SU Downdraft carburettor in place of Solex

David2021
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SU Downdraft carburettor in place of Solex

Post by David2021 » Wed Mar 30, 2022 11:47 am

I am thinking of replacing the Solex on my 12hp P2 1947 tourer with a D3 SU as used pre war. I have acquired a suitable Rover 10hp used SU (which has little apparent wear on the spindle) and also a brand new SU post war one for a Wolseley12. They are basically similar...A few significant differences though!
The throttle controls are "on the other side" ..easily sorted by turning the spindle round, The air cleaner has a 2 bolt flange not a plain fitting (not a problem) and the moving jet assembly is more complex externally with the later type return spring. The other big difference is that the pre war carburettor has no piston damper (round slotted brass damper cap). The parts diagram in the SU catalogue does not show it with a damper, nor does the Rover works manual) but the post war Wolseley carb certainly has one (hex brass cap with damper piston!) Was this a post-war development? Piston and suction chambers are identical numbers as are the float chambers to all intents and purposes (different top).The damper piston does not fit into the pre war piston.
I have a number of options:
1. Fit the 10 carb as it is.
2. Fit the 10 carb, but fit any interchangeable new parts from the Wolseley carb including the later Piston & suction chamber with its piston damper), replace the float chamber, but keep "tickler top". Maybe somehow fit the sprung pattern later jet...it seems to have a lot of levers & may be a step too far for me! Alternatively take and use the "Jet Bearing kit" parts from the new carb. I note that the 37-40 Ten and twelve used the same Jets, so no change there. The existing Dash choke control should work.
3. MAKE the Wolseley Carb fit as is (with a Rover spec needle) and the spindle changed round to meet the Rover throttle linkage & operate the right way & somehow make the Rover choke cable fit & operate the sprung jet lever.
I do like the idea of fitting a brand new carburettor...., but the 10hp one will fit pretty well straight on I believe as it was fitted to some 1946 cars... so option 2 is on balance my preferred route.
Any ideas out there?

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luli
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Re: SU Downdraft carburettor in place of Solex

Post by luli » Wed Mar 30, 2022 4:10 pm

I am about to get the rover D3, restored by Burlen. Hope to share impressions.
IMG_8383.jpg
IMG_8383.jpg (40.41 KiB) Viewed 3774 times
Last edited by luli on Fri Apr 01, 2022 7:59 am, edited 1 time in total.
Rover 10 1946 RHD
Rover 10 1947 LHD
Rover 12 1947 tourer LHD
http://lulisml.wordpress.com/

TonyG
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Re: SU Downdraft carburettor in place of Solex

Post by TonyG » Thu Mar 31, 2022 8:35 am

Hi David,

I’d suggest fitting either one or the other, rather than muddling the parts up. In fact, you could easily get both working and try to see which operates best with your car.

However, there are a couple of things you need to do. The 10 carb should be a straight fit for your 12 but you need to change the needle. If you look on the Burlen site I believe it lists the appropriate needles for different years of the Rover 12. Because your car has the later cylinder head and is, therefore, more powerful, you would be best going for the latest 12 needle listed I should think. Also, you will probably need a different spring . You can always call them for guidance and they are very helpful. It would also be worth getting a D3 service kit from them, which includes all the parts, seals and instructions to strip down and rebuild the carburettor. This will also ensure that the carburettor is equipped to handle unleaded fuel. They really are very simple to do and, having done it, will hold no mystery for you in the years to come. You will also need a can of carburettor cleaner, available from any motorists parts shop but be careful not to get it on any of your car paintwork.

Without examining it I cannot be totally certain that the Wolseley carb will work but, I agree, a new unit is an attractive option so well worth a try. Again, you will require the needle and spring best suited to the Rover 12 from Burlen, but swapping is a quick and easy job as you will discover. The Rover SU carbs are downdraft so the piston is horizontal and does not have an oil filled damper so this is not required. Whether or not it will work with it I don’t know. As you say, swapping the spindle around is an easy job but make sure the butterfly is fitted correctly so it closes properly. Burlen can advise if this carb requires seal changes etc to work with unleaded long term. The different air filter fitting means the Rover unit won’t be an easy fit but it does mean you can probably fit an aftermarket unit for an MG Midget or similar, which may help with performance and economy. I think Holden list a number of these but, again, Burlen may be able to help.

In the past I’ve also used Southern Carburettors for parts so they are worth trying as an option to Burlen.

I’ve seen posts before about SU carbs and float chambers; my experience is that the valve adjustment is not critical to the running of the car providing it opens and closes effectively and doesn’t overfill, thereby overflowing onto the engine and exhaust! It works like a domestic water tank or lavatory cistern which, as we all know, needs to be full enough to work but not too full that it overflows.

Hopefully, this will result in two carburettors that will work with your car. Always useful to have a good working spare as these units are getting harder and more expensive to find.

Hope that helps.

Tony.
Tony Gilbert

P1 12 Tourer
P2 12 6 Light Saloon
Discovery 3
Discovery Sport

David2021
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Re: SU Downdraft carburettor in place of Solex

Post by David2021 » Thu Mar 31, 2022 10:30 am

Luli....and that is BEFORE it goes foe overhaul....!?
Tony.
Wise words. One step at a time! I will fettle the 10 carb with a kit. Needles are the same for 10&12 from 1937-39 (according to my SU catalogue from Burlen) Weak #5.Std #CN & Rich #56 /AG for 1939. I will take advice from them...is the later (post war) head higher compression than the pre-war or with a modified chamber? I see the gaskets are the same.
The Wolseley certainly has a piston damper (as well as the top up oiler) I will ask Burlen if they know & if there is any advantage...
Meanwhile I will get back on the road with the Solex!
David

TonyG
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Re: SU Downdraft carburettor in place of Solex

Post by TonyG » Fri Apr 01, 2022 8:53 am

David,

The earlier cars had a different head with two inlet ports rather than the four that became standard 38/39, I believe. This made the 10 as powerful as the old 12 and the 12 as quick as the 14- if I remember the details from James Taylor’s book correctly. Both my cars have the early head so I know little about the later ones, but it sounds right that the later pre and post war ones would have the same gaskets. I have no idea regarding the compression ratio of the later design.

However, I do know that the early head was capable of modification to increase compression. My P1 Tourer head had a drastic 4mm taken off it as a ‘works’ modification, so I guess the later head would also allow some skimming but I have no personal knowledge of this being done. I seem to recall a post saying that the later 10 head can be used on the later 12 engine to increase performance (Mike M, was that you?). The skimmed head does present a couple of challenges on my car; specifically the need for shorter reach plugs, the water pump fouling on the support casting on the block plus the need for fresh petrol to give easy starting and smooth running. As such, it is doubtful that the mod is worthwhile now as our cars are rarely used in the tough competition environments this was aimed at pre War. And, of course, they had high octane fuel back then.

Good luck with the carbs. With the Wolseley unit you could just bolt it on, without modification, to see how the car runs and only bother with linkages etc if it seems ok. Nothing to lose. Let us know how you get on.

Tony.
Tony Gilbert

P1 12 Tourer
P2 12 6 Light Saloon
Discovery 3
Discovery Sport

David2021
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Re: SU Downdraft carburettor in place of Solex

Post by David2021 » Fri Apr 01, 2022 9:41 am

Will do!
priority at the moment is to get the tank back on (Fingers crossed no more leaks....) and the dash refitted, start up so I can move it round easily! Then the front lights/horns/bumper etc back on. Now has the trimmer lost the tonneau cover? Hours of harmless fun!
David

David2021
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Re: SU Downdraft carburettor in place of Solex

Post by David2021 » Fri Apr 01, 2022 9:47 am

Luli
Looking at your D3....I see it has the spring loaded jet assembly like my Wolseley carb. Do you have a piston damper on your piston?
Very helpful picture, it shows where levers & stuff should go!
David

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luli
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Re: SU Downdraft carburettor in place of Solex

Post by luli » Fri Apr 01, 2022 5:25 pm

Right now it is stuck at customs. As soon as I will get it I will be able to answer the questions.
Rover 10 1946 RHD
Rover 10 1947 LHD
Rover 12 1947 tourer LHD
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TonyG
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Re: SU Downdraft carburettor in place of Solex

Post by TonyG » Fri Apr 01, 2022 10:08 pm

David,

The big spring behind the jet adjustment nut is there to maintain pressure on the nut once the desired mixture has been determined. The black spring on the side of Luli’s carb is the choke return spring. The choke operating via the mixture knob on the dash and simply pulling the jet out a bit to enrich the mixture. In tandem with this is the throttle knob which increases engine revs by moving the brass lever that sits against the throttle stop, effectively opening the butterfly.

In case you were unaware, the SU carb is a clever device having a constantly variable Venturi (air way) and works by the accelerator opening the butterfly which, in turn, creates more vacuum thereby causing the piston to move. This opens the air way and moves the needle within the jet to allow in more fuel, making the engine rev more. As such, the shape of the needle has a critical effect on the progression of fuel delivery and explains why there are different numbered needles. Similarly, the tension of the piston spring determines the piston movement in relation to the vacuum that operates it. In reality it is a matter of trial and error to find the best combination of spring and needle. The jets are all the same size.

I doubt Luli’s D3 has an oil filled brass damper attached to the screw in end plate. Partly because Rover didn’t use them and because the orientation of the downdraft SU means that the oil wouldn’t remain in the damper. What puzzles me is that the Wolseley carb has one. I assume it is a D3 designed for downdraft operation? Very few cars used them this way around with side draft or angled being more common, as on an MG midget or Morris Minor. Both of these have oil dampers I believe. However, I did buy a pair of downdraft SUs at the NEC Classic Car Show a few years ago. No idea what car they originated from though and they had a different filter fixing arrangement like yours. Also they had a different number on the side cannot recall what it was. The Rover ones have something like 1492/2 on them. I bought them with a view to converting my 12 Saloon to twin carb operation but it runs so well that I have yet to do anything about it and I would need to fabricate inlet manifolds etc, which is more trouble than it’s worth really!

Tony.
Tony Gilbert

P1 12 Tourer
P2 12 6 Light Saloon
Discovery 3
Discovery Sport

David2021
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Re: SU Downdraft carburettor in place of Solex

Post by David2021 » Sat Apr 02, 2022 8:26 am

Thanks for the information Tony.
The piston damper is a bit of a mystery! The Wolseley carb. is an "SU factory rebuilt" and is absolutely brand new! Certainly all my previous cars with SU's (Frogeye, TR3A, ZA Magnette etc) were/are "normal" side-drafts with dampers. I cannot understand the Wolseley set up either! ....How much oil has to go in through the oiler on the Rover downdraft to keep it's suction working? How does one know? Can it be overfilled? If you fill it to the level of the piston it will be awash! All exciting questions!
I see in the SU booklet that there was a twin side-draft SU carb 9HV") set up on the Police specification Rover 10 in 1936...WOW!

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