SU Carburettor

[Pearls not a Singer]

I'm currently working on a fellow members car fitted with an 1¼ SU carburettor.

There are symptoms of fuel starvation (engine under load loses power for a few seconds then picks up again). I know that the zenith 26VA runs on 1½ p.s.i. but I suspect that a  SU electric pump would supply with a higher pressure.

Is this  a common problem when using an SU carb? and if so what was the solution?

Thanks in anticipation of a helpful discussion

[jpsmit]

Hopefully not teaching granny to suck eggs but is there oil in the dashpot and if so what weight?

[John Cornforth]

Hi

It's not clear from your post if the SU setup is using a mechanical pump or an SU electric pump.

As far as I know, both the standard Austin Seven AC mechanical pump and the SU low pressure pump put out somewhere between 1.5 and 2.5 PSI which the SU HS2 will be happy with.

I know Austineers even gravity feed SUs, and with maybe 12 inches of head the pressure must only be about 0.3 PSI, but they seem to work.

Fuel starvation would normally show up when running at sustained high power, e.g. climbing a long steep hill in 3rd gear with foot to the boards. Assuming 15 BHP you will be consuming fuel at about 1 gallon per hour which should be well within the flow capabilities.

I suspect loss of power for a few seconds has another cause, e.g. temporary lean-out caused by the piston rising too quickly (undamped) as has been suggested.

[Chris KC]

Neither pump should have any problem keeping up with the carb. 
Check the dash pot oil at first - should be full of 3 in 1 or similar (I think 20 SAE is the official grade).

[Hugh Barnes]

Pop your finger in the throat of the carburettor. Lift the piston and release. It should fall with a satisfying 'clunk'

[Pearls not a Singer]

Thanks to all

The dashpot is full of oil and the piston is free. The car still has a mechanical AC pump which is why I wondered if incompatibility with the SU was causing lack of fuel delivery .

I will change the spark plugs before investigating the pump output.

[Peter Clayton]

Make sure the dash pot and needle are centralised and can rise and fall without binding.

[Robert Leigh]

It could be useful to know whether we are talking about an HS2 or an earlier carburettor, and is it newly fitted with the problem only showing since that fitting.

[Ian Williams]

More information is needed to answer this correctly. Besides dashpot oil, the piston weight and spring strength affect how quickly the piston responds to throttle openings. Set correctly this acts like temporary enrichment device such as the fuel pump in carbs like Webber's. A lag in response indicates a weak mixture on initial throttle opening, possibly due the afore mentioned items or it could be the wrong needle choice, or even a combination. I have in the past suggested reading a couple of books on SU tuning and set up that will explain everything you need to know if you can be bothered researching. I will reiterate past advice that there is no one size fits all solution with A7s as there are so many variables. You CAN NOT set the mixture correctly just with the jet adjustment nut unless you have the correct needle profile, and just because so and so said they use an AN or a No 5 or whatever dosen't meant that will be right for your engine unless everything else about the two engines and carbs is identical. The jet adjustment is only to set the idle mixture, everything above idle is set via needle profile. If you ignore this advice your mixture will always be a compromise at some point in the engines rev band and the cars performance will NEVER be optimal.

[Malcolm Parker]

My 1931 saloon is fitted with a 1 1/8" SU on a Ruby manifold with a direct gravity feed from the scuttle tank.  The head of pressure cannot be more than about 6" but it works very well even in hilly North Yorkshire.
I do not have any sort of filter in the fuel line but there is a fine gauze filter in the banjo inlet to the carb.   At one time I was having some problems with fuel starvation but this was cured by replacing  the needle valve with another from an SU.
When I had my first Ulsteroid it had an SU fed from the standard crankcase mounted SU fuel pump.   At sustained high revs (it had a pressure fed Reliant crank and rods) it would suffer from fuel starvation.