XJ700 Fuel Sensor Questions...

'85 700 airhead. I am currently fixing some wiring, mostly for the headlight, but the wiring for the fuel sensor has completely dislodged its wire harness.

Will the sensor give you actual readings on how full, or empty, your tank is (obviously not to be relied on if it did/does) if I were to go for an aftermarket gauge cluster? Or does it only function as a "you have approx. 30 miles to find a gas station" light switch?

Is there a way to test the sensor? And if mine does need replacement, and I decide to replace it, should I trust the eBay ones (I'm not paying $80 for a fuel sensor from XJ4EVER)?

 

It is only a low fuel level warning sensor, it does not provide a reading of current fuel level.
If the harness is broken, what makes you certain the sensor is inoperable, as well? Why replace it?

 

Since it's only a low fuel sensor, I'm not even gonna bother with it, one less thing to worry about! Just need to get a good ballpark MPG figure this spring so I know not to drive over ___ miles without filling up.

 

IIRC the sending unit on the later 700 is the same as on the earlier 750. What makes it into a low-fuel warning is it being hooked up to a light that is triggered when the resistance of the sending unit meets a value that indicates the tank is low.

You can measure the resistance of the sending unit and see if it varies, or if it just acts like a switch. If the value varies with the fuel level then you can hook up a fuel gauge.

 

I have the same bike and I recommend that you get it working. If you get the bike finally tuned you can hit reserve any where between 80 and 100 miles depending on how you drive it. Mine is in good tune and on the road at 60-65 the light comes on between 95 and 100 miles and I hit reserve fairly quickly after the light comes on.

 

How many miles do you get after your light comes on before you switch to reserve? Cuz that don't sound right....if I ride sensibly (shift at 4k and cruise at 3.5k in 5th), I tend to get around 37-42 mpg all in stop-and-go; this means at the 3.43 gallon main tank size, I should, theoretically, be able to knock out around 130 miles (rounding up) before hitting reserve based on 37mpg. I can only imagine that fuel economy is greater on the highway, especially if I keep it well below the usual 75+ most people ride. If I ride 60mph, I want to say that I should be able to receive a respectable 45 mpg at least (about 155 miles before reserve). If I rip on it during acceleration, which I plead guilty of doing.....a lot.... I tend to get anywhere from 27-35 mpg.

If I were traveling on ANY bike with a <5- 6 gallon tank, I would make sure to fill up around 100 miles anyways, which makes for great timing to stretch the body out (about 1.5 to 2 hours depending on speed). I think I am going to get another bike for traveling on and keep my XJ as my "weekend warrior" bike...not quite sure if I would trust it on a road trip yet, it seems that I fix one thing and another thing goes awry with it! As is, like I said, I don't do any long distance riding right now. I check my fuel level every time I hop on, if it's too low for comfort (which isn't much: maybe a gallon and a half), I just ride to the closest station and fill 'er up and be on my merry way, so again, why bother?

 

The problem with your math is that the 3.34 gallons is the total capacity of the tank. There is no seperate reserve tank. The reserve is a portion of the total capacity. You also can't actually access every bit of that 3.34 gallons; some of that capacity is used as settling space to help keep crap out of the carbs.

As to why bother. I ride daily for 9 or 10 months of the year. You'd think that I'd never run so low on fuel that the warning light comes on given that I pretty well know how long I can ride before needing to top up. I've been caught nearly going past a station more than once, and only been saved from walking by the light flashing (the Seca 750 light works differently from the 700 Max light). To sum up; being tired happens, getting lost in the moment happens. The light helps. Besides I'm pretty sure that at some point you'll want to head east from Spokane. There will be occasions when 100 miles of range won't quite get you to the next open station if you have any desire to go off of the main highways.

 

Like I said, I don't travel on this bike, when I do, it's only 40 or 50 miles, fill up before I leave and fill before I return. City travel, like I said, hop on bike strand it up and check tank, if I don't like the looks, I fill up. I've filled up around only .7 gallons before. I can believe the 3.34 for the whole tank, it is pretty small, however there is technically a reserve, why else would they have a "reserve" slot on the petcock? Even if it's a two tower set-up like most dirt bikes do, it is still a "reserve". I'll looking into reconnecting the wiring somehow if you guys seriously insist on it though. But if it still don't work, which I likely will never be able to test, simply because I never use more than a half tank before filling. then I'm won't be sourcing money for the part unless I want to try and sell it eventually.

 

Quentin, while the tank is a bit over 3 gal, there is an amount that stays below the petcock that you can't use. Your effective amount is more like 2.5 or so..... It also depends on how your fuel level sending unit is set. On mine ( 700 ) the light comes on at roughly 100 mi, and when that light comes on, I'd better get to gas pump QUICK, cuz I got about 2-3 miles left. I've coasted down 2mi-long hills with either in neutral or even off just to get to the pump.... A different 700 that I had was such that the light came on consistently around 97-100 and I had about 10-15 mi left.

I do have to adjust the shims in my 700 this coming summer and I expect it will bring the numbers back up, but I'm sure not going to be running 130+ mi on an xj700. Maybe 110 -or so....

 

My set up must be exactly like hogfiddles--light comes on between 98-100, and I have maybe 3-4 miles before it dies if I don't switch to reserve. Since I would hate to have it die in traffic I go ahead and switch to reserve around 85 miles. At my ripe old age of 70 I cruise between 60-65 unless on the interstate where I run about 70. I've never been hard on any machine I've owned so in town and acceleration is fairly gentle. I've had this 700 for about 3 years and I may have had it over 7000 rpm's once. If you are getting 45 mpg with today's fuel you are doing great. My fuel light stopped working about 6 months ago and since all the wiring is original and in good shape I assumed the sending unit was bad. The next time I had the tank off I removed the sending unit and found that the wire that runs from the top of the tower to the base had broken loose. A little solder and all's well. I have to admit that I didn't like not having the light although none of my other bikes have one. But both of them have larger tanks and get better mpg.