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MG MGB Technical - R 45D4 euro spec dizzys manifold vacuum?

Hi all, in my hunt for a new dizzy, seems that everyone has the euro 45D4 available but I need to be certain that it's manifold ported for vacuum and my twin HS4 conversion is off of an early 70's B and has a manifold port for vacuum.
So are these new 45D4's manifold ported? I'm particularly interested in a dizzy with a pertronix setup from British Parts Northwest(fairly close to Vancouver, decent rate on shipping if I cross over to Point Roberts to pick it up).

thx
Ron

Ron:

There is a 45D4 distributor designed for use on twin HIF equipped later UK MGBs. It uses manifold vacuum, but is not widely distributed in the US. This is not the same distributor that is on the general market as a replacement for the MGB. The Pertronix equipped distributor now available under the Flamethrower name is set up for use with ported vacuum only.

However, that said. If you have your centrifugal timing retarded a bit to prevent detonation under load, the ported vacuum advance unit seems to work really well when hooked up to manifold vacuum.

The vacuum advance pulls in at idle, improving operation as originally the engine was running a bit retarded at idle. Once you put your foot in, the distributor goes to centrifugal only until you hit cruise, then the vacuum unit again comes in and improves low load running efficiency.

Not exactly what the designers had planned, but it seems to work pretty well.

I hate it when you are taught one thing, then find out in practice that there are other variables at play.

kelvin.
KJ Dodd

Ron. The "Euro-Spec" distributors I have examined are the same dizzy that was used in the UK versions of the car from about 1976 onwards. Paul Hunt lists that distributor as being capable of being used on both the ported (off the carb) and manifold vacuum take offs. I have four of these distributors in current use, one off manifold vacuum and two off ported vacuum. I cannot detect any noticable differences between the two vacuum sources myself.

Les
Les Bengtson

Because of the never ending discussion, not to say argument, over carb and manifold vacuum and how compatible the two are I have just published some comparative tests on my web site, see http://www.mgb-stuff.org.uk/wn_ignitionframe.htm and click on 'Vacuum Advance - Carb vs Manifold'.

I fitted my UK 73 roadster with a vacuum solenoid 'changeover switch' with connections to both the carb and inlet manifold. A simple on/off switch enabled me to switch between the two vacuum sources, and a vacuum gauge Teed into the pipe from the solenoid to the distributor enabled me to see the signal at the distributor.

Basically, once the throttle is open just a few percent, i.e. anything more than a steady cruise at 30mph in 3rd gear, the two signals are identical. As far as the distributors go there is absolutely no difference in the vacuum advance *mechanism* between one originally fitted to an engine with carb vacuum and one fitted with manifold vacuum. The *curves* may vary, but then they vary between distributors fitted to carb vacuum engines, and those fitted to manifold vacuum engines anyway. When accelerating anything other than gently there is no vacuum signal left anyway, and hence no vacuum advance, it only comes into play during steady state cruising and gentle acceleration.

The final proof has to be that for UK cars the vacuum take-off changed from carb to manifold in September 76, but they retained the same engines and distributors throughout rubber bumper production. This change, like for North America in 1971, was purely an emissions measure. The full vacuum from the manifold at idle allows a smaller throttle opening to maintain a given idle speed, hence less fuel is used, hence less pollutants are emitted.
Paul Hunt 2

Ron, a good guy to talk to would be Jeff Schlemmer [jschlemmer1@mn.rr.com]He is working on building a business rebuilting Lucas distribors and is a regular here. He has helped me on a few occasions and knows what he's talking about.
Geoff Lund

Here is a new mail address for Jeff Jeff Schlemmer, Minnesota, USA, jeff@advanceddistributors.com give him a try
Geoff Lund

I was going to pipe in earlier, but Kelvin pretty well explained it. I've sold quite a few of the Flame Thrower distributors and now that they seem to have come up with a consistent curve (they changed them around for a while) results have improved with manifold vacuum. If that's the unit you're looking at buying, I highly recommend it!

To verify that you have one of the newer units, You can lift off the points plate held in by 2 screws and check the tiing plate. It should have a "12" stamped into it. The older units had a "10." I believe the part number is PE-D176600.
Jeff
Jeff Schlemmer

This thread was discussed between 29/09/2006 and 01/10/2006

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