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MG MGB Technical - Tank Capacity / Gauge Wizard

Hi everyone,
I am about to help my brother-in law calibrate his Gauge-Wizard to get his fuel gauge working properly on his 77 mgb roadster (Ex R/B but converted to chrome bumpers)

I have recently done this on my MGA and the gauge now (for the first time since I got the car) works brilliantly.)

Does anyone know the what the fuel tank capacity of a 77 B should be? (I know this could vary depending on the tank)

If not, what is the best way to drain it so that I can fill it from empty to find out what its capacity is. I have a spare fuel pump that I could rig up at a pinch)

(My MGA has a tank drain plug and so it is easy to drian)

Cheers
Colyn



Colyn Firth

From memory there were only ever 2 tanks, the early strap mounted one 8 gallons and the later bolt on 10 gallons. Draining even a 1/2 full tank results in scary quantities of fuel to keep safe.
Stan Best

Like mine often is Stan's memory is a little off - tank capacities were 10 & 12 (UK) gallons, the later was 12 gal.

Capacities are often nominal anyway but especially if the tanks has dents.

I think original tanks had a drain plug but wouldn't know about after market ones.

https://www.mg-cars.org.uk/MGB/mgbspec.html
Nigel Atkins

Several tanks, but only two capacities as per Nigel.

It would have to be a helluva dent to make much difference as far as draining a tank goes.

Reputedly tanks had drain plugs until Jan 74. However my roadster came to me at 15 years old with a old tank that soon rusted through and that didn't have one.

If the fuel level is above the position of the outlet on the sender you could remove the hose from the pump and bring the blocked open end down below the tank, unblock it, and it will siphon the tank. This is always the problem with the chrome bumper pumps, but on RBs the pump is higher then the tank which means it won't siphon unless you want it to. That does mean a container with a large base area to height, so it is lower than the bottom of the tank (unless the whole car is raised), so using a spare pump into a more conventional container may be more practical. Be patient.
paulh4

Thanks everyone,
looks like I will have to drain the tank and then carefully refill it to work out the capacity.

Cheers
Colyn
Colyn Firth

My ‘67 has its original tank, complete with drain plug.
Dave O'Neill 2

I replaced a couple of tanks last year. Just syphoned as much as I could. As we were changing the tank we removed it and poured out the rest. But as syphoning got to where "Empty" would be, it might be sufficient.
Michael Beswick

We haven't started the job yet because I would rather wait for some better weather so that we can do the job outdoors so as to prevent any build up of fuel vapour.

My plan is to run the fuel tank as empty as we dare, then take a fuel pipe off under the bonnet and use the cars own fuel pump to empty the remaining fuel into a can. Even if there is a little fuel remaining in the tank, I suppose that this "empty" point is the only one that really matters for this exercise.

Then I will put a measured amount of fuel into the tank (say 5 litres) and drive it to a (very) nearby service station to completely fill the tank and also note exactly where the level of fuel is in the filler neck. (The fuel station is around 2 miles away)
Adding the amount needed to fully fill the tank to the 5 litres will give us the exact capacity.

To be absolutely exact, on our return home we could always top up the tank until the fuel level is back up to the position in the filler neck seen at the garage and take a note of how much fuel is needed to to this. Probably around 3/4 pint.

Then we would have to drain the tank once more so that we can then begin the calibration process

With the Gauge Wizard you then have to fill the tank from empty in exact 1/4 steps and programme the wizard each time to calibrate it.

Cheers
Colyn


PS

GAUGE WIZARD INSTRUCTIONS

I personally found the Gauge Wizards written instructions a little confusing and so I watched their YouTube installation video which I found much easier to follow.

I then transcribed my own step by step instructions to make it simpler to do.

If anyone would like a copy of these for an MGB (or a slightly different set for an MGA) I would be happy to send them one.

Colyn Firth

Colyn,
I've run out of petrol in several cars B and Spridgets included without subsequent problems so I wouldn't worry too much about completely running the tank out of petrol.

Do allow for heat expansion (probably not the technically technical word), avoid going from when its hot weather etc. to when its not for getting petrol from petrol station and measuring in tank filler tube and calibrating. There can be a (possibly very small) difference from even being in the shade or not with the car, station petrol pump, spare can and its storage, ect., especially when sunny and hot.

Imagine the fun of being with me on a mpg test.

As my midget tank holds less than 6 gallons I often fill right up the tube to the back of the filler cap - not in summer obviously - so I know you need to let the petrol settle to get a fixed fill point on the filler tube neck.

I've run out of petrol on a motorway, a dangerous thing to do as the hard shoulder is a very exposed and dangerous place to be let alone putting petrol in on the side nearest the live lane (I was stupid to allow it even if it was unexpected).

I've run out within yards of a petrol station I was pulling into as my wife suggested that we couldn't trust the gauge on a classic car new to us - she was correct as it showed a quarter tank as we rolled in.

She refused to push any of my cars many years ago, the one that caused it was only about quarter to half a mile and it was on the flat and I was doing the steering too. I blame the bloke I asked to borrow his can as he was mowing his lawn with a petrol mower, it was the look he gave us.

