In the motorcycle world, having twin carbs aided acceleration, but generally didn't boost max bhp much, if at all.
And the jetting for twin carbs was the same as for a single carb - it just takes a breath more (or less) often.
The potential for carby icing with twin carbs would be greatly reduced, however.
Being closer to a source of warmth.

Remind us again, what is this to be used for ?
3 bhp is a miniscule amount, for a 350cc engine.
The revs would have to be curtailed, vastly, to hold it down to that. ?

First thing to say here is, I fall about in various stages of ignorance in even asking this! It may be a very poor idea, and if you know, then do tell.

In my experiences with the Douglas T35 (stationary engine version), it is clear that it was a grossly tuned down version of the Mk1, MK2, MK3. It used a AMAL 174 feeding a truly tiny manifold pipe to a shared pair. At best, it would be good for about 3HP. Of course, in this setup, the whole constant speed governor linkage was removed.

I do know that the T35 MK1, MK2, MK3 used left-hand and right-handed versions of the AMAL274, but here we get to that the engine is not on a motorcycle, but instead, it turns a propeller. I don't have such carburetors, and there were things in the motorcycle engines detail that were different anyway. Things like valve spring length and rate.

Suppose I just put two AMAL 174 carburetors on it (they do fit OK), and perhaps adjusted the jets if need be, and ran them via throttle cables on a common lever. Could such an arrangement actually run? Also, not having to run two cylinders through a shared 1/2" bore pipe, might one hope for a little bit more than 3HP? 

Bump

The motor is too big to fit into an EW350 frame. 
The gearbox for the 600 is a WG... and has a wider spacing underneath to mount on the frame. 
Yes the heads are detachable. 
Paul. 

Do we know if all the cams and gears are inside that (unbolted) timing cover ?

Will this bolt into an EW350  ?
Same gearbox ?

5 pics of the front, none of the back or sides or underneath !!
Do the 'heads' bolt on, they seem to be separate ?

Hi , can anyone help with replacement belt for my Duggie. Keeps breaking the leather straps evertyime I go for test ride, using an old spare belt but needs better quality one.

I need an 80+ crankcase, the front half anyway, the LH cylinder base bolts have pulled out and cracked the case. Judging by the welding already present, this has happened before. 

Or recommendation for a really good welder near Swindon!

Thanks

looking for spare parts list and manual for the 4-speed gearbox of my T6
Regards
     Manfred 

Thanks mate! Updated ad!

I am not a tunner so I cannot offer useful carburetor advice, but you might want to mention what carburetors you have fitted. Douglas used several types over the years. AMAC and track Amal to name but two. Or it could have any other make & model carburetor fitted in the intervening years. 

On the cinder track they originally ran a 50-50 mix of Pertol-Benzol mix. But the same engine as a road model ran happily on petrol, and in sprinting events they have run on petrol, methanol, and 10% nitro-methane (briefly!) It is a matter of having the right compression ratio, carburetor, timing, and ignition for the fuel you are using.

Douglas used the same cams (there were several) for cinder track racing as for the road models; though usually with a single carburetor on the road models.

With the anti-fouling pockets for the spark plugs, they tended to run a little more ignition advance than normal. My 1934 OW1 model (descendant of the DT) likes 40-35 degrees advance. If those pockets have been drilled out, the timing is going to be more in line with other marques on a given fuel. 

There is no source for the original style head sealing rings that I know of. Myself and others have made them using the method shown in this post:

https://www.douglasmotorcycles.net/index.php?msg=31133

-Doug