Replacing Stromberg carbs for SU

Hello every one, I have checked the forum and cant really find a definite answer.

I wander if any body can help me with a question, I have a Rover V8 90 running on very worn, leaking original Stromberg carbs. So I have just purchased a cracking set of SU carbs complete with linkages and manifold of a Rover SD1 car that have been overhauled.
My question is, the Strombergs have a fuel return pipe back to the fuel tank, but the SUs do not any outlet for a return pipe.
why is this? can I fit them or will fuel keep filling them up and then over flow, or will the electric pump stop when the carb is full.
Any advice would be great or has anybody else done this conversion as I want to crack on and fit the carbs.
many thanks Andy

I had this problem years ago but couldn't remember answer

The V8 Owners Forum - :: View topic - Fuel Return line

Never really thought about it since I got the 110 V8 but I know it has a return. I had SUs in the past (single on minis and twin on my midget) and they do have a float chamber that cuts the fuel pump.

I don't use one back to the tank - there is a float valve on SUs (HS6s anyway) which comes up when it the chamber fills and stops any more fuel getting in, but if the float valve sticks open it worth knowing so I leave the return either off or to a pipe that drops to the ground. Petrol in the pipe means I have a problem and out comes the hammer!

MattMatt said:

I don't use one back to the tank - there is a float valve on SUs (HS6s anyway) which comes up when it the chamber fills and stops any more fuel getting in, but if the float valve sticks open it worth knowing so I leave the return either off or to a pipe that drops to the ground. Petrol in the pipe means I have a problem and out comes the hammer!

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An expensive sticking-float-valve indicator indeed if it pours all that precious go go juice on the floor!

the su's do have a return to tank..atleast all mine have..inc lr110 and rrc's..its a little 6mm 1/4 pipe that returns to front of tank ..

Sure the SU carbs have a float valve but it is designed to work on very low pressure. So the fuel lines themselves need to have a restricted return pipe system. Similar or even the same as the strombergs should have.
The SUs have an overflow pipe system that will dump fuel if they are over fed fuel. This often happens off road when the floats bounce and even the 3 or 4 psi fuel pumps are too much without the restricted return pipe system.

Might I ask why you are changing carbs? People often think they have carb problems so change to SUs yet they never spend any money on a rolling road to get the right needles, jets and springs so they often end up worse than when they started! Strombergs are even more accurate than SUs. Knackered camshafts and ignition problems are likely to be the culprit rather than strombergs with good diaphrams.

Strombergs rely on a rubber diaphram, SU's rely on engineering tollerance, I'd choose SU's every time.

You hit the nail on the head there mate. When you replace the diaphram you have a brand new carb again! SUs have the edge on tunability and needle selection though. Shame no one ever fits the right needle for their engine though!

spitfiremk1uk said:

Shame no one ever fits the right needle for their engine though!

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Should be a book on that somewhere - I had to do loads of research and testing to get anywhere near for SUs on my 3.9. There was a little program by Scot Bevis called WinSU but I don't think he's upgraded it to run on Win8