middle with a reserve on the right side of the tank and a reserve on the left side of the tank. reserve on the right is located higher up in the tank (?) and it just happens based upon the geometry to have less (?) reserve while the reserve on the left is located lower in the tank and it just happens to have more reserve - basically solely based upon the geometry of the tank, is that right?

and standard operating procedure seems to be to turn on the right reserve, use it up, then turn on the left reserve and use it up (assuming you are not on the highway or going to be merging in traffic). in this way both carbs get fed through the crossover and you have an extra reserve (or reminder) in the left reserve in case you don't get to a gas station on the first reserve?

so - when you turn the right petcock ON - it is 'sipping' from the tall straw and catching all fuel that is above this level. and then in the case of both petcocks being ON, you are catching all the fuel at the lowest level of the pair of tall straws is that right?

then when the fuel gets below this level - which i am guessing /could/ be at a height above where the reserves are "cut off" by the "dividing line" in the tank - you would turn the right petcock to RESERVE and it would basically sip up any fuel that is above the cutoff for the right reserve and when this is done it would start using up just the right reserve fuel. then turn on the left to RESERVE and it uses up all of its reserve tank?

if so, doesn't this mean that taking a measurement by turning on the right RESERVE /first/ will show more fuel in the right "reserve" and conversely turning on the left RESERVE first will show more fuel in the left RESERVE?

i mean, i realize that the total amount would be the same in both cases but is it the case that by turning on the right RESERVE first give you some amount and then turning on the left RESERVE will give you the ACTUAL reserve in the left side of the tank. and then conversely turning on the left RESERVE first will give you some amount and then when this is used up you turn on the right RESERVE and this tells you the ACTUAL amount left in the right RESERVE?

i know this is not consequential for slab riding or riding in the city but in west texas you don't have a lot of gas stations (with some closed on sunday or even saturday) and even if you look on a map and it tells you that you have some amount of miles to get to the next station that the topography is so steep in some areas that you may actually be getting a great deal less per mile than you would on a normal straight slab.

anyway, i'd love to at least make sure i understand how much i have in the reserves (and how they work) so in case i have to loan out fuel at least i do it with the understanding of how far i can get with what i have in the tank.

THANKS in advance for any good solid info on this.

- jon

1993 r100gs BMW

2. Yes - you can drain the tank down till no fuel flows, then measure what will still flow on reserve. Don't over think straws and left vs rigt. Just measure what will flow on reserve.

Or get a can of gas - go on reserve - and measure the miles till you sputter to a stop. Add from your can and go home.

Where in West Texas? In the Big Bend we gas early and often.

3. ## Don't over think the reserve thing

PGlaves is as usual spot on with a reliable reserve measuring method. Always use the KISS (Keep It Simple Stupid) method with these bikes. Having said that, there are a few things to consider when your trying to stretch your gas or plan a route based on gas stops. You can face head winds, have extra weight in gear, hit lots of rain, ring the bike out a little to much and a few other unforeseen issues like mechanical trouble. All could and probably would have an affect on gas mileage so get a baseline for the reserve miles and carry a couple 1-2 liter fuel safe bottles for those just in case times.

I used to travel solely on my trusty and still trusty R 60US and knew that when I hit reserve at 180 miles I had 25 miles to find a gas station or I could count on some cardiovascular bike pushing exercise.

Just treat the later two petcock bikes as if they had one fuel petcock and when you hit reserve put both on reserve, remember just keep it simple

4. I have never used a direct measurement method to determine the amount of fuel in each reserve on my /7. I just use the hundreds of fillups and the calculated miles per gallon to determine how many gallons were burned to get to the point where I turned the first reserve on, that is how much is in the main tank above the level of the straws. I have far fewer measurement points where I've had to turn the second reserve on, but those gas log entries have let me calculate the amount of fuel I expect in the first reserve and second reserve. With those numbers in my head, I do quick calculations on my gas mileage at the moment I have to switch to reserve to figure out if my mileage is more or less average or if it's below average due to head winds, altitude, etc. I then make adjustments on how many more miles I can go on reserve.

I generally go for the left petcock first as it's the easiest and quickest to get to when the bike begins to stumble. A little trickier to drop the right hand off the throttle since I'm "working it" to keep the bike running.

I don't know if the '93 GS tank internals are different than my /7, but I've found that for all my calculations, the amount of gas in each saddle is the same number. I think the dividing wall inside the tank is high enough that when I'm on the first reserve, there is no gas that can move back and forth between the two halves. Even though my bike has the under tank master cylinder which results in a change of shape of the inside wall of the tank, that hasn't made enough of a difference in my calculations.

And there is a third reserve in the tank...if you get to the point, you're in serious need of a gas station. Depending on the height of the reserve straws in each half of the tank, there will be some trapped gas. If you remove the tank and slowly roll it to say the left side, you can move the trapped gas on the right into the left which will put fuel above the lowest straw on the left side. Might get you another 5-10 miles and make all the difference on how much walking you have to do.

