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Fitting an S.U. carburettor

Author: JRB

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It had been recommended to me that fitting an S.U. carburettor on my box saloon would give it a little bit more power. Power always being in short supply I thought I would give it a try. Searching on line I found a 1 1/8" throat size S.U. with a nice brass oil cap that looked the part so I bought it. It seemed to fit the bill but unfortunately it was semi down draft which I thought would look a bit modern and BMC mini-like. So then I found a true side draft one that was 1 1/4". But swapping parts around I ended up with a presentable 1 1/8" side draft with brass oil cap.

Fitting the S.U. in place of the Zenith is actually quite easy except for one thing, the manifold. The S.U. fixing holes did not line up with the Austin inlet manifold, so an adapter piece was necessary. To make this adapter a friend donated a large cylindrical piece of aluminium. To cut this block down to size I had either a hacksaw or a bandsaw. The hacksaw route seemed a bit rough and ready so I attempted to cut this roller shaped block by offering it up to the band saw hoping to cut a slice off. As I pushed it onto the blade it cut into the block and spun it up to a frightening speed, then it shot off the bandsaw table and out of sight. Once my nerve returned I decided a better bet would be to offer the block up to the saw blade clamped in a vice – to stop it spinning. This worked well with slow, very careful pressure.

Fitting an S.U. carburettor

I roughed out a block with just four cuts and then came the problem of drilling the big hole (1 1/8") for the inlet. Not having any metal working machinery I decided to drill a hole using a pillar drill and a Forstner bit that I had in a cheap set purchased for woodworking. I put a pilot hole through first, then gently applied the Forstner bit and to my delight it gently cut its way through leaving a nice clean hole.

Fitting an S.U. carburettor

I proceeded to drill the holes for the mounting bolts (with a twist drill) and thought 'things are going well'. I drilled and cut out the flanges using the band saw again – with care.

Fitting an S.U. carburettor

At this stage I attempted a rough fit of the manifold and found to my dismay that I had been working from the air-inlet side of the carburettor. It's fixings are at 90 degrees to ones on the other side so I had just wasted a lot of time and effort so it was back to the bandsaw. The second attempt was easier, I had now got the hang of things although the Forstener bit was a bit blunt by the time I had finished the second one.

One thing that did surprise me how the aluminium work hardened as I filed it into the final shape. I thought afterwards that maybe I should have heated it up to anneal it?

Fitting an S.U. carburettor

The second one fitted well, as shown in the photograph. The accelerator linkage just fitted straight onto the SU without any modification. The choke cable was very easy to adapt. This just left the fuel supply. I bought an inline filter and a length of fuel hose that, after cutting and clamping completed the job.

Note: I find the transparent fuel filter that I fitted very useful for viewing the fuel to see if any pump priming is needed.

Fitting an S.U. carburettor

I just left the needle that came with the SU (or one of them) in the carburettor and started the engine up. The first thing that surprised me was how easy it was to start the engine and then how smoothly it ticked over. I took it out for a spin and found that it behaved very well. There was not a great increase in power apparent but it did run smoothly and I feel that if the needle was optimised it could run even better. The more modern S.U.s have a spring that fits inside the piston chamber and a choice of springs is available to vary something or other. This carburettor has a weighted piston instead of the spring and so that is one thing less to adjust. One difference I have noticed with the S.U. is that engine speed pick up is slower than with the old Zenith, this maybe because the piston weight is too high, any suggestions on this matter would be appreciated.

The other aspect of the carburetion that I wanted to improve was air filtering. The tin lid of the Zenith is little more than a bumble bee deflecter. I spent a long time on the internet looking for some thing suitable to fettle into shape but in the end resorted to adapting something from the kitchen.

Fitting an S.U. carburettor

I am still running around with the same needle in the carburettor despite finding a few more in a drawer in my garage and trying them out but I would welcome any suggestions for selecting something better.

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