1.5" SUs

[brian]

I picked up my new engine from MED today. Very shiny and rather lovely. I will be able to run it with my (beautifully rebuilt) 1.25 SUs but 1.5s would be better suited I think.
Does anyone know of a good source? Someone changing to Webers, perhaps???
Brian

[Mknight702]

I have a pair of nice shiny 1.75" SUs. Did look at fitting one of them or a single 1.5" SU (both will handle the air flow nicely), the problem I found was finding a manifold that would fit under the bonnet without cutting a hole. You may be able to get one from Maniflow, but if you're going that far why not just get a Weber 45 DCOE, they make a wonderful noise! May even work out cheaper to source the single DCOE than twin 1.5" SU's, and I can recomend Peter Baldwin for a tune up. (I can let you know my settings for starters, Ebay Item 350185779771 is like mine)

[brian]

Thanks, Matthew.

I thought the 1.5" would be the same external size as the 1.25's. Obviously not... I think I'll get it going and run it on the 2 x 1.25" for now and think about a carb upgrade later.
Brian

[11admire]

When I get my new MED 1380XT, my plan is to fit twin HS4 (MGB 1973) on a 20 dgr Maniflow steel manifold.
The SU's will be tuned as David Wizard suggest. Filed down butterfly shafts and instead of epoxy fill before and after the ramp my plan is to use aluminium welding material. Tested OK on an old HS2 body.
To get the carburettors to fit under the hood I plan to saw 4/5 from the underside of the manifold tubes, bend down and weld, until they fit.
If carb's get too near the exhaust, I have several sets of the fiber distance peaces.

[Westfield 129]

You can order your manifold with any angle you want from Maniflow. I have done this on a couple of 5 and 6" manifolds for the 45 DCOE, and have no problems at all with clearance.

Order a "flat" manifold and skip the sawing and cutting. It's really hard to get that nice port match with all that home modification.

[11admire]

Hi 129 here is 52
How long a manifold at 0 drg will clear float chambers from the exhaust and filter from the frame ?

[Westfield 129]

I don't know. Here, a good set of 1.5" SUs runs about the same as a Weber. So I just went Italian.

I am sure that a little measuring will give you the answer you are looking for. It would help to have the carbs available when you do your measuring. You probably want the manifold to be a bit longish, and you don't want the air cleaners crashing into the frame. I have found that if the carbs stay below the level of the top of the valve cover, everything fits. I am sure that Maniflow can make you exactly what you need.

I have found that wrapping the exhaust header to keep the carbs cool is a sure way to make friends with your local welder. The Y pipe cracks. I have made an exhaust heat shield of alloy. It works quite well, and can be insulated for more heat mitigation if you wish. It does require one trip to the welder, though. I welded 2 studs to the center branch of my modified Maniflow large bore Mini header to hold the shield.

[11admire]

Have just ordered a 7.5 dgr 4.5 inch long manifold for my HS4's from Maniflow. Will be shipped to MED for final fitment to engine
Standard price for the 20 dgr. is £130 and a special one is £140. The make them with either 36mm or 38 mm hole at the cyl. head. I have ordered the 36mm

[11admire]

Manifold is at MED and they have polished it and mated it to the head inlet.
I got 2 sets of HS4 on ebay and have started the rebuild and modifying.
Next week-end I am going to the Copenhagen Historic Gran Prix and last year there was a swede who demonstrated aluminium welding.
I have been trying it myself, but I will try to get the swede to fill out the holes before and after the bridge with aluminium.

[Westfield 129]

You might consider doing a good rebuild of the stock carbs, with a proper set of needles to match your tune.

Those 1.5s do flow well in standard form, and you should consider that tuning with an unmodified stock carb might get you into the right neighborhood when it comes to the initial tuning. "Bridged" SUs don't idle well, and that might be a problem for the road, and you might loose your bottom end performance as a sacrifice to top end power.

By all means, match the manifold, narrow the throttle shafts and clip the throttle plate screws, make sure that the throttle shafts don't leak, but until you are used to your new engine, it is easier to work with a standard carb that has a predictable characteristics. Install a set of "stub stacks" to slightly improve flow inside your air cleaners.

Don't be a victim of "Too much science". Once you start welding and grinding on the inside of an SU, you have an experimental carburetor pair to sort out.

When you have the engine running well, and properly tuned, then decide if you really want to "bridge" the SUs, and likely kill the low end response and idle performance. I follow all of Vizard's recommendations, but carb mods are often unpredictable when it comes to gross changes in airflow. Especially if you are adding weld material.

I build my own engines, and do all the tuning as well. I always try to eliminate some variables, especially when it comes to the initial tuning.