Topic: Separate gas gauge on Hondamatic gas tank + vacuum assist?

[gnarlycharlie4u]

So I bought a 78 hondamatic gas tank from a fellow forum member. Apparently the CB750A tank is vacuum assist?
Not really sure what that means in terms of running it on a stock cb750. Do I need to modify it at all?

Also, there are a couple of wires sticking out of the tank for what I can only assume is the fuel gauge.
Can I just hook these up to an external gauge like the one I have for my boat? It would be pretty sweet to have the option of putting a little gas gauge near the steering head.

If you don't know the answer to either question but know anything about the tank you'd like to share with me, please feel free to do so.

[raymond10078]

Fuel tanks are not "vacuum assisted" in any way.  Just normal gravity fed fuel tanks.

The in-tank fuel sender should be able to connect to a guage.  The difficultly may be in finding a gauge that will work with the sender - the two have to match.  The full resistance value is 6 to 10 ohms, and empty is 75 to 80 ohms, so the range from full to empty is about 70 ohms.  You'd have to find a gauge that will work (from full to empty) across that same range.

The carb idle system utilizes a "vacuum assist" of sorts to incrementally open the throttle with the bike in gear.  Otherwise, no other vacuum system on these bikes.

[raymond10078]

This gauge is the closeset match I could find.

http://www.summitracing.com/search/Department/Gauges-Accessories/Part-Type/Gauges-Analog/Gauge-Type/Fuel-level/Sender-Ohm-Range/73-ohms-empty-10-ohms-full/?keyword=fuel+gauge

The only other thing needed is a 7V power supply (5V may work).

The Honda manual says to not hook up to 12V.  However - you may get lucky.  I recall that most car systems use 5V for fuel levels.  If so, I would expect that a gauge may connect to 12V, but only provide 5V through the sender.  I'd call the gauge vendor and ask them.  If its 5V, then the gauge identified above will be very close.  It may a little off when empty - it should actually indicate lower than it really is - so, it is "off" in the right direction.

The voltage limitation in the Honda manual could mean that the resistor in the sendor is not rated for the higher currents (if 12V were used).

[gnarlycharlie4u]

Fuel tanks are not "vacuum assisted" in any way.  Just normal gravity fed fuel tanks.

The in-tank fuel sender should be able to connect to a guage.  The difficultly may be in finding a gauge that will work with the sender - the two have to match.  The full resistance value is 6 to 10 ohms, and empty is 75 to 80 ohms, so the range from full to empty is about 70 ohms.  You'd have to find a gauge that will work (from full to empty) across that same range.

The carb idle system utilizes a "vacuum assist" of sorts to incrementally open the throttle with the bike in gear.  Otherwise, no other vacuum system on these bikes.

It appears I was misinformed about the vacuum assist.
That's excellent info and exactly what I was looking for. Thanks so much.

I already have a gauge I bought a while back for my boat that I was hoping to adapt. I'll have to figure out some way of checking the range on it to see if it will work with the 750A tank.
Any suggestions?

[gnarlycharlie4u]

This gauge is the closeset match I could find.

The only other thing needed is a 7V power supply (5V may work).

The Honda manual says to not hook up to 12V.

This could mean that the resistor in the sendor is not rated for the higher currents.

Ah good to know. I'll have to spec the wiring diagrams for the 750A and see where the power for the sending unit is derived from and try to either source the parts to make that happen or figure out a power source and if necessary a voltage drop.

[Spanner 1]

The 750A has a 7volt regulator for the fuel gauge; i.e. the fuel gauge gets 7volts to it ........