SU fuel pipe orientation and route

[Howard Wright]

Hi All

A friend recently commented that the fuel pipe from gravity tank to the SU carburettor should be routed so that it enters the float chamber from the front of the car. He couldn’t tell me why although I respect his vast knowledge of Austin Sevens.

So I rerouted the pipe down under the carb and up to a banjo union.  I had to do this as there was no room to run it around the carb. Since then I’ve had problems with what seems like fuel starvation on steep hills and when I put my foot down. I think this may be down to possible vaporisation in the pipe.

Can anyone confirm my friend’s idea that the pipe should enter the carb from the front of the car and also confirm my suspicions that a loop down and up might cause vapourisation?

Cheers

Howard

[Steve Jones]

Yes and, most likely, yes. Your carb is gravity fed. To do it any other way works against gravity.

Steve

[Bluebird7]

Why not fit a 6V f/pump ????

[Howard Wright]

Thanks Steve

I’m still curious as to why the fuel pipe has to enter the float chamber from the front of the car?

I’ve rerouted the feed pipe as shown. 


  IMG_4402.jpeg (Size: 108.81 KB / Downloads: 240)

I’ll go out for a test run later this afternoon.

Cheers 

Howard

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Why not fit a 6V f/pump ????

Hi Bluebird,

I don’t have a 6 volt pump and I’m not keen on adding unnecessary complexity unless I have to.

If the problem persists I’ll consider the expense of a pump.

Cheers

Howard

[Reckless Rat]

Howard, that looks fine to me - your SU sits a lot lower than a standard side draught set-up so it should feed OK from the tank. I would rotate the top of the float chamber a tad anti-clockwise just to ease the apparent tension in the fuel pipe, but apart from that it should work fine. Fingers crossed!

[Steve Jones]

Yes, agree with Bruce. What I was meaning and, I suspect what your friend was referring to, is that the fuel should enter the carb via a float chamber to the front of the car. It is perfectly possible to turn it through 180 degrees using an appropriate float chamber and that would cause difficulties. Turn the lid through 90 degrees or so to make life easier on the pipe and all 'should' be well.

Steve

[Ruairidh Dunford]

Have you done a flow test on the fuel coming out of the tank, there is a lot of pipework/connections which may be causing lack of fuel, when most needed...

[Terrytuned]

Howard the float chamber at the feeds the jet when you accelerate if it was at the rear it may flood the jet when you brake very noticeable when racing. Jack the front up or stop on a steep hill and check the the fuel still has enough flow from the tank. Terry.

[Howard Wright]

Thanks Steve, Bruce and Ruaridh

We’ve done just over 20 miles in some hilly terrain this afternoon and all went fine. In fact the car drove reasonably well considering it has done less that 500 miles since the rebuilt engine and box were installed.

Thanks for your comments.

Cheers

Howard

PS Ruaridh, I haven’t done a flow test as such but copious amounts come out when I detached the float chamber and accidentally left the tap on!

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And Thanks Terry
Cheers

Howard

[Bob Olive]

I had this problem many years ago on my gravity-fed Ulsteroid.  Fitted a 6V S.U. pump and never had a fuel problem in the ensuing 25 year or so of very regular use (and abuse...).