Miles per Tank and Miles on Reserve

[Kurt in S.A.]

Today was a day of discovery!  I've been wondering how many miles I could get until I went on reserve.  I also wanted to know now far I could go on reserve...I guess I found out today!

Bike is a R25/2...I was able to put essentially 3 gallons in the tank when I was doing the first fill in my garage.  Last time I rode, I got the indication that I went on reserve after about 225 kilometers (~140 miles).  I was near home, so I decided to just head there and get gas the next time.  I did some calculations and thought that I probably had about 0.5 gallon left.

On the way to my gas stop today, I ran out of gas  .  The station wasn't that far away, maybe 10 miles.  Hindsight says I should have peeked in the tank before I left, but I figured I could make it with the 0.5 gallons less what ever could not be drawn from the tank.  I also assumed that there would be some unused fuel on either side of the saddle and I could lean the bike or take the tank off and tilt it to get all of the gas to one side.

Well, those were wrong assumptions!  Amazing, the tank was bone dry when she ran out.  I got help from a local who went and got some gas for me.

I haven't had the bike on the road that many times and this is only the second time I've filled the bike at the pump.  Calculations show that:

- 2.64 gallons to reserve and about 140 miles
- ~0.3 gallons to empty and only about 10-12 miles

I'm getting slightly under 50 mpg.  I guess I've learned...when she goes on reserve, better be looking for gas!! 

Kurt in S.A.

[Kurt in S.A.]

[Taz]

Hello Kurt.

What does your driver's manual say on this? As far as I can recall it should mention somewhere how many gas is left in the tank before you go on reserve...and keep in mind that after some 50 years this may just be considered a mere guideline (my R25/3, e.g., is supposed to offer 1.5l reserve...in fact I have to go on reserve after roughly 7 litres spent so in my case reserve is good for more than 100kms ).

If you are not happy with the 10-12 miles possible on reserve just replace the reserve brass tube with a longer one...less miles before reserve but peace of mind once you have to switch

Best regards, Tas

[Kurt in S.A.]

Tas -

My manual basically says the same thing - 12L total with 1.5L for reserve.  I'm guessing I have a little less reserve, maybe 1.3L.

Being the most recent owner, it's hard to tell what's been done before.  Obviously, I now have a more accurate data point.  I wondered about the internal straws but was unable to get the petcock off.  I know about the right-left hand threads on the connecting nut but I just wasn't able to get it to budge.

As I said, I was a little surprised that when on reserve, it took all of the fuel.  On my other two twins, the reserve straw is actualy a few millimeters up from the floor of the tank.  And with the two halves of the tank, once it gets down that low, it's feasible to tilt the tank to combine the remnants of the two to get enough for another few KMs.  Again, now I know what to expect from this tank.  Fortunately, it happened in a harmless situation, not miles from home on a dark, stormy night, temperatures near freezing, etc.

Kurt in S.A.

[BenW]

Kurt,

The question is whether the 'original' petrol tap is still in place. The amount of gasoline in reserve depends on the length of the shorter brass tube, so if it is the wrong petrol tap or the tap has been modified by shortening the brass tube, then you will get less miles running on "Reserve (R)".

Cheers,
BenW

[berndr253]

Seems, that there ist a misunderstanding towards the function of the tubes of the petrol-tap inside the tank.
The long one allows the fuel to get out - until the height of this tube ist reached. At this point, engine will show the user: it is time to change the valve to the "reserve" position. Now the fuel can flow through the short tube and engine will continue to run.
We, as professional users of our BMWs know that the time has come to look for a petrol station, not immediately but soon - but I think most of us had made the experience in the past, that the content of the "reserve" is much too low to reach the next station.

In fact - changing the length of the LONGER tube will push up the fuel-content in the reserve mode and reduce the fuel-content in the "open"-mode

Regards
Bernd

[Taz]

Hello Bernd.

