reserve switch

[racerrad8]

Remember, you don't need to run the bowls dry, when the bike is running on the needles the fuel level needs to drop just to a point below the mains before studdering begins.

Dry, empty whatever you want to call it...

The distance from the bottom of the main jet to the bottom of the bowl is about 6mm, so while they are not dry or empty, there is only the well of fuel in the bottom of the bowl when the main jet becomes uncovered...

Might as well be empty...

Randy - RPM

[FJ_Hooligan]

Remember, you don't need to run the bowls dry, when the bike is running on the needles the fuel level needs to drop just to a point below the mains before studdering begins.

If you're just cruising, then you only need to uncover the idle jet to feel the engine surging.  I had a tank vent problem once and went a few miles as the carbs ran out of fuel.  Once I noticed the slight surging, I twisted the throttle and the engine picked back up as it transitioned from the idle circuit (which was starving for fuel) to the main jet and needle (which still had fuel).

I'm not sure what's at work with the reserve, but I've run on 2 cylinders before and it is a VERY noticeable change in engine response and exhaust note.  I do recall that the low fuel light works as a variable resistor circuit, which is why the light changes intensity as the fuel level gets lower.  The sensing element has a current running through it when the ignition is on.  When surrounded by fuel, the circuit stays cool and has a certain resistance which keeps the fuel light off.  As the fuel level drops and the element begins to be uncovered, it heats up and the resistance changes causing the fuel light to begin to glow.  The more the resistance changes, the brighter the light gets.  When I first heard this circuit description, I remember thinking that it can't be a good idea.  The only place I want to mix fuel and electricity is in the combustion chamber!

[Capn Ron]

I'm not sure what's at work with the reserve,  I do recall that the low fuel light works as a variable resistor circuit, which is why the light changes intensity as the fuel level gets lower.  The sensing element has a current running through it when the ignition is on.  When surrounded by fuel, the circuit stays cool and has a certain resistance which keeps the fuel light off.  As the fuel level drops and the element begins to be uncovered, it heats up and the resistance changes causing the fuel light to begin to glow.  The more the resistance changes, the brighter the light gets. 

The highlighted above is interesting.

I just went out to the garage to take some voltage readings off the fuel pump in several different states.  My bike is a '92 with ABS.  The ABS light is where everyone else has their low fuel light, so I don't (believe) I have any dash indication of low fuel.  The fuel tank is currently at about 1/4 full.

I put the multimeter probes into the fuel pump wiring harness connector alongside the wires without disconnecting it from the wiring harness.  I verified that the probes were making contact by switching the ignition on and off again.  I'm using "12V" below to indicate full power even though voltage readings would vary +-a volt or two.

Reserve switch in ON position
Engine off, key off: 0V
Engine off, key on:  12V for 5 seconds then 0V
Engine on, key on:  12V

All the above makes sense and is behaving as I would expect.  I then attempted to simulate a low-fuel condition by depressing the float on the level sender down to the bottom of the tank with a 2' length of PVC pipe.  The PVC was just heavy enough to hold the float level sender at the bottom of the tank for as long as I wanted.

The fuel gauge dropped well past empty, but did not reach the stop pin.
There was no light indication on the dash of a low fuel state (ABS model)
The engine continued to run at 1,100 RPM smoothly
The fuel pump continued to run with 12V indicated on the multimeter

I let this run in this way for ten minutes with no change.  Pump still had 12V, engine ran normally.

I then flipped the tank switch to RES with no change.

In short, either the reserve function is no longer working on my bike, or I'm not able to simulate a low fuel condition just by lowering the level sender float.  Possibly due to what FJ_Hooligan stated above.

If someone can verify that I'd have to drain the tank to simulate the low fuel condition, I'd be happy to go try that.

Cap'n Ron. . .

[FJ_Hooligan]

I don't think reserve is triggered by the float position.  That's too variable, the pump (or whatever) would be cutting on and off like mad as the fuel sloshed around.  Damping in the fuel gauge (assuming you still have damping in the fuel gauge) prevents the gauge from jumping around.

I had an old Yamaha with a low fuel light that drove me crazy when the tank got low.  I'm pretty sure it was activated by the float.  It would light up and I would reach up and push a button to cancel it only to have it come back on after the fuel sloshed around a little.  I eventually jumpered that sensor.

I think the FJ reserve function gets its signal from the low fuel circuit.  Even though that light is missing from the ABS models

[movenon]

I'm not sure what's at work with the reserve,  I do recall that the low fuel light works as a variable resistor circuit, which is why the light changes intensity as the fuel level gets lower.  The sensing element has a current running through it when the ignition is on.  When surrounded by fuel, the circuit stays cool and has a certain resistance which keeps the fuel light off.  As the fuel level drops and the element begins to be uncovered, it heats up and the resistance changes causing the fuel light to begin to glow.  The more the resistance changes, the brighter the light gets. 

The highlighted above is interesting.

I just went out to the garage to take some voltage readings off the fuel pump in several different states.  My bike is a '92 with ABS.  The ABS light is where everyone else has their low fuel light, so I don't (believe) I have any dash indication of low fuel.  The fuel tank is currently at about 1/4 full.

I put the multimeter probes into the fuel pump wiring harness connector alongside the wires without disconnecting it from the wiring harness.  I verified that the probes were making contact by switching the ignition on and off again.  I'm using "12V" below to indicate full power even though voltage readings would vary +-a volt or two.

