SU Carburettor Conversion

[John Cornforth]

I recently removed the AC mechanical fuel pump from my Pearl for cleaning and overhaul.  Before I put it back, I measured the outlet pressure which was about 2 PSI.  This equates to a head of about 40 inches, considerably more than normal gravity feed.  

As I understand it, a low pressure SU electric pump gives about 1.5 PSI which is comparable.  Presumably SU carbs from the 1930's were designed for this sort of pressure.

[squeak]

http://s1173.photobucket.com/user/austin...2.jpg.html

Hopefully the above link will show what Dave refers to . The upside down early inlet manifold lowers the SU relative to the tank. 
My 28 roadster with scuttle tank and no pump runs OK on an SU mounted in a much higher position. It seems the Austin is very forgiving with less than optimal fuel pressure      cheers  Russell

[Austin in the Shed]

That's saved me putting up another photo,thanks squeak.There is about 5"between the bottom of the tank and the level in the float chamber.
I think that if you left the fuel pipe off the fuel would come out faster than you could use it through the carburettor on full throttle!

[Robert Foreman]

This is what I have used on the RN in the past before modifying a 22FZ that now goes like stink. Bronze prewar manifold with an 1" S.U. Direct feed with a flexible hose from Burlens, Gravity fed and never had any problems with starvation, I can empty the tank completely.

[JonE]

Robert - nice manifold. What sort of mods did you do to the FZ out of interest, and did you change or alter the original manifold? (not sure if that needs another thread!)

[Robert Foreman]

I breathed on the inlet manifold, fitted a larger choke tube and jets plus one or two secret mods. Still returns 35/40mpg. Plus an incorporated throttle return spring for trials and VSCC use.

[Bob Culver]

The topic of gravity feed to SU was covered at length some time ago. Someone must be able to find the posts. As I recall some found feed inadequate with a part fill tank. 
Incidentally the listed pressure for underbonnet mounted SU elctric pumps is generally less than for AC pumps.

[Stuart Giles]

Here's one I prepared earlier. It's a 33 onward manifold, I cut out the centre section of the inlet manifold and rotated it to suit this 1 1/8" semi downdraft  SU carb. I brazed the joints once I had pre-heated the manifold in the domestic oven (the oven was set to self clean temperature when SWMBO was out shopping) The joints never showed any signs of cracking in the couple of years this manifold was fitted. I did fit an SU pump for this carb.

[Robert Foreman]

Good work as ever Stuart. 
   Bob I'd just like to reiterate I ran our SU for about fifteen years purely on gravity feed with no issues whatsoever and as I said before I could empty the tank completely without any starvation.

[JonE]

I also had a report from someone in the A7OC (London) who had exhaustive tests with a car on various ramps to recreate the steepest hills. He found things ok with a sidedraft zenith and gravity feed, apparently.