289 build

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amulheirn
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Re: 289 build

Post by amulheirn » Sat Jun 06, 2020 9:24 pm

I think I should've heeded Clive's warning which I remember from another thread, but the canoes are now well installed behind the dash now so perhaps I'll change them when they start playing up.

Work today was to remove the fuel tank. I've purchased the goo that Stu recommended to seal the joints, and am waiting on a new fuel sender to arrive. In dismantling it all I discovered a few issues with the sender:

1. Although I adjusted it, the arm is still too long and hits bottom of tank so will never read less than about 1/4 full (d'oh!).

2. I can feel the arm's action against the potentiometer through most of its swing, but for the top quarter of travel. I've got a couple of suspect meters to test resistance and I'm not sure I got accurate readings as a result, but I think the range should go from 20 ohms to 200 ohms, but I am getting about 40 ohms to 200 ohms. So the gauge would never read full either.

3. Arm movement is not free. Arm itself is covered with white corrosion, so I wonder if the innards of the potentiometer are also corroded. Its stiff enough to make me wonder if it moved at all.

4. I'd applied some kind of black gasket stuff to both sides of the cork gasket which evidently allowed the cork to squish flat and also go out of shape. It also didn't adhere to the tank so I am assuming it wasn't fuel resistant and is probably the cause of the bad boot smell. I think I'll put the cork on dry next time.

Anyways - all good fun! Now I've got to decide what to do with a load of dodgy petrol I don't want to re-use....

Hope you're all doing ok and keeping well.

Andy

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Re: 289 build

Post by KevinW » Sat Jun 13, 2020 11:53 pm

There is a fundamental problem with all of these sender/gauge systems, in that the senders normally give a non-linear response, even when the impedances have been matched between sender and gauge. A couple of summers ago I built some maths models to try and understand this, as the Crendon tank is wedge shaped, and at an angle, which is more awkward to understand then the rectangular task in the Hawk and the Sumo. The sender tends to produces basically an 'S' shaped response with fuel depth, so very little change from actual full to actual 3/4, then you will see a rapid change in apparent fuel level going down the S followed by fairly small changes as the tank approaches empty. I have heard some people say it doesn't matter what the gauge says, there is only one point that matters: empty. But if you want to get rid of this non-linear behaviour, Spyda do a gadget to address this: https://www.spiyda.com/fuel-gauge-wizard-mk3.html

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peterc
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Re: 289 build

Post by peterc » Sun Jun 14, 2020 7:36 am

I can agree with Kev. 3/4 when it is really 1/2 etc etc.
The important first step is to ensure that the arm can traverse its full movement without bottoming out. It should be adjustable for length. Mine was.
Then fill the tank with known quantities of fuel ( either gallons or litres to your choice) and take ohm readings from the sender. Then you should be able to program the Spyida unit. I have already got readings for every 2 litres but it shows not a lot changes around empty.!
I note that there is a discount for two units so Andy if you feel the desire to buy a Spyida unit then buy two and I will split the cost of the postage with you.
I must have missed the point but what is wrong with the petrol?
Peter C

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clive
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Re: 289 build

Post by clive » Sun Jun 14, 2020 8:59 am

I would point out that the Hawk petrol tank is narrower at the bottom than the top, so a swing arm sender will never be 100% accurate.
Cheers, Clive.

(If I'm not here I'm in my workshop or on the golf course!)

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peterc
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Re: 289 build

Post by peterc » Sun Jun 14, 2020 2:11 pm

Hi Clive, that’s the reason for using the Spyida unit. You adjust the fuel gauge reading by manipulating the correct ohms to reflect the correct fuel level you have put in for calibration purposes. It is designed to take into account tanks with changing shapes. The Hawk tank is actually pretty simple in only changing in cross section.
Clearly it needs the sender arm to make a full sweep. E.g fully down when tank is empty and not when there is significant quantity of petrol left. Even then if you know that there is say one gallon left then you can treat that as your reserve.
Even a vertical tube gauge would need calibration if the tank tapers or changes shape.
It would be nice to see the gauge a tad more linear than the rapid changes at the top and bottom as Kev describes.
I’m certainly prepared to give it a go.
Peter C

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Re: 289 build

Post by CobStang » Sun Jun 14, 2020 4:02 pm

I have a hardwood stick to dip the tank through the filler, kept in the boot. From empty I put in a gallon at a time and marked the stick. I have had my BRA since 1994 and in all that time it has never gone wrong or failed to read correctly.

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clive
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Re: 289 build

Post by clive » Sun Jun 14, 2020 4:45 pm

My only concern is the E end of the gauge. My reserve starts when the gauge reads 1/4 as that is when I start looking for a petrol station. From experience (Over 46,000 miles) I have a fairly good idea what the range of my car is on a long run, but it's the series of small runs that can lead to doubt. For example, I know that from home to Silverstone takes two tanks of fuel and I know that West Aukland is half way, which is where I refill. Because there are no further filling stations between there and Edinburgh at night, it can lead to a few twitchy moments if I get stuck in traffic!
Cheers, Clive.

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peterc
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Re: 289 build

Post by peterc » Sun Jun 14, 2020 5:43 pm

As you say Clive it’s the series of shorter runs that leads to doubt. I’m afraid I often fill up when it get close to E but often find there was plenty left and i could have driven home and chosen a cheaper option of petrol station.
Peter C

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clive
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Re: 289 build

Post by clive » Sun Jun 14, 2020 5:49 pm

peterc wrote:
Sun Jun 14, 2020 5:43 pm
As you say Clive it’s the series of shorter runs that leads to doubt. I’m afraid I often fill up when it get close to E but often find there was plenty left and i could have driven home and chosen a cheaper option of petrol station.
Peter C
And they say Scotsmen are tight! :P :P The day you gamble on how much is left, is the day you run out. :lol:
Cheers, Clive.

(If I'm not here I'm in my workshop or on the golf course!)

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peterc
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Re: 289 build

Post by peterc » Sun Jun 14, 2020 9:06 pm

Just prudent!
😉
Peter C

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