You are right, of course...the longer one defines after which amount of gas consumed "reserve" will need to be selected, the shorter one simply makes sure that not all the rubbish on the tank floor can reach the gas filter in the petcock (provided there is still one installed after all these years)-altough this has also an influence on the reserve volume-the longer this straw, the more fuel will remain in the tank even if the the engine cannot get any more out...

Best regards, Tas

[Kurt in S.A.]

This latter point, about the height of the reserve straw, is what surprised me.  There was no residual gas which seems to indicate that the reserve straw is missing.  Which also means that the petcock has been off at least once in it's history.  Not that having a reserve straw at all would have helped me.  Once below the level of the input to the petcock, I couldn't get any more gas (petrol, right?  ).  And as I mentioned initially, I thought there would be trapped gas on the left half of the tank which I could then migrate back over to the right and get me another couple of miles down the road.  But this tank must have a very low saddle, especially at the very rear which allows all the gas to be used.

That appears to be a difference in tank design.  My R69S on petcock on the left side and has a small fuel line that connects the two halves...not sure how effecient that is at using the gas trapped on the right half.  My /7 has two petcocks, so they must be needed because of the deep tunnel in the tank, preventing any trapped gas from getting to the other side.  This is where my idea of tilting the tank to get unused gas over to combine with the other half for some additional distance...hopefully to find a petrol station!!

Kurt in S.A.

[berndr253]

Hello Kurt,

also the petrol-tank of the 25/3 is like the same as for your R69. The small pipe/tube which connects both "parts" is absolutely sized big enough.

Relating your problem it might be helpful to know, what kind of petcock is installed. Best would be to get the petcock off - you wrote, that this procedure might be difficult - please make a photo. You should be able to deinstall the petcock by turning the female screw off - turn it anti-clockwise  - this should work.

Are you sure, that both "lines"of the cock are free?? If the bike was standing for a long time somewhere in "dirt and mud" there might be something which throttled the throughput. Put a long flexible tube onto the petcock - put the end above the level of fuel and into your mouth and blow. Then you can test the throughput of both positions of the cock.

Regards

Bernd

[Kurt in S.A.]

Bernd -

I will try and take a picture of the petcock.  I don't seem to have a problem with flow through the petcock.  What my "experiment" this weekend showed me is that I get about 200km on the main part of the tank and only about 20km on the reserve portion.  When I ran out of gas, there was absolutely no gas in the tank.  I could tell by looking into the opening in the bright sun.

I wish I had more range; I'm only getting about 50 miles per gallon...if I did my math right, that's about 21 km per liter.

Kurt in S.A.

[berndr253]

Hi Kurt,

in fact, the consumption of fuel seems to be too high. Following diagram shows the throuput of fuel depending on speed and the solo/sidecar-mode (the upper of the two curves shown in diagram.

Having a look into the "Handbuch" I saw, that the petcock is designed in a way, that "reserve" fuel-stock is low.

Regards

Bernd

PS 1 US liq.gallon := 3,8 liters

[Kurt in S.A.]

in fact, the consumption of fuel seems to be too high.

My handbook indicates around 80 miles per gallon might be expected which is also stated on this page:

http://www.bmbikes.co.uk/specpages/R25-2.htm

I have tuned the bike as well as I can.  I can't say the performance is "snappy" but I have to be realistic with the small displacement bike.  I have been able to achieve around 85-90 kph maximum speed on level ground, tucked down on the tank.  I weigh around 90 kg.

I've been considering advancing the timing a small amount to see if that makes a difference in performance.  My flywheel only has a small mark for top dead center (TDC).  I've had to make some guesstimates as to where the static timing point should be since there is no such timing mark.  I believe I set it up so that the TDC mark is near the top of the timing window when the points open.

Kurt in S.A.

[berndr253]

May be, that the position of the ignition point is not as good as it should. After my opinion it is necessary, that you find the ignition points "S(tatic)" and "F(dynamic)". Without the knowledge of these points tuning becomes impossible.
If "OT" is marked on the fly-wheel the work can be done at a saturday afternoon with a beer or two, a graduated disc and some "special parts" like wire, adhesive tape, and all the things which one have in a work shop.
If you are familiar with that and you don't need help in that case please write me, otherwise I will translate the procedure, what can cost a while.

