Fuel pumps and carbs?

[Paul in NZ]

Just asking for someone else...

Say a bike like an 1100 Virago. Runs carbs BUT has a very odd system with a small aux gast tank low down and a pump to push fuel back up to the carbs. Now assuming somone modified the whole bike a lot and the new tank no longer needed and aux tank and was well above the carbs would the pump be needed and more importantly would you need to modify the carbs? (shut off valve in float chamber?)

Cheers

[Grumph]

Shouldn't be a problem. Gravity feed gives around 1.2 psi at the carb (measured many times) The yamaha and Honda pumps usually cut out around 1.5 psi. Unless the carbs he wants to use have an exceptionally small float needle and seat, flow should be sufficient.
Usual test - bowl off carb, fuel on, you want around 3 litres in 1 minute for road use on petrol.

[bogan]

My bike has high carbs and a pump, no issue with running pumpless until the fuel level gets below the carb level, so you should be right.

[Fergus]

Shouldn't be a problem. Gravity feed gives around 1.2 psi at the carb (measured many times)

I think your gauge needs calibrating...
1.2 psi equates to a static pressure head of over one metre, I can't think of many bikes where the tank is 1 metres above the carb...


(working for nerds:
1.2 psi = 8274 Pa
density of petrol = 750 kg/m^3
gravity = 9.81 m/s/s

pressure = density*gravity*height
P = pgh

h = P/(pg) =8274/(750*9.81) = 1.12 metres )

[carburator]

I think your gauge needs calibrating...
1.2 psi equates to a static pressure head of over one metre, I can't think of many bikes where the tank is 1 metres above the carb...


(working for nerds:
1.2 psi = 8274 Pa
density of petrol = 750 kg/m^3
gravity = 9.81 m/s/s

pressure = density*gravity*height
P = pgh

h = P/(pg) =8274/(750*9.81) = 1.12 metres )

Im not going to argue maths ( not my strong suit ) most bike carbs are low pressure but a high volume of flow.
most tend to be in the range of 3psi delivery pressure.

[ducatilover]

It's fine to run without a pump, it's not ram air and shouldn't suck more petrol than gravity can manage

Interesting math mentioned though

[Fergus]

Im not going to argue maths ( not my strong suit ) most bike carbs are low pressure but a high volume of flow.
most tend to be in the range of 3psi delivery pressure.

well my bike consumes 5 l/100km on the open road (@ 100km/hr) so 5 l/hr = 5l/60mins = 5l/3600seconds = 0.00139 l/second = 1.39 ml/sec Pretty low flow rate if you ask me. Of course that might double/triple/quadruple/whatever at high rpm and WOT but still..

To get 3 psi delivery pressure at the carb with gravity alone the tank must be 2.8 metres above the carb..

As Bogan said, you actually don't need any head/delivery pressure at all; The lowered pressure in the carb is enough to draw the few mls of gas a second required..

[jonbuoy]

I guess its possible the float valve might be smaller if it was designed to work with a low pressure lift pump.

[Grumph]

I think your gauge needs calibrating...
1.2 psi equates to a static pressure head of over one metre, I can't think of many bikes where the tank is 1 metres above the carb...


(working for nerds:
1.2 psi = 8274 Pa
density of petrol = 750 kg/m^3
gravity = 9.81 m/s/s

pressure = density*gravity*height
P = pgh

h = P/(pg) =8274/(750*9.81) = 1.12 metres )

I'm not going to argue with your math.
The background to my answer goes back to the early 1990's when I imported a 0 -5 psi gauge from the states specifically to set up pump fed carbs on Speedway TQ's and roadrace chairs as they were at that time a large part of my business. The gauge cost around $70 landed.

i have no doubt your education cost rather more but i'd point out that carbs set up using this gauge have been winning for a long time now...

[badlieutenant]

hey paul, the vfr tank is high enough that it can gravity feed the carbs, but still it has a low pressure pump. When the pump failed intermittently I by passed it. It worked fine for a while but after 10kms or so I would have full throttle, with a really loud bike and traveling progressively slower. If I pulled over, disconnected and then reconnected the fuel line I would be fine for another 10kms or so.
Weather I was starving the engine of fuel due to a slightly higher consumption rate vs flow rate or getting an airlock in the fuel line Im not sure.
I don't know about carb modification but by the sounds of it the carbs will get fuel so something will burn in the engine but maybe something like what happened to me will be your experience as well.