back to basics... fuel reserve question

Ok, I admit this is going to sound pretty stupid, but I can't figure out the fuel reserve system. Im a super dooper platinum certified whoopty doo automotive technician working in a dealer doing complex electical diagnostics on a daily basis but I just can't wrap my head around this one. Had my gas tank apart over the winter and I always just assumed the reserve was a separate partitioned off section in the tank with a tube running from it to the petcock. When I got the petcock off I was stymied when I didn't see anything hooked to it other than a debris screen. Where is the reserve fuel held? When you switch over to "reserve" when riding where does the mystical reserve fuel come from? Now I know you guys are all probably having a good belly laugh at my incompetence but I work on cars. No fuel reserve systems there. Help me out. My curiosity is keeping me up at nights.

 

afaik, there is no separate section for the reserve fuel there just is a certain limit inside the petcock which stop's fuel when reached on the on/prime setting. u need to switch to Reserve for the leftover fuel. im sure others would chime in for a better detailed description.

p.s. nothing is stupid if it bothers u

 

= on /prime
=
=---------------------------------fuel level
=
= = reserve
= =
= =
-------------
on/ reserve is gone now it's on reserve...........long pipe, short pipe no high tech here

 

I took the liberty of drawing a picture!

It's not exact, but it pretty much describes how it works...

 

OIC.... its so simple. Thanks for the visual Polock. Really cleared things up. Can't believe I've been puzzling over this. All makes sense now.

 

It's just two different openings at the top of the petcock. The main, or ON position, is a siphon tube that sticks up 2 or 3 inches. Once the fuel level drops below the top of that tube you have to switch to reserve, which is an opening that's flush with the top of the petcock, giving you the last bit of fuel.

When you get to the point where you switch to RES, consider it "fair warning" that you need fuel now. Since most of us don't have an actual fuel gauge, we reset our trip odometers to zero when we fill up, and stop for gas again at a predetermined mileage. The fuel "dummy light" will normally come on before you get to reserve.

Z

 

Wow! I just responded to polock's post and then I saw gremlins.... very impressive display. Thanks again for helping my pea brain figure this one out.

 

Great explaination,
Now I have a "question".
Why not just operate the petcock in the reserve position all the time?
What is the true "purpose" of a reserve if one keeps up with their tank level?
:lol:

 

If you can keep track of your mileage, then you can run on reserve all the time. Just don't come crying to anyone here about running out of gas.

It's really just meant as a warning signal that you need gas. To keep you from pushing your bike to the nearest station.

 

When my tank ran low and was sputtering, I switched it to reserve. I didn't have enough gas to make it to the next exit.
The first time in my life I ran out of gas on the road.

Don't expect much of a reserve.

 

Very nicely done, but there's one exception to your drawing...

The main supply of fuel in the Prime position is through the Reserve opening, bypassing the diaphragm within the petcock, which allows the fuel to free-flow.

Z

 

ya my reserve didnt take me more than 10 miles

 

Ah! Thats good to know. I didn't realize that. Thanks!

 

If you've never had your petcock off before, and never inspected the siphon tube closely before, there are a couple of things that can happen.

The first is that the siphon tube may just be laying in the bottom of your tank, which means there is no reserve - only fuel or no fuel - regardless of the petcock lever position. The reason for this is that the siphon tube is only pressed into the petcock. Vibrations and such work it out over time.

The second thing is that the material used to make our plastic siphon is very weak, and the fact that it's mostly a fine mesh makes it even flimsier. This means it could bend/crack/break at a point lower than what's intended for the Reserve switch-over, thereby reducing the mileage you get when you make the switch. (At one point I was lucky enough to find a NOS replacement petcock for my bike - still in its original packaging - but bent/cracked in this way. But it could have happened at the factory too.)

Z

 

And one other note - gas floats on water. If you get water in your tank then it builds up at the bottom of the tank. Meaning if you don't use reserve you will never know if you have it.

My reserve has lasted well over 10 miles.

IIRC on the 650 maxim the On position is about 10 Litres and the reserve position is 3 litres. Meaning you should be able to go almost 30% as far on reserve as you did on main.

 

It's a good idea to let you bike go low on gas and find out just how far your reserve will take you. You have to carry a gallon of gas with you to do this, or just have someone follow you with some (I don't ususally recommend carrying extra gas on your bike except for this) You will go a lot farther if you take it real easy and go slow after switching over to reserve.
Another idea I use with my other bike (has big saddlebags) I carry a 10' length of clear gas line with a squeeze bulb on the end for syphoning gas from another source (another bike or auto) incase I run dry out on the road. I keep it rolled up in a little zip-lock bag. I'd rather have it and never need it, then to need it and not have it!!

 

I've gone as far as about 15-20 Km's (to pick up a paycheck... and then gas) on my reserve tank (keeping the engine RPM's low).

 

Well - I've never had the misfortune to actually test it, but;
My 650 with the 13 litre tank gets me around 180km on the main portion (less in the city, more on the highway) - 10L, so that's about 18km / L. Given that, my reserve 3L should get me another 54km.

Adding some fudge-factor to that, and a quick guess says once I hit reserve, I need gas within 40km.

 

OT - but Altus, how many Klicks do you get out of a tank on the highway? My XJ650J is giving me 170-177 so far, and I'm trying to get an idea of how much below average that is.

Another related question for anyone to answer, when I bought the bike I was told not to fill above the level where fuel touches the bottom of the fill tube in the tank. Can I safely go above that to add a little more range between fills?

 

if you overfill your gas tank, expect it to run down the side of the tank and on your pant leg

that rubber seal might not be as good as it was brand new.

also, the reserve should be right at the bottom ~30% of the tank... If you're only getting 10 miles or so out of it, the bottom of the petcock filter might be clogged.