Bernd

[Kurt in S.A.]

Bernd -

You make a good point.  I have notes at home (I'm at work now  ) where I determined the circumference of the flywheel and computed the distance around the edge to be equivalent to the static timing point.  I could do the same thing with a "degree wheel" and make corresponding marks on the edge of the flywheel.  Do you have the number of degrees for S and F marks on the R25/2?  Maybe I should confirm the diameter of the flywheel...anyone have that dimension?

When I get a chance to tear down the bike for slinger inspection and cleaning, I will do this more precisely.  But it might not be a bad idea to do the best I can now and recheck during the teardown.

Kurt in S.A.

[berndr253]

Hi Kurt,

as I wrote you need a graduated disk – diameter about 150 mm – diameter is not really important.

Important to now are the values:

5 degree (Static)
38,5 – 41,4 degree (dynamic)

Fix the disk with the screw of the centrifugal governor. With a peace of wire you can arrange something what makes it possible to ,,fix" the position of the disc.
Now you have to turn the fly-wheel to the point of ,,OT) and arrange the disk that the end of the wire shows ,,zero" of whatever the markage you choose.
Now turn the fly wheel anticlockwise to the markage of 5° and sign that position on the fly-wheel. Now its time to open the first can and relax for some seconds.

Next step
Turn the fly-wheel again anticlockwise to the position 40 ° and mark the fly-wheel again.
It is useful to sign the fly-wheel with a center-punch with different signs. May one point at the static and two points at the dynamic position.

Now put away all the things which will not needed any more (graduate disk, peace of wire and the empty bottle).

Ignition point has to be installed on the position (DYNAMIC) what is possible when you ,,open" the weights of the centrifugal governor and fix them with peace piece of wire (thick one!) The dynamic-mark now shoud be seen in the ,,spy-whole" - turn the ground plate of the ignition to ignition point, then fix the ground plate.

If you don'want to make a running test it is possible to open the next can

Cheers

Bernd

[Kurt in S.A.]

Bernd -

Now you make me wish it was the weekend already!  Thanks!

Kurt in S.A.

[rolf.soler]

BMW handbooks say that in the R25/2 the late ("static") ignition timing is 5° before OT; early ("dynamic") timing is 40°. All post-war single cylinders (R24 - R27) have exactly 35° variation between late ("S") and early ("F") ignition timing - however, due to wear of the advance unit, it is more likely to be 37, or even 40°....doesn't matter, just adjust the early (dynamic, "F") timing at 40°, even if "S" is then only 2-3° before TDC , or even at TDC.  
Instead of using a degree disc, you can also measure distances: 1° corresponds to 1,5 mm on the flywheel; so in the R25/2 the late ignition point "S" is 5x1,5 mm = 7,5 mm before top dead center TDC ("OT"); whereas the early ignition point is 40x1,5 mm = 60 mm before TDC. Marking "S" is not too difficult;  for the 60 mm for "F" you would have to stick a piece of adhesive tape measuring  60 mm length on the flywheel, starting at "OT"

Your fuel consumption is not optimal;  should be around 3 l /100 km (o.K. in truth it is maybe 3,5 l...), that is at least 100 km with a (US) gallon...A full tank should carry you  300 km or more before switching to reserve
So, I guess some optimizing is due....ignition timing first, then carburetor settings. Top speed could increase a bit, you could expect 95-100 km/h of everything is fine.
Reserve is 1,5 l, i.e. about 50 km.
Sorry for using SI-units, but you can easily transform them I think

[Kurt in S.A.]

Well, I put enough miles on the tank to recheck my gas mileage.  I went from around 50 mpg to a little over 55 mpg.  Not a huge change (~10%) and I don't know how consistent that will be.  But, getting the timing set correctly for full-advance has helped.  I need to do a compression check at some point.  I suspect the engine probably needs a refresh...I plan to tear it down for slingers maybe about this time next year.  I'll check everything - valves, guides, rings, etc.

I'm still waiting on different sizes of main jets from Bing...

Kurt in S.A.

[berndr253]

A short notion towards comparing the definitions mileage (MilesPerGallon) and consumption (LitersPer100Kilometer).

These information can easily transformed by the magic factor 235,2 - based on 1 mile:= 1,609 km and 1 gallon:= 3,785 Liter
what means

Consumption[l/100km] := 235,2/mileage [miles/gallon]

If the mileage is 55 the consumtion is 235,2/55:=4,28 liters/100 km, what surely is too much.

Bernd

[rolf]

Too much? I (better my /2) needs 4-5 L /100km...I never need less, I think it is a question how you drive...I drive digital(ly?).

[berndr253]

May be driving "digitally" should be annotated - what that means is nothing else than driving the engine on status "no load" and "full load" what only can be done in the wilderness of the US or the nothern states of Germany.
In fact - my 25/3 runs with a consumption of 3 l/100 km - with a load of 75% - what seems to be more than ok, but there is no doubt that it is possible to rise that value up to 5l/100km by running on the autobahn (on the right hand side of the road - hunted by the big trucks which need to carry their load as fast as they could (and of course not should).

Regards

Bernd

[Kurt in S.A.]

I don't think I'm driving "digitally".  But I do seem to be running pretty high in the RPM range which would tend to translate to always running on the main jet.  Wouldn't running on the main jet also translate to higher consumption?

I think of the my R69S or /7 and what Bing writes about regarding the jets and how they overlap to a degree.  Idle jet is primarily in play up to say 1/4 throttle; 1/4 to 3/4 is the needle jet; above that is the main jet.  For the twins, with a top speed of say 170 kph, if I get to the main jet in top gear, I suspect I'm going to be going pretty fast...I estimate maybe 130 kph or more.  I generaly don't ride that fast, in fact hardly ever, so I get pretty decent mileage (or consumption).

But given the low top speed of the R25/2, and the fact that I'm likely to be operating more on the main jet trying to keep up with traffic, I suspect that I'm on the main jet more often than I am with the twins.

That to me might explain the higher consumption than expected.  Bernd, what kind of speed are you running at (75% load?) to get 3 l/km?  Would that be mostly at say 60 kph?

Kurt in S.A.

[rolf]

I think ....80 to 85 Km/h (because of being h(a)unted by Trucks).....at this speed you had to have more than 3 Mirrors to watch the traffic behind you...at my speed (95-105 Km/h), 3 mirrors are not so urgent...even on the Autobahn....you can flow with the others ...in the city...you must drive digitally....and then you are king of the amps(?)...even with 12 HP's....which it is a joke (among people who has cars not under 100PS)....my best experience: on the Beschleunigungsspur (acceleterating?) Spur....Rolf (me) with High Light flasher to get the sleeping(?) cardriver before me a litttle bit faster to get a place in/on the "Autobahn".
Rolf

[berndr253]

Kurt,

what Rolf wrote is correct - speed is about 80-85 km/h to realise that consumption of about 3 liters/100 km. Do you know the diagram of the fuel consumption depending on the running speed?? If not, let me know, it is shown somewhere here in the "stock" (Baureihe --> Betriebshandbuch --> R25/2 --> page 43 - or attached jpg (poor qualitiy).

After my opinion it is not really necessary to be the "king of the (traffic) lights" - driving too fast results, that no one has the time to admire your wonderful bike  .

Regards

Bernd

[Kurt in S.A.]

Bernd -

I had downloaded that a while back...the figure in question is on page 83.  I bought an English version from one of the larger suppliers here in the US.  Unfortunately, it doesn't have the same number of pages and that particular graph is missing.  They have replaced it with a table, which I conveniently plotted for myself.  Thanks...

Kurt in S.A.