Weird problem?

[Gene]

 I have a '85 650sc Nighthawk.

I've rebuilt the carbs myself, (they still work fine after about a year) and checked everything else over and fixed some minor problems.

I have two problems, perhaps related.

1st, the fuel gauge *seems* to be inconsistant. with what is really in the tank, although it seems to work fine sometimes. When I removed the fuel gauge, it looks a bit rusty. I tried running it through it's travel, and the gauge seemed to be accurate and not have any skipped segments.

Another weird thing.

 Most gas tanks go over a frame tube, the 650 is no different. Sometimes I have to switch to reserve when the tank is approx half empty. This is after a sharp right turn. I find it hard to believe, but the turn seems to partially drain the left side of the tank, stupid as that seems. I remember bikes that I had that had a balance tube to equalize the two sides, this bike doesn't.

Anyone else with a 83 to 85 650 have the same problem? Am Imissing something?

[Bob H]

1st, the fuel gauge *seems* to be inconsistant. with what is really in the tank, although it seems to work fine sometimes.

That is an issue for modern fuel injected motorcycles as well. Most people just go by the odometer and reset trip miles with every fill up.

Most gas tanks go over a frame tube, the 650 is no different. Sometimes I have to switch to reserve when the tank is approx half empty. This is after a sharp right turn. I find it hard to believe, but the turn seems to partially drain the left side of the tank, stupid as that seems. I remember bikes that I had that had a balance tube to equalize the two sides, this bike doesn't.

Here is an image of your petcock from Ebay.



It works like the late model 750, the copper tube limits the amount of fuel that can be accessed. The remainder is obtained when you flip reserve on, then the rest of the tank drains.

Old Harley's with separate Fat Bob tanks, they will have connecting tubes between the tanks and I had one with an odd reserve arrangement. But most modern bikes do a pretty good job of having the left and right tanks "connect" where the lowest rear section of the tank is located. There isn't any other hose connecting the sides.

I rent and ride modern bikes for a week at a time, and trade off with friends who do NOT have to flip a petcock valve to access the rest of the tank. They know to fill up around 150 miles or whatever. Fuel injected bikes don't have a reserve per se.  My petcock is very difficult to reach and move the valve while riding, so I just leave it on reserve 24-7 and reset the trip miles at fill up.

[mollusc]

When I first got my 700S, I was having some issues with the reserve setting.  Regular was fine for the most part, but would sometimes starve a little.  Turns out that the PO had lined the tank -- poorly -- and the liner material had blocked up the lower intake part of the petcock.  That is, only the standpipe was pulling fuel.  In addition, the standpipe had eroded so it was starting to collapse on itself and was partially obstructing the fuel flow.  I cleaned all the tank liner out of the top of the petcock and sleeved a new brass standpipe over the stump of the old one (get this at a local independent hobby store).  Since then I have had no issues.
I can see the possibility of the lower intake point getting clogged with sludge or debris even if your tank isn't lined.  It might be worth pulling the petcock out and giving it a thorough cleaning -- and maybe flushing the tank while you're at it.  As far as I know, no Nighthawks have a crossover pipe like early Harleys or Indians did.

Regarding reserve vs. regular setting, I never leave it on reserve.  I can't change mine while riding, but leaving it on reserve defeats the whole point of having a spare gallon or so in the tank.
If your odometer is off for some reason (e.g. you forget to reset it when you fill up) and you do run out of fuel, you can pull over and change to reserve so you can get to a station.  If it's already on reserve and you run out, you're SOL on the side of the road.

[Bob H]

leaving it on reserve defeats the whole point of having a spare gallon or so in the tank.

Modern motorcycles (Fuel injected) don't have a spare gallon in the tank.
My reserve was FORCING me to run out of gas prematurely, painfully evident when riding with friends that had more fuel efficient (fuel injected) bikes, no hidden gallon of fuel they couldn't access.

I grappled with this issue for a long time. Of course to each their own - but I resolved that all vehicles are capable of getting stranded and running out of gas. Cars, trucks, motorcycles. Cars and trucks don't have a reserve valve. I just carry extra fuel if I know I am going somewhere remote, with sketchy availability of fuel.

[mollusc]

Of course all vehicles can run out.  Fuel injected tend to have relatively accurate fuel gauges though, which at least goes some way towards mitigating the issue.  I never found the fuel capacity on my Nighthawk to be a limiting factor, even with other bikes in my riding group.
It all depends on your personal preferences.  I prefer to carry the extra gallon inside the tank instead of outside it.