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1935Hillman
Joined: 06 Apr 2010 Posts: 257 Location: Hampshire
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Posted: Thu May 30, 2013 3:22 pm Post subject: No damper SU's |
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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
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Posted: Thu May 30, 2013 6:46 pm Post subject: |
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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 |
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peter scott
Joined: 18 Dec 2007 Posts: 7113 Location: Edinburgh
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Roger-hatchy
Joined: 07 Dec 2007 Posts: 2135 Location: Tiptree, Essex
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Posted: Thu May 30, 2013 11:12 pm Post subject: |
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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
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Posted: Sun Jun 02, 2013 1:08 am Post subject: |
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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
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1935Hillman
Joined: 06 Apr 2010 Posts: 257 Location: Hampshire
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Posted: Sun Jun 02, 2013 11:26 am Post subject: |
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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
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Posted: Sun Jun 02, 2013 12:44 pm Post subject: |
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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
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Posted: Sun Jun 02, 2013 10:23 pm Post subject: |
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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
_________________ http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon |
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Roger-hatchy
Joined: 07 Dec 2007 Posts: 2135 Location: Tiptree, Essex
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Posted: Mon Jun 03, 2013 11:19 am Post subject: |
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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
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Posted: Tue Jun 04, 2013 9:02 pm Post subject: |
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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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