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SU Float Chambers
Following on from my previous posting about HS2 Carbs.: I have now got mine running really well, except that it has a tendency to stall after either a long downhill on trailing throttle, or a long uphill on full throttle and then stopping on the slope. It always settles to a steady idle when stopping on the level.
Maybe the float level needs checking again, but a more basic question is does it make any difference which way round the float hinge pivot is orientated? There are 3 options if you are not limited by pipe runs. My float chamber is in front of the carb., ie nearest the radiator and the pivot hinge is that side too. Is this correct?
SU Low Pressure petrol pump fitted on bulkhead, pulling from a rear tank through 1/4" copper pipe. All clean, not a dirty fuel/pipe problem for sure.
First thing I’d look at is if the piston is sticking. Unscrew the damper and then lift the piston with your finger and let it fall. If there isn’t a metallic thud as the piston closes, well there should be. Most likely then is the jet isn’t centred properly. An internet search will tell you how to put it right.
Alan Fairless
No, I tested that, all OK
Any other ideas anyone?
Bob is the HS2 a modern Mini type or an earlier type with the hole through the centre of the float?
 Sounds like the fuel level in the bowl could be the issue. Check fuel level by looking down the jet (level should be just below the bridge) with the dashpot and needle removed and/or set the float level according to the type of HS2 you have. I have also had issues with new replacement needle and seats both sticking and flooding.
Probably nothing to do with it, but all earlier SU discussions seemed to suggest the 1 1/4" is too large. 

Most recommended the 1" or 1 1/8" SU for good performance on an Austin 7.

The earlier models with weighted pistons, no springs and with damper pistons seem to work well, even with gravity feed.
Hi Bob

This thread interests me as I’m just fitting a 1 1/4 SU to the special.  I think the HS2 is the standard Carb of the mini era with a flexible pipe between the float chamber and the main body of the carb. I think the H type with the hole through the float was used from the late 30’s on.

I can’t see how the float lever orientation would affect the running of the carb but it does sound like fuel starvation is the problem. One thought is that the angle of the carb is set incorrectly on the manifold.  From Burlen’s catalogue there is a myriad of float connecting pieces for every conceivable float chamber angle.  The float chamber must be level in all directions.

Hi Tony, I’ve heard the same regarding the 1 1/4 but also reports of them working well.  I guess it depends on the state of tune of the engine?  I’m hoping that a sports inlet manifold, Ricardo head and bunch of bananas exhaust will provide suitable breathing requirements for the 1 1/4 H type I’m fitting? I’d be very interested in comments from those running the larger SU!


A friend in university days used to run a Rover 16. This has a down-draught SU. After prolonged full throttle driving the piston would stick up and the engine would stall as soon as the speed dropped. A quick loosen of the holding screws fixed it and all was well.
Maybe yours is sticking. I think another test is to remove the piston and its housing spin the piston holding it horizontal. If it's touching the side it can be heard.
Hi Howard how big do you want to go I have run H1, H2, HS2, H4, and HS4 in the search for more power mostly in semi updraught using an inverted 30degree alloy inlet manifold there is no float chamber adapter for this setup as the float chamber needs lowering for the correct fuel level. (see photos) at present using an H4 with a remote float.  Terry.

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OK, back in the workshop today:
I was happy that I had centred the jet properly, but having slept on it I thought that as Alan was so certain, I should check again! I have found that usually the piston dropped down perfectly, but maybe 1 in 4 times  it was sticking.  So I went through the centring procedure again and it is much improved. However I also found that a slightly out of true piston damper rod was also a contributing factor, so I swapped that and all appears good now.
The petrol level does seem correct, as far as I can see, nearly up to the bridge. The piston does drop into its housing ok when I removed and inverted it out of the car.
So thanks Alan and to the rest of you who gave your pennyworth too - what a great resource this forum is!
Hopefully dry enough for a decent test drive on Friday...........
No further news means good news!
Hi Terry

Thanks for this, gives me some reassurance ?.

I’ve just rebuilt the carb and set up the linkages. Float chamber dropped about 1 1/2”. Here’s a pic.  Access will be more limited when the exhaust manifold is fitted!



PS good to have positive feedback from you Bob

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