SU conversion

[Peter Naulls]

Robin, Derek,

I endorse the suggestion of starting with a new jet.

For comparing needle profiles take a look at the excellent http://www.mintylamb.co.uk    If you want to get very exacting but don't have access to a rolling road (my nearest is miles away!) set up a camera pointing into the carb inlet to record what stage (height) your needle is sitting at for various revs. By using roadside adjustments to the mixture, this allows you to calculate quite accurately how rich or weak the needle is at each stage and choose a more appropriate one.

Peter.

[Robin Gooders]

Thank you again folks for all the advice. I will get out in the garage and start experimenting!

[Bob Culver]

From various spec sheets I have, SU seem to have avoided the term dashpot. However other books apply it to just the oil filled damper fitted to late 30s and subsequent SUs. The main body has the construction of a dashpot but does not primarily function as one.  The term is best avoided for carbs with solid guide rod and undamped pistons.
 
An attraction of the SU is that with most needles, part operating the start enrichment device richens the mixture in all situations.  On other carbs with strangler chokes this convenient test is compromised.
 
I stumbled across a print of Jeffs original article, Feb 2014. The lack of identity numbers on the carb bodies complicates identitification.

[Robin Gooders]

Hi, 

I originaly started this thread back in November when I was contemplating replacing my up-draft Zenith with an SU. After taking advice from this forum, and also finding some needle info on the old forum, I have just completed the job.

I'm just posting this now to thank those who offered their advice, and also to state what bits I ended up using incase anyone else is searching for the same information.

I managed to buy an SU 1 inch OM type carb on ebay. I know these are quite difficult to come by, but it was worth seeking one out as it made a nice neat compact installation with no flange adaptors etc. I used a standard late side-draught type combined inlet/ exhaust manifold, and an AN needle. (I tried a couple of other needles but the AN gave the best pick-up and mid-range acceleration).  I've road tested the car with a full scuttle tank and no pump, but I plan to fit a Moggy Minor type LP SU pump.

So thanks again folks for all your advice. This forum really is a great asset to 7 owners.


Robin in cold West Sussex.

[Ian Williams]

Thanks for updating us Robin, it is always nice to hear about positive outcome from a thread.

[Colin Swinbourne]

Thanks for the update.
This is useful and interesting, particularly the fuel pump part, as I have had problems with my 1929 Rover that has a scuttle tank and an SU that I have fitted in place of its original AMAL. 
I have been having problems with it running under any load, and wonder is an LP fuel pump may do the trick.

[GK5268]

Burlen supply the 1" OM body NEW, all the other component parts can be fitted new (they fit a number of carbs), dash pot & piston can be obtained secondhand (with two ear fixings) as one size fits a multitude of carb bodies. As Robin says, these are a straight fit without the need for any adapters, and a new body avoids any problems relating to distortion of the mounting flange, I have seen several oval shapes, which means that the butterfly will not operate properly!

http://sucarb.co.uk/om-carburettor-body.html

[Robin Gooders]

Thanks for the update.
This is useful and interesting, particularly the fuel pump part, as I have had problems with my 1929 Rover that has a scuttle tank and an SU that I have fitted in place of its original AMAL. 
I have been having problems with it running under any load, and wonder is an LP fuel pump may do the trick.

Hi Colin, I don't know the relative heights between the carb float bowl and the base of the tank on your car. But on the Austin, with a carb bolted straight onto the side-draught manifold, I believe a lack of pump is unlikely to cause that problem if you maintain a good head of petrol in the tank. The pump only being necessary when the tank is nearly empty and you are going up hill.  I'm going to fit a pump so it works well under these circumstances, but the car drives perfectly on a full tank with no pump, relying on gravity to keep the fuel filling the float bowl.

Rgds, Robin

[Colin Swinbourne]

Thanks Robin,
I think my problem is when the level is low...it was ok(ish) at low level but then I introduced an inline filter into the mix!
The first job though is to change the pipe work as it’s currently flexi that hangs down too low then has to climb back up to the carb, a straighter run with copper pipe will hopefully improve things.
Colin.

[Terry McGrath]

Those cheap plastic fuel filters do not work well with gravity feed, sometimes causing starvation.  They are designed for pressure systems.  My fabric racing special ran for 25 years on a Chummy tank, no pump and a Mini semi-downdraught SU which I converted to semi updraught by inverting the Nippy inlet manifold to increase the gravity-drop.  SUs will work in any direction as long as one adjusts the float bowl height.