Gravity fed carb, fit a pump?

Gravity fed carb, fit a pump?

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Discussion

Tyre Smoke

Original Poster:

17,725 posts

228 months

Saturday 5th June
quotequote all
The little Trabant gamely goes everywhere, but struggles up long or steep hills with less than a full tank of fuel.

The reason is the tank sits against the bulkhead under the bonnet and holds about 26 litres. It gravity feeds the carburettor, a drop of perhaps only 12 inches. You can see the tank here, and the arrow shows the carb position.



I'm wondering if fitting a cheap fuel pump would make things better or worse? A little in line one for about a tenner on the bay. But it's not needed all the time, just up steep or long hills of which there are plenty here in Devon! And only when there is perhaps half or less in the tank.

Any ideas for a cheap and easy fix?

caiss4

1,642 posts

164 months

Saturday 5th June
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Put the fuel tank on the roof? I joke! Just brings back memories of my youth running around the church hall car park in old Mini Traveller with a metal fuel funnel strapped to the windscreen pillar feeding the SU carb. The fuel tank had long since rusted out, the fuel pump didn't work, there was no windscreen and the funnel held 1-2litres so I'd get 10-15 mins of looning around with petrol spilling out of the funnel before topping it up.

Sorry that's of absolutely no help to you but it brought back good memories smile

Tyre Smoke

Original Poster:

17,725 posts

228 months

Saturday 5th June
quotequote all
Sounds like when I fill up at Tesco. Open the bonnet, dip the tank - no fuel gauge, fill with petrol, note number of litres, around to the boot, take out 5L can of two stroke oil, calculate approx 40:1 mix, top up fuel tank, put can away, head in to pay.

It's a mini service! And that's only if they haven't shut the pump off and yelled over the tannoy!

gazza285

7,588 posts

175 months

Saturday 5th June
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That should be ample head to fill the float bowl. I presume you have checked all the obvious stuff, clean filter, cleaned the carb, checked and adjusted the float height, checked the feed pipe for blockages, fuel tank vent etc...

Tyre Smoke

Original Poster:

17,725 posts

228 months

Saturday 5th June
quotequote all
Yes, it's all brand new and flows fine. There just seems to be less flow up steep and or long hills. Helpfully Sachsenring fitted a flow meter for a bit of decadent Western gauge type stuff. The flow drops right away the long the car is going up hill. I've proven it's simply lack of pressure (I'm with you, I thought that was ample head of fuel) because as soon as you turn around, or it flattens out, things return to normal.

tapkaJohnD

1,610 posts

171 months

Saturday 5th June
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If you fit a fuel pump then a pressure regulating valve will also be necessary, as most pumps will deliver more pressure than the 12" (?) you have. But most valves cannot be adjusted to less than several times the pressure that your gravity system will give, so carb flooding may be expected.

If the pump were to keep filled a small reservoir as high under the bonnet as possible, with excess redirected back into the main tank in the manner of a 'swirl pot', this would maintain the gravity feed rate.

JOhn

GreenV8S

28,859 posts

251 months

Saturday 5th June
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Tyre Smoke said:
Yes, it's all brand new and flows fine.
The problem may be within the carb. With the fuel level low and on a slope where you'd expect the problems, do you get enough flow out of the disconnected fuel pipe to feed the engine at full throttle?

If you want, you can add a low pressure pump to the circuit with a return back to the tank. The pressure at the carb will be determined by the geometry of the connection to the carb, and the restriction of the return feed. For example if the supply line points the flow at the carb inlet and the return is taken off the side, you'll get a little ram pressure; if the supply flows across the carb inlet it'd tend to suck fuel out of the carb. But when the pump isn't running, it'll offer quite a big restriction so if you plan to run like that you'd need the 'return' to act as a supply when the pump is off. It's perfectly doable, and relatively easy to set up if you can understand the pressure and flow of the fuel, but also quite easy to get wrong if you don't.

Tyre Smoke

Original Poster:

17,725 posts

228 months

Saturday 5th June
quotequote all
I was thinking that. Almost like having a bypass pipe in the supply to the carb, which goes via the pump?

Would splitting the line into two via a Y piece, half direct to carb (as now) and the other via a switchable pump, then back to another Y piece and into fuel line, work? That would permit excess pumped fuel back into the tank I think. Plus the pump wouldn't be required all the time.

I guess it depends how close the pump is to the carb will make all the difference with regards to how it works best?

For info, all fuel lines, fuel tap, filter and carb are all brand new. The drop isn't so huge when going up hill, there is probably no more than 18-20 inches laterally from fuel cock to carb inlet. Then remove half the fuel in the tank and the gravitational effect isn't going to be massive.

Error_404_Username_not_found

330 posts

18 months

Saturday 5th June
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Hi TS. Nice to see Trabitha again.
Left field suggestion:
Next time it happens take the tank cap off and try again.
A mate in Missouri had a similar issue with his Midget (yep, I know it's not gravity fed but even so...).
Borderline breather cap issue.
Quick, easy and free test and you never know...
Cheers.

Tyre Smoke

Original Poster:

17,725 posts

228 months

Saturday 5th June
quotequote all
I will try that, the cap is the tightest of all things on her!

Error_404_Username_not_found

330 posts

18 months

Saturday 5th June
quotequote all
Good luck.
But think about it:
If the tank is half empty the breather needs to pass more air to equalise with atmospheric pressure than if it's near full. So it seems plausible to me.
OTOH it might be bks.
Interested to know who wins!

oakdale

1,272 posts

169 months

Sunday 6th June
quotequote all
The carb is gravity fed but the fuel pipe appears to come from the top of the tank, how does that work?

Tyre Smoke

Original Poster:

17,725 posts

228 months

Sunday 6th June
quotequote all
No, that's the breather/return. The fuel tap is off the bottom in the middle.

oakdale

1,272 posts

169 months

Sunday 6th June
quotequote all
Tyre Smoke said:
No, that's the breather/return. The fuel tap is off the bottom in the middle.
Can't see it being a return on a gravity feed, it must be just a breather, is it clear?

If it's a breather, changing the fuel cap won't make any difference.

oakdale

1,272 posts

169 months

Sunday 6th June
quotequote all
I've just looked at a diagram of the fuel system on these, there seems to be a small fine gauze filter in the tank where the tap is located, is that o/k?

Tyre Smoke

Original Poster:

17,725 posts

228 months

Sunday 6th June
quotequote all
Yes, all clear and brand new.

The breather is odd, it tees off the fuel line and returns to the tank via a banjo bolt. Again, all clear.

oakdale

1,272 posts

169 months

Sunday 6th June
quotequote all
Tyre Smoke said:
Yes, all clear and brand new.

The breather is odd, it tees off the fuel line and returns to the tank via a banjo bolt. Again, all clear.
The pipe must be to prevent vapour lock, I'd imagine the fuel system must get pretty warm with it all being under the bonnet.

You still need to find the breather then, which may be in the cap.

Tyre Smoke

Original Poster:

17,725 posts

228 months

Sunday 6th June
quotequote all
I will check the cap, I imagine the breather will be integral with it. These are very simple cars and built as cheaply as possible.

Obviously Comrade, you have waited five years for your car, if you don't like it won't go up steep hills, give it back. We have many more worthy party members who would be delighted to own such a fine example of our glorious engineering prowess. hehe

Tyre Smoke

Original Poster:

17,725 posts

228 months

Sunday 6th June
quotequote all

tapkaJohnD

1,610 posts

171 months

Sunday 6th June
quotequote all
What I said above.

That pump is rated to 4-6 pounds per square inch of delivery pressure.
That is equivalent to a gravity feed with the tank more than EIGHT FEET above the carbs! (100inches)

Unless the carb has a valve, and seals, that will stand that pressrue you can expect some flooding and leakage.

JOhn