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MG MGB Technical - dizzy connection to larger SUs

Hi All
I will be fitting 1 3/4" carbs to a 1950cc engine with flowed head and piper 270 cam. My question is whether to retain the vacuum advance from the distributor to the rear carb, as per the original setup. If so the carb does not have the connection and I will have to drill and rob one off an old 1 1/2" carb.
Alternative is to ditch the vacuum dizzy in favour of one normally used for a weber conversion i.e. no vacuum.

Many Thanks
Mark Dollimore

If you are doing all this work, to allow the engine to breathe better, you should also upgrade your distributor. You can send it out and have the mechanical advance curve set to match your engine's upgrades, or you can purchase a new distributor. However, having your old one rebuilt to the proper specifications will give you much better performance. Drilling out your carburetor should be avoided as the possibility of irreparable damage is high. The main function of the vacuum advance is to improve fuel mileage and lower emissions. It can be deleted with no loss of power. RAY
rjm RