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Pre-war SU Carb q?
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1935Hillman



Joined: 06 Apr 2010
Posts: 257
Location: Hampshire

PostPosted: Thu May 30, 2013 3:22 pm    Post subject: No damper SU's Reply with quote

I fitted an 1-1/4" downdraft SU to my Aero Minx when I got tired of having to mess with the old Stromberg every week to get it run right and with a bit of experimentation with needles got it to run very nicely with little effort. the car now starts easily and goes better than it ever did. However, on spying a complete 1-3/8" downdraft SU at Beaulieu I couldn't resist it and have rebuilt it with the assistance of a new gasket set and a lot of elbow grease. I am about to fit it to the Aero to see if I can achieve an even better performance. The holes on the bottom flange are unfortunately in a different orientation to the little one so a new adapter plate will be required to avoid having to make a hole in the side of the bonnet! The main point of my message is that I was equally surprised to find no damper, just a brass cap on the end which looks entirely original. It does have a spring however. I think these larger downdraft SU's were fitted in pairs to some of the larger engined prewar cars, possibly the odd Lagonda. It may mean a bit of messing about with needles although I will try the needle from the 1-1/4" one first as a starting point. Unless of course anyone out there has any bright ideas about needle selection???
All the best
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Bitumen Boy



Joined: 26 Jan 2012
Posts: 1733
Location: Above the snow line in old Monmouthshire

PostPosted: Thu May 30, 2013 6:46 pm    Post subject: Reply with quote

Well, you need a needle to use as a starting point, so you might as well try the one you've got. Pictures would be great, don't think I've ever seen one of the SU downdraft carbs Smile
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peter scott



Joined: 18 Dec 2007
Posts: 7113
Location: Edinburgh

PostPosted: Thu May 30, 2013 8:18 pm    Post subject: Reply with quote

Dampers are a post war thing.

You might find this excel spread sheet helpful.

www.clancy.ch/Library_files/SUSEARCH.XLS‎

Peter
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1939 SS Jaguar 2 1/2 litre saloon
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Roger-hatchy



Joined: 07 Dec 2007
Posts: 2135
Location: Tiptree, Essex

PostPosted: Thu May 30, 2013 11:12 pm    Post subject: Reply with quote

I have twin 1-1/4 MC2 SU carbs on the DLM as fitted to Aerial SQ4 and some BSA motorbikes

There are no Damper pistons, but there are springs, the early SU's relied on the weight, and fit, of the piston with no spring or Damper, just a drop of oil to keep things smooth is all thats needed.
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1935Hillman



Joined: 06 Apr 2010
Posts: 257
Location: Hampshire

PostPosted: Sun Jun 02, 2013 1:08 am    Post subject: Reply with quote

Changed the carb today. Bit of a business as I had to create an adaptor plate to get the body to sit the right way on the intake mainifold and keep the dashpot and float chamber clear of bonnet side, distributor etc. etc. Once done I fitted the existing CY needle which after a little fiddling about proved more than satisfactory. Running was smooth and acceleration good but improved steadily through the revs and I have never driven the car so fast before, reaching about 70mph on a nice section of road. A problem became apparent when I later spent some time 'tidying up' my handiwork. The piston sticks at rest and can only be freed by levering it through the downdraft chamber. Through trial and error I discovered that by loosening one of the dashpot fixing screws the piston became free suggesting that when tight the piston is somehow out of line and the needle is sticking in the jet. Not at all sure how o cure this one. It was going so well too!
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Roger-hatchy



Joined: 07 Dec 2007
Posts: 2135
Location: Tiptree, Essex

PostPosted: Sun Jun 02, 2013 6:21 am    Post subject: Reply with quote

Might be the jet needs centering.
Or, as you have found, the suction chamber will also need centering

Are these of any help

http://britishclassicmotors.com/3301.html

http://britishclassicmotors.com/media/b542a3b3e31aeb72ffff806affffd524.pdf

http://www.classiccargarage.tv/Resources/S.U.Carb.pdf
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1935Hillman



Joined: 06 Apr 2010
Posts: 257
Location: Hampshire

PostPosted: Sun Jun 02, 2013 11:26 am    Post subject: Reply with quote

I'll give them a read through and see what I can glean, thanks. The thing I have gathered with the early downdraft carbs is they are crudely simple with very little adjustment possible.
Thanks again, let you know how I get on
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1935Hillman



Joined: 06 Apr 2010
Posts: 257
Location: Hampshire

PostPosted: Sun Jun 02, 2013 12:44 pm    Post subject: Reply with quote

Jet centred and returning quite nicely now, thanks for the help everyone who chipped in
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peter scott



Joined: 18 Dec 2007
Posts: 7113
Location: Edinburgh

PostPosted: Sun Jun 02, 2013 10:23 pm    Post subject: Reply with quote

1935Hillman wrote:
I'll give them a read through and see what I can glean, thanks. The thing I have gathered with the early downdraft carbs is they are crudely simple with very little adjustment possible.
Thanks again, let you know how I get on


I don't think it's really true to say that downdraft SUs were crudely simple.

Ok, after jet centring there are only three simple adjustments:

Float level.
Offset relation of jet to jet needle.
Butterfly end stop (idle speed)

But there is a very large range of adjustment of fueling dependent on engine load by virtue of the needle profiles (which are carefully adjusted at multiple different levels.)

Peter



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Roger-hatchy



Joined: 07 Dec 2007
Posts: 2135
Location: Tiptree, Essex

PostPosted: Mon Jun 03, 2013 11:19 am    Post subject: Reply with quote

Even a standard set of needles per engine are different
There are 108 'Standard' needles for the Rover V8.

Also the piston / suction chamber springs are another factor in performance.

Not often realised with regards float/fuel level, this should be a constant throughout the whole rev range, even under load.
Not many bother cutting the engine at full revs and checking the fuel/float level, as it means part dismantling the carb/s.
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1935Hillman



Joined: 06 Apr 2010
Posts: 257
Location: Hampshire

PostPosted: Tue Jun 04, 2013 9:02 pm    Post subject: Reply with quote

OK, perhaps crudely was not an accurate adverb (?) to employ. I was making a comparison with more modern SU's.
Anyway all is good although a definite 'powerband' is evident and which I would love to know more about and perhaps even out a little. My problem as you all know is that the carb / engine combination is not original so needle selection etc. is quite simply trial and error.
Cheers,
Tim
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