My next weird experience

[Hojo]

My gas gauge reads very oddly. I put 6 gallons slowly into my standard new design tank behind the seats and eventually it reads almost full. I drive around 100-120 miles and the gauge goes down to a little under 1/2 tank, just as it should. Then, as I drive I watch the gauge go down to below empty with the empty warning light on over the next 10 miles or so. I put 6 more gallons in the tank. No more will go in. The gauge doesn’t show that any gas has been added for 20 miles or so.

I removed the seat and firewall and pulled the 2 rheostat/float mechanisms. Both worked perfectly up and down to the book resistance specs. I replaced the oval one and manipulated the float up and down through the other round opening. The float was free at all times and clear of the round opening float. I was thinking of using a boroscope to look down the gas fill pipe to see if the tank is really full. Other than that, I don’t have a clue of what to try next. Why does the gauge go from just under 1/2 to empty on 10 miles? Is it really empty? Am I filling the tank with 6 gallons and the gauge is showing the correct amount? Am I fooling my self about the floats not binding and working properly? Should I have stayed home instead of going to the counterinaugural ball when Nixon was elected?

[RPGs818SNA]

Hojo, those are some weird symptoms. I don’t know what’s causing them, but here are a few bits that might help.

My new style factory tank holds just over 14 gallons, but only 7.8 below the top of the fill pipe. So if I fill it without letting the air in the top of the tank out, it only holds 7.8 gallons. The tank has to be vented somehow, preferably through a charcoal filter.

My two floats worked fine individually, but bumped into each other as the tank filled. I put a stick of ¾” wood across the front of both float covers while they were installed and marked how the covers aligned with the stick using tape. I did the same on the back side. Then I took the floats out, clamped them to the sticks as marked, and clamped one of the sticks in a vice so that the floats hung in position from their covers. When I raised the floats at the same time, they bumped into each other a few inches up. I had to bend each support wire a little to keep them from bumping.

Also, the two floats have to be wired in series to read correctly.

I don’t quite see how a combination of these 3 issues would cause all your symptoms, but it may give you a few things to check.

Good luck getting everything resolved.
RPG

[Hobby Racer]

The issue I had with the dual senders was that they could contact each other at some places in their arc of travel. Each one independently could move through its arc fine. Since FFR placed them so close they would some times tangle and give weird reading. I ended up bending the arms to give them more clearance relative to each other. Its still not perfect but its much better.

It was very hard to know where they contacted since you needed to have both installed to get the issue to happen, and then you can't see in the tank to know where the issue occurred!

If I remember correctly, the sender attached to the fuel pump also scrapped the edge of the tank.

[J R Jones]

RPG. I have a resto-mod 1929 Mercedes SSK with Lexus running gear. The Lexus fuel tank was a saddle-like configuration originally under the rear seat. It required remote pumping and sensing. I retained all the evap system and it has two tank vents plumbed to the filler neck near the cap. I had to transfer all the pump/plumbing bits to the new custom tank.
It seems to me that venting from the high point of your tank to the filler neck would resolve your fill issues.
jim

[RPGs818SNA]

I agree, Jim, that a good vent from the top of the tank to the filler neck is the rock solid solution for slow filling issues. That’s what Subaru uses.

Instead, I converted the fuel outlet on the round topped fuel level sender unit to an extra vent by drilling small holes in the tube near the top. I plumed that and the fuel pump vent to the charcoal canister, hoping that will be enough venting to allow a reasonably fast fill. If not, I’ll add the filler neck vent.
RPG