Hmm fuel gauge not working

[ThePowster]

Afternoon fellow Nighthawkers

Ok so my problem goes something like this...

Bought this second hawk (first one I had is a spares bike) and when I rode it the needles on the rev counter and Speedo just snapped, they were super brittle.

Over time the gear indicator shows gear 1 then goes blank after that and the fuel gauge isn't working when it once was.

Sick of not having a Speedo today I swapped them over and used the clocks from my #1 Nighthawk.

I started her up took her for a spin and... Speedo works, tacho works, gear indicator works, but still no fuel gauge, so this has to be elsewhere on the bike.

I know there are 2 wires under the tank but they are plugged in and I'm at a loss now where to start and fault find why the fuel gauge isn't working.

It has in the past, especially when I filled up but after a while it stops responding. It can't be the gauge itself as this is a new low miler set of clocks and it's still happening.

The new clocks are American I think as they go up in 5mph increments, which is awkward for me as in the UK we have speed limits of 30-70mph in 10mph increments so I'm going to have to read between the numbers here.

Something else odd with the clock change, the idle before was loud, 3-4000rpm, now it's idling around 1,500rpm, has to be coincidence surely, no clock would change a mechanical idle, I did ride it before so maybe it cleared itself out a bit.

If the fuel gauge works again then I have a full set of instruments and I'd be a happy bunny 😁

[mollusc]

The fuel level sender is a mechanical float with a rheostat, inside the tank.  It's pretty simple to remove and clean, then make sure that the sweep arm makes contacts with the rheostat windings across the full arc.

[ThePowster]

A-ha! I'd never of guessed that and looked inside the tank.

I guess I'm going to have to remove the tank and drain it, right ? To get this sender out.

Could you elaborate about the sweeper arm and rheostat please as I've never heard such terminology before.

[Larry Fine]

See part #17: https://www.partzilla.com/catalog/honda/motorcycle/1984/cb700sc-a-nighthawk-s/fuel-tank

[ThePowster]

Thanks a lot 👍

[mollusc]

Yes, you have to pull the tank and drain it.  The unit is held in to the bottom of the tank by four machine screws.
Once you carefully extricate the mechanism, it should be pretty self-evident what goes on.  There's an arm that has a float, and it moves.  From memory, the pivot end goes into a small metal "house" which you can open by carefully bending some tabs.  Inside it you'll find a set of copper windings and a small sweeper arm that's connected to the float arm.  The further along the copper windings the arm is, the higher the resistance.
The idea is to clean up the copper windings and the contact patch on the sweeper arm, and then ensure that they make good contact across the whole arc of movement.  You can check this by putting the probes of a multi-meter into the wire connecter (where it would plug into the wiring on the bike) and setting the meter to measure resistance.  You should see a smooth change as you move the arm.

[Pete in PA]

Ive done exactly this to a V65 Sabre.  Even bent the arm slightly because it went to "reserve" much too early.

[ThePowster]

That's brilliant guys thanks so much, it's a lot clearer now and I feel more confident in giving this a go.

I'll fire up some pics too when I can and I have another tank and I may compare the two so we can identify the problem.

[mollusc]

I had to bend the sweeper arm on mine very slightly to get it to make good contact all the way across.  Worked perfectly after that.  There's nothing particularly special or unique about the system.