When we first got my present Midget I was lining up to overtake a draggy-arsed Merc in our group when my wife asked if it was a good idea as she'd heard the pump tick as we turned off the roundabout, we went by and back in then stutter and coast to a stop. Of course we had the spare gallon in the boot as my wife won't go in the car if it's not there and at least half full and wants me to fill the Midget's tank when the needle goes below half - a very unforgiving person. :)
Nigel Atkins

Colyn, I am not sure what the Gauge Wizard is, but I may need it!

The current project has a rubber bumper tank and sender and a chrome bumper gauge.

Does that mean the gauge will be inaccurate?

Colin
Colin Parkinson

Not necessarily, the sender has to match the gauge obviously, have a look at Paul Hunt's mgb-stuff site under Hammer & Spanner and then Fuel and gauge and sender.
Nigel Atkins

This is a post that I put into the MGA Forum last year about the Gauge-Wizard.
Colin, it is a very clever electronic (digital) gadget that will let you correct the position of your fuel gauge so that when the tank is full, the gauge shows full and when the tank is empty, the gauge shows empty. With the full range in between.

I have got it working brilliantly on my MGA and I now have to sort the gauge on my brother in laws 77 MGB.

So long as your fuel gauge needle varies in position from between when the tank is full and when empty, even if it is not showing full movement on the dial, then you should be able to calibrate it using a Gauge Wizard.

You MGB guys may be more fortunate with your replacement sender units, but even the best MGA sender units usually don't work accurately.

This is the post I put on the MGA forum about the Gauge Wizard last year. Its quite long, sorry.

"Thread: Gauge Wizard
Posted 13 July 2018 at 22:57:49 UK time
Colyn Firth, South Yorkshire, United Kingdom
I fitted a new fuel sender unit a year or 2 ago which was a vast improvement on the old unit because its resistance range was almost correct.
Its resistance range is 0.5 to 72 ohms which is really close to the correct range according to MGA Guru) of 0 to 70 ohms.

The old sender unit had a resistance range of 15 to 98 ohms which meant that the gauge showed full most of the time and dropped to about 1/3rd full when the tank was empty.

The new sender unit was much better in that the gauge did drop to about 1/8th full but would never quite drop to empty.

The trouble was that I started to trust the gauge a lot more than I did before which was a mistake, I normally keep an eye on the odometer to see how many miles I have driven since refilling the tank and doing this combined with an eye on the gauge has worked fine.

However, if you have ever attended any of the MGCCs MGA Weekend Tours or similar, you are often told to zero your trip at a certain points so that you can follow the route.
This meant that it is really easy to lose track of your actual mileage and this meant that I ran out of fuel on the way home from 2 of these events!


So a few months ago I bought Gauge Wizard (From Spyda) to see if its digital processor would let me correct the gauge so that it read correctly.

I had read about it in Safety Fast Magazine when the MGA Scribe Mr Edward Van Dyke wrote about the Gauge Wizard by Spyda. He had arranged with Spyda to include the MGAs sender unit (0 to 70 ohms) to be included in its range of built in pre-programmed range (or Pre-Sets)
This means that you install the gauge wizard, select the MGA pre-set and off you go.

Well, its no surprise to know that using the pre-set, my gauge didnt work any better than it did before!

So I had to then go through the Spyda recommended procedure of filling the tank in one quarter steps (2 1/2 gallon steps on my my MGAs 10 gallon tank) and programming the gauge wizard each step of the way.
You start with an empty tank and work your way up to the tank being full.

It is a bit tedious and I must admit that I was not totally comfortable with the process as you have to handle a lot of fuel to do this. (In did keep a couple of heavy duty fire extinguishers handy.)

You have to input the sender units output to the gauge wizard in 1/4 steps, from empty through to full so that it remembers each setting.

Then you programme the gauge wizard to move the gauge needle to match the newly memorised settings from the sender unit.
This means that when the sender unit sends empty, the needle also shows empty and when the sender unit send full, the gauge then also shows full. etc

At first I was unsure if there was any real improvement, the gauge showed full for longer than I expected. But this was because when I checked the resistance reading of the sender unit on the 3/4 full mark, there wasn't so much difference from the full resistance reading.

However, once the fuel level dropped, the gauge began to show much more accurate readings.
I found that when the gauge showed 1/2 full there were exactly 5 gallons remaining in the tank and when the gauge showed 1/4 full there were exactly two and a half gallons remaining.

I have also installed the warning light option to act as a low fuel warning light, this illuminates continually when the needle drops to the 1/4 level and then flashes when the needle drops to 1/8 full.

It is a refreshing change to be able to look at the fuel gauge and know that what you see is what you have really got!

Cheers"

Colyn
Colyn Firth

Colin P - the tank sender and the gauge both changed together for the 77 model year so it's possible the CB and RB systems are not compatible. However the CB gauge at least has a very wide capacity for calibration.

Although the UK only got the electric temp gauge for the 77 model year North America had them some time earlier. Those gauges changed for the new dash similar to the UK dash, and the sender was changed to suit.
paulh4

This thread was discussed between 27/01/2019 and 08/02/2019

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