Reserve switch in ON position
Engine off, key off: 0V
Engine off, key on:  12V for 5 seconds then 0V
Engine on, key on:  12V

All the above makes sense and is behaving as I would expect.  I then attempted to simulate a low-fuel condition by depressing the float on the level sender down to the bottom of the tank with a 2' length of PVC pipe.  The PVC was just heavy enough to hold the float level sender at the bottom of the tank for as long as I wanted.

The fuel gauge dropped well past empty, but did not reach the stop pin.
There was no light indication on the dash of a low fuel state (ABS model)
The engine continued to run at 1,100 RPM smoothly
The fuel pump continued to run with 12V indicated on the multimeter

I let this run in this way for ten minutes with no change.  Pump still had 12V, engine ran normally.

I then flipped the tank switch to RES with no change.

In short, either the reserve function is no longer working on my bike, or I'm not able to simulate a low fuel condition just by lowering the level sender float.  Possibly due to what FJ_Hooligan stated above.

If someone can verify that I'd have to drain the tank to simulate the low fuel condition, I'd be happy to go try that.

Cap'n Ron. . .

The sensor doesn't move with the float. It is in a fixed position. I will try to get you a picture in a few min.  
George

[Capn Ron]

The sensor doesn't move with the float. It is in a fixed position. I will try to get you a picture in a few min.  
George

Perfect George!  Also, can you verify that it's not a mechanical switch of any sort and is more like a thermal sensor as FJ_Hooligan suggests?

Cap'n Ron. . .

[Pat Conlon]

The thought of a thermal sensor in a enclosed steel shell with fuel......makes me nervous.

I think I'll stop reading now......

[movenon]

The sensor doesn't move with the float. It is in a fixed position. I will try to get you a picture in a few min.  
George

Perfect George!  Also, can you verify that it's not a mechanical switch of any sort and is more like a thermal sensor as FJ_Hooligan suggests?

Cap'n Ron. . .

Kind of a cruddy unit but that's what it is in my 1990.
George

[ribbert]

The thought of a thermal sensor in a enclosed steel shell with fuel......makes me nervous.

I think I'll stop reading now......

What about all the cars, and probably some bikes, that have in tank electric fuel pumps?

[movenon]

This is a 1991 wiring diagram showing what I am looking at. Sorry it's not the best photo. I also forgot to highlight the reserve switch. 


George

[Capn Ron]

Good stuff George!  Now that I know what I'm looking for, I poked a flashlight into my tank and I can see that entire sensor is completely submerged in fuel.  I'll drain some fuel out of the tank until that sensor is hanging in open air...and redo my tests.

Doesn't look like your wiring schematic came through...Might be helpful!

Cap'n Ron. . .

[ribbert]

When surrounded by fuel, the circuit stays cool and has a certain resistance which keeps the fuel light off.  As the fuel level drops and the element begins to be uncovered, it heats up

What about when the fuel tank (and fuel) gets so hot you can hardly touch it?

Well, mine will run on the reserve function for almost 30 miles...that is a long time to be "lean" and will damage the engine. So the EGT (exhaust gas temps) rise immediately...

It's only running lean from when it starts to splutter to when you switch to reserve, at which point normal operation is restored.


I don't know why everyone's making such a meal of this, it's simple enough to check, and I just have.

My bike happens to only have a cupful of fuel in it and has been on reserve for a few days.
I just started it on the side stand, switched it back to main, waited for it to splutter, probed the live wire to the pump, nothing. While holding the test light on the terminal, switched to reserve, light immediately comes on, power restored to pump.

Repeat cycle 3 times, same results.

Noel

[movenon]

When surrounded by fuel, the circuit stays cool and has a certain resistance which keeps the fuel light off.  As the fuel level drops and the element begins to be uncovered, it heats up

What about when the fuel tank (and fuel) gets so hot you can hardly touch it?

Well, mine will run on the reserve function for almost 30 miles...that is a long time to be "lean" and will damage the engine. So the EGT (exhaust gas temps) rise immediately...

It's only running lean from when it starts to splutter to when you switch to reserve, at which point normal operation is restored.


I don't know why everyone's making such a meal of this, it's simple enough to check, and I just have.

My bike happens to only have a cupful of fuel in it and has been on reserve for a few days.
I just started it on the side stand, switched it back to main, waited for it to splutter, probed the live wire to the pump, nothing. While holding the test light on the terminal, switched to reserve, light immediately comes on, power restored to pump.

Repeat cycle 3 times, same results.

Noel



Ron, diagram should show.

George

[RichBaker]

G'day
  Ok my fuel light is on the dash on the LH side between the speedo and tachometer,up high but below the neutral and turn signal. It gets brighter the less I have, and you can make it go out by stopping  hardish. I have ridden it too empty a few times without flipping the reserve switch. I honestly think it makes no difference on my bike. Know this for certain as I have a reserve switch no more. I am losing fuel though, not sure all this talk of fuel systems makes me think its time to have a bo peep in there maybe replace a few odd hoses. ( this will possibly be another thread)
  Just to throw a spanner in the works.
Cheers
Neil

Something is likely broken, or disabled, in the reserve circuit....

[RichBaker]

.....Still got my money on fuel not spark

Yep, me too   again, what do the wires to the 86/87 petcock do...??

Good discussion....all good points, I will learn something here....

I would start a "Poll" but I am afraid it would start another thread.    
George

Opinions mean NOTHING, they won't change the facts....  I'll write a note to myself to look this up in the manual, pretty sure it explains how these work. I'll check it this weekend, unless someone here beats me to it...
I'm pretty sure Yamaha wanted to make it feel like everyone grew up with, meaning, the dual-height fuel inlets on manual petcocks.

ETA: Looks like Noel just proved it's a FUEL-cut, not spark, for the pump-equipped models...