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MG MGB Technical - Down draft or side draft

What would be better for a 79 MGB a side draft or down draft. I have both
Jim

I was told that the down draft provided better fueling to the back to cylinders
Bob

Jim-
If it's Daily Driver reliability that you're after, then the Weber DGV downdraft carburetor is the way to go. However, don't look for performance with the Weber DGV. It's strictly an econo-carburetor. If you have performance ambitions, then the Weber DCOE sidedraft carburetor is the better of the two. However, if you choose the DCOE you'll have to use a centifugal-advance-only distributor.
Steve S.

There's a whole lot of information missing that would allow one to provide any meaningful answer. How about this...I'll make the following assumptions:

1. '79 B is otherwise stock, but has an earlier exhaust manifold available, as well as the rest of the exhaust system required.

2. Both Weber carbs have the appropriate intake manifolds available

3. You've managed to find an air cleaner assembly for the sidedraft that would clear your brake servo/master cylinder (hmm).

4. The "downdraft" is a Weber DGV 32/36 (I think I have that right)

5. The "sidedraft" is a Weber 45DCOE.

OK, with these assumptions in place (and you should consider them, especially with the sidedraft), you'll find the following:

1. The sidedraft is exceptionally hard to tune and far exceeds the requirements for any stock '79B, unless you've had cylinder head work, a very open exhaust, and larger bores w/HC pistons (I'm probably missing something here, but then again, these requirements fly in the face of the assumptions above...bottom line, the sidedraft will not likely work well for a stock '79B). Not a good choice without extensive complementary mods (read $$$).

2. The downdraft will be ok, and will work consistently after it's dialed in (which, as I understand it, isn't really terribly dificult). However, you'll likely find the carb somewhat anemic during the first 2/3 of the throttle (before the other jet kicks in), with a nice kick after you have the advantage of both jets. Some find this effect introduces a "flat spot" (the archives are filled with those who have evidently found a way around this effect). On the other hand, those converting from a ZS on a STOCK integrated manifold, tend to note significant improvement, due to the change in manifolds (rather than carbs, as is often described on this BBS). The long and short of it is that the DGV is not a performance carb, but is probably adequate for the less discriminating in the application assumed above. It is most certainly not the best flowing setup around, to say the least.

So, you have two carbs, with the DGV probably being a better fit, but neither being an optimal solution. I've implemented a very good single ZS solution (that probably breathes better than most folks' twin SU setup), but it involved a great deal of custom work and I'm now about to embark on a different intake setup (which I'll describe after everything's together). That being said, I've never heard a complaint about a well dialed in, properly tuned, twin SU setup. You might wish to consider selling both of these carbs and purchasing a set of HS4s (which you could probably pick up new, right out of the box for what you can get for these two carbs), a used intake manifold, linkage, and the conical filters intended to provide brake booster clearance. (HIF4s are more expensive, but do have some advantages.) I'll qualify the comment on the conical filters...I'm only recommending them because they're a commonly available (out of the box) solution for late model Bs. I'm switching to a modified set of twin SUs and will be going with a custom air box intended to clear the booster AND provide excellent breathing (upon examination, the conical filters were poor, at best). Until I get it all together and running, I will limit my comments on this setup for now. Hopefully, this will provide enough information to at least solicit the input necessary to obtain the answer you seek.

Have fun,

Barry
Barry Kindig

Thanks

I have 2 complete sets. Both have an intake manifold and I have a header. Both have all the linkage and air cleaners that fit. (Although the side draft is very close to the brake) This is going on a stock engine that was just rebuilt. Less than 100 miles on the engine. It has a mechanical advanced Mallory dual point dizzy.

The down draft only has about 50 miles on it the side draft has about 300 miles on it. Any idea what they are worth if I should decide to sell them.
Jim

Jim: The terms "econocarb" and "performance carb" are really quite meaningless. The Weber downdraft, sidedraft, and the SU's can be jetted to work well on a B engine in all stages of tune. Good authority says the Zenith can also (with intake/exhaust mods) but I have no experience with this.
For your stock engine the downdraft or dual SU's would be better. If your sidedraft is a 40 you can get it to work but with no advantage over the other choices and more setup difficulty. A 45 would be tough.
I would sell the sidedraft- a new one is $600-$700 so you should get about $400 or so. There is very little difference between the SU's and a Weber DGV downdraft. The Weber delivers a bit more HP but the SU's give a bit more torgue so low rev response is a little better. Little or no difference on timed runs between them. The Weber is easier to set up than SU's. Since you already have the DGV, try it out and I think you'll be satisfied. If it came with standard jetting (almost certainly did)all you have to do is install and set the mixture. Any flat spot that MAY appear can be fixed with an accelerator pump jet change. You will need to use a header or earlier model B exhaust manifold and twin downpipe (recommended)in place of your stock intake/exhaust manifold.

Terry

Thanks But I gave up and took it in to the shop. I told the mechanic to use whatever carburetor he thought would make it run the best. This is a British car shop with the exact same car in the show room so they should know what they are doing. I hope. IF I can I will install the other carburetor on my VW trike.

Thanks for all the ideas

Jim
Jim

Let us know how it turns out and ENJOY YOUR B!
Terry

Your downdraft will fit on your VW if it is the 32/36. I had several on several VWs over the years. I currently have a 79 B with a 32/36 that is missing a part. I don't know how to describe the part but it is square, small and on the side. Anyway, if I find the part, a bad carb to rob the part or a good used set of carbs, or any carb that will fit, I may have a complete B someday. In the meantime, I am robbing trim etc. for my 74 B. Good luck with whatever you do.
kids1

kids1: go to aptfast.com (Advanced Performance Technology) for parts diagrams.
Terry

This thread was discussed between 25/03/2002 and 28/03/2002

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