5. hi gents,
thanks as always. I'm never disappointed.
so roughly in order, I do plan on a big bend area trip. I'm sure it has changed a lot (at least Marfa) since I have been there. in my pickup I found the topography to be a game changer in terms of calculations so I do want to start with a 'known' quantity so to speak. I may /see/ 40 mi to the next station on the map, get 10 miles in and then have to turn to reserve and still not be likely to make it - given topography - even if I was getting 40 MPG. I think the Topos out there could drop your MPG by 1/3 personally.
also. I am planning on mount a 2 liter can. right now it is going to go to the /front/ of a right hard aluminum pannier from TT. I got some wobbles when I loaded up and mounted these panniers so I'm thinking mounting this in front (and maybe some shock and/or spring work) will keep things smooth.
I /do/ have to get some saddle time and start figuring out what kind of mileage I am getting.
first the plan is to drain the new petcocks while on reserve and just verify that I get 1 gallon when I switch over.
I should also say I do like the method of turning on the left one first (it's easiest) and then I still have a little in reserve if I have an oh **** moment in the city or while traveling under texas sun.
the advice to actually take the tank off and move all you can to the left or right side is pure genius.
this is why I am so thankful to be able to post here.
THANKS
- jon

6. hi paul, kurt, all.

one thing I am wondering is if the reserve "straw" for the left or right petcock actually picks up more fuel if you turn it on FIRST. I mean, if there is a dividing line then the amount of fuel left in the /actual/ reserve is only correct if it is turned on last. this is assuming that the first reserve that is turned on is 'sucking' gas from the reserve straw from ABOVE the dividing line which seems to be the case.

for instance I got this explanation recently:

"Fuel drains to the top of the longer "straw" on main. The difference between the long and short "straw" is the reserve capacity. The tank tunnel divides the tank so there's a marginal amount of reserve left in the side that isn't switched to reserve. It's not an exact science"

if so, this means that it is in fact an issue only of /total/ reserve and one may only practically be concerned with the actual amount in the LEFT reserve since this is the one you are turning on last...

I know this may sound pedantic but I find understanding the system to be part of the joy of these bikes.

THANKS

- Jon

7. You really are over thinking this issue. The crossover fuel line will keep the gas level even on both sides of the tank - as long as the petcock is "open".

When you start out with a full tank, the gas will be provided until the level reaches the lowest of the two main straws. Then when the engine stumbles you turn one of the petcocks to reserve and you will get roughly 15 to 20 miles from the gas on that side. When the engine stumbles again you flip the other petcock to reserve and you will get roughly another 15 or so miles from that one. If reaching a supply of gas was a concern, you could modify your ride style and use of the throttle to maximize mpg.

The short straw on the petcocks could be shortened slightly to increase the amount of accessible reserve. But keep in mind you are drawing gas from the bottom of the tank where water and debris have collected. If you intend to lower the reserve straw then regular cleaning out of the tank is recommended to make sure you haven't collected a mess at the tank bottom. Cleaning out the tank is a good regular practice anyway...

8. Originally Posted by m_stock10506
You really are over thinking this issue. The crossover fuel line will keep the gas level even on both sides of the tank - as long as the petcock is "open".

When you start out with a full tank, the gas will be provided until the level reaches the lowest of the two main straws. Then when the engine stumbles you turn one of the petcocks to reserve and you will get roughly 15 to 20 miles from the gas on that side. When the engine stumbles again you flip the other petcock to reserve and you will get roughly another 15 or so miles from that one. If reaching a supply of gas was a concern, you could modify your ride style and use of the throttle to maximize mpg.

The short straw on the petcocks could be shortened slightly to increase the amount of accessible reserve. But keep in mind you are drawing gas from the bottom of the tank where water and debris have collected. If you intend to lower the reserve straw then regular cleaning out of the tank is recommended to make sure you haven't collected a mess at the tank bottom. Cleaning out the tank is a good regular practice anyway...
hi michael,

thanks. this helps. for some reason i was not able to visualize the behavior. i am probably going to measure the actual reserve at some point which i am guessing just involves literally turning on the left petcock until it stops and then turning on the right petcock until it stops to see how much gas i get once i get to reserve. the issue is mission critical out in west texas.

i also just ran into this idea of shortening straws - one rider seems to have shortened his main straws because he felt the new ones were too long and caused him to hit reserve too early. now would be a good time to do this if i put in new petcocks so thanks for the visualization - it helps!

- jon

9. Originally Posted by 20774
I have never used a direct measurement method to determine the amount of fuel in each reserve on my /7. I just use the hundreds of fillups and the calculated miles per gallon to determine how many gallons were burned to get to the point where I turned the first reserve on, that is how much is in the main tank above the level of the straws. I have far fewer measurement points where I've had to turn the second reserve on, but those gas log entries have let me calculate the amount of fuel I expect in the first reserve and second reserve. With those numbers in my head, I do quick calculations on my gas mileage at the moment I have to switch to reserve to figure out if my mileage is more or less average or if it's below average due to head winds, altitude, etc. I then make adjustments on how many more miles I can go on reserve.

I generally go for the left petcock first as it's the easiest and quickest to get to when the bike begins to stumble. A little trickier to drop the right hand off the throttle since I'm "working it" to keep the bike running.

I don't know if the '93 GS tank internals are different than my /7, but I've found that for all my calculations, the amount of gas in each saddle is the same number. I think the dividing wall inside the tank is high enough that when I'm on the first reserve, there is no gas that can move back and forth between the two halves. Even though my bike has the under tank master cylinder which results in a change of shape of the inside wall of the tank, that hasn't made enough of a difference in my calculations.

And there is a third reserve in the tank...if you get to the point, you're in serious need of a gas station. Depending on the height of the reserve straws in each half of the tank, there will be some trapped gas. If you remove the tank and slowly roll it to say the left side, you can move the trapped gas on the right into the left which will put fuel above the lowest straw on the left side. Might get you another 5-10 miles and make all the difference on how much walking you have to do.
hi paul, kurt, all,

anyone have any last minute reservations on my /removing/ the reserve straw on the RIGHT petcock on a 93 r100gs? one rider on another forum has apparently done this with both petcocks.

i got to thinking about this and realized that if i do this i would only be using the right petcock in something that would be approaching an emergency - say if i was out in west texas and got into a jam. also, i do have the new strainers on this petcock since i am putting new ones in.

it seemed to me that this would be a good strategy and - worst comes to worst - the last step would be to use the remove tank and get all fuel that is below the straw on the left hand side to the right hand side of the tank where there is no straw.

not sure if this makes sense as i do know the right hand side of the tank is cutout a bit for electronics but it seems like a decent strategy as long as i am in there today replacing the petcocks.

thoughts?

THANKS

jon

10. There might be some advantage to lowering the reserve straws so that you can more readily get to the fuel in the bottom of the tank. Too low and you'll get more debris as previously mentioned.

Honestly, I can't think of any place in the US where I would be concerned about finding a gas station. Except maybe the Alaskan highways. Knowing how many miles you can go on a tank of gas, along with real-time calculations when you hit your first reserve, should let you look at the map and find the next town that you need to get gas at. If you're in a somewhat sparsely populated area, you should fill early and often...don't drive the bike into reserve.

I'm talking about riding on paved roads...if you plan on spending 90% of your time off pavement, well then that's a different kettle of fish. But again, you should be aware of your general gas consumption and where the nearest town is so that you can head that way if the situation presents itself.

11. Originally Posted by m_stock10506
You really are over thinking this issue.
I think so as well. In remote areas, I like to get fuel- as suggested, whenever I can. If the area is remote enough where I think there may be a problem, I carry spare gas.
On the TW200, I have 2 "tool tubes" each has a refilled SeaFoam container with "spare" treated gas.
OM

12. Originally Posted by Omega Man
I think so as well. In remote areas, I like to get fuel- as suggested, whenever I can. If the area is remote enough where I think there may be a problem, I carry spare gas.
On the TW200, I have 2 "tool tubes" each has a refilled SeaFoam container with "spare" treated gas.
OM
hi OM,

I'll carry spare gas. can you help me what you mean by 'treated'? right now I'm planning on just a 2 liter with 91 octane in it strapped to panniers.

- jon

13. Originally Posted by hotwheels22
hi OM,

I'll carry spare gas. can you help me what you mean by 'treated'? right now I'm planning on just a 2 liter with 91 octane in it strapped to panniers.

- jon
Sure, just treated for aging and stability. Usually a mix of Techron and StaBil. Once a year I rotate the gas into something else. With the now easy availability of no-ethanol gas, even if it's a 2-stroke mix, I will use that next.
Good luck.
OM

14. A few comments. You risk crud in the carb by shortening or removing the reserve straws. Before I did that I would lengthen the main straws to increase the size of the reserve.

Next - look at the map. There is gas in Alpine, Study Butte, Presidio, Fort Davis, Marathon, Marfa, Lajitas, inside the National Park at Panther Junction and Rio Grande Village, and of course up at Ft. Stockton and over east at Sanderson. The longest stretch between stations is about 80 miles - Alpine to Study Butte. I live on that stretch 53 miles south of Alpine and 25 miles north of Study Butte. I have had a few cars passing by need gas but in ten years, never a bike.

If we go to Alpine we gas up. Return home 53 miles, back to town 53 miles, so we gas back up at a few over 100 miles and go back home. Always gas early and often.

15. ## methodology for measuring reserve

Direct physical measurement of gas tank supply capacity will give you raw data. Experience and records will give you real world data of distances possible under varied conditions. Trimming reserve straws, or entirely removing them, will increase the probability of introducing settled water in the tank and other "splooge" (TM Rubber Chicken Racing Garage) at the tank bottom into the carbs with less than desirable results. Keep the reserve straws, keep records, you'll figure it out shortly. Carrying some spare gas safely is acceptable if you are truly worried, like crossing the Gobi desert.

Remember. you can ride a bike farther in 5 minutes than you can push it ALL day long !

Friedle

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•