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MG Midget and Sprite Technical - Downdraught SU carbs? No Spridget content

I've built up a pre war car and have made a nice manifold system but need a downdraught SU for it. Anyone got one? Looks like a Frogeye carb but the float chamber sits at 90 dgrees to normal and the dashpot has an oiling plug screwed onto the side.

I've dropped myself in the poop since I can't shorten the manifold enough to keep the sidedraught H2 from touching the bonnet side panel.

Doh!

[IMG]http://i82.photobucket.com/albums/j279/foggythomas/chris5.jpg[/IMG]

r thomas

Rob,
Couldn't get your link to work. But it's on an Austin 7 special IIRC isn't it?
You probably don't want 2 of them, and even more probably don't want to pay this price!

http://www.ebay.co.uk/itm/NOS-D3-1-1-4-DOWNDRAFT-SU-CARBS-MANIFOLD-FILTERS-/121009491706?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item1c2cba4efa
Guy W

http://i82.photobucket.com/albums/j279/foggythomas/chris5.jpg
Nigel Atkins

I thought it was a Morris.
Dave O'Neill2

Yes, this one is a Morris (I have 8 cars at the moment)
I turned down a NOS downdraught for under £50 last year. I could kick myself!
r thomas

Could you not redesign the inlet manifold - side draught carb breathing across the engine, turns through 90 degrees to enter the top of the U-piece, rather than the side.
Dave O'Neill2

The dynamo sits on top of the engine (not in the picture) along with the ignition leads and some radiator tie rods so fitting the carb over the top won't reeeeeally work. There ought to be a few of those downdraught SUs about since quite a few cars used them before the war. Good suggestion, though.

I'll have to make the Mk2 manifold a bit tighter to the block so I can get an H1 in there. All part of the design-cockup-redesign process! Burnt fingertips, arc-eye, sooty nostrils. Aaaaaah, the pleasures of classic car ownership.




r thomas

Rob, looks great. Can't help on the carbs but I love your manifolds. Is it based on a normal pre war Minor chassis? Please throw up a few more pics!
Matt1275Bucks

Dave. You've got me thinking....is there any valid reason for not having an 18 inch long extension to an intake? I could curl the intake right over the rear corner of the engine via 2 90 degree bends and then use a standard-ish carb.

Are there any other engine tech types on here who would like to add some wisdom? I was thinking about icing and droplet puddling.




PS Some midget content! I've removed the 3 foot 6 inch axle and put a 4 foot Midget wire wheel axle in the Morris that I had in a Morris 8. Took bloody ages to file out the rounded bits. Standard 1932 Minor chassis but Morris 8 front axle, Wolseley brakes, MGPA body and screen.


r thomas

So, I've found a downdraught 1 1/4 SU and have mocked up flanges for both the downdraught and a long-intake sidedraught. Any tech reasons for selecting either option? Would a longer manifold be a problem???


Downdraught version...


r thomas

Sidedraught version...

(although obviously it would be trimmed down a bit. :o)


r thomas

I know it was my suggestion, but...I'm not sure what effect that length of pipe would have on throttle response.
Dave O'Neill2

Long manifold good! Bill Heynes lamented the fact that he could not put the SUs on the XK engines on a two foot pipe, no room. XKE pretty much accomplished that, when you include the filter box - and they don't work without it. You will note that nearly all modern cars have long induction tracts - for good low end running. Vizard tested long ram pipes for single carb cars, something around 30" total length to the valves; doesn't matter much where the carb is in the pipe. Icing not a problem, that happens at the throttle plate. Pipe ID should not be any bigger than the carb bore. Plus, you could always replace the pipe with a blower!

FRM
FR Millmore

Alternatively, a clever lad such as thee ought to able to modify an H series to have a remote float bowl with a banjo fitting on the carb, sort of like an Amal GT or TT carb.
Bit of fiddling can even restore most of the damper action without oil in the damper tube. (air restrictor - you could cheat and get a little industrial pneumatic flow control, adjustable for resistance going "up", but free "down". I have some that are brass, about 1 1/4" long, 5/8 W, 1/4" thick, with a needle valve to adjust).
Very period!

FRM
FR Millmore

This thread was discussed between 26/01/2013 and 02/02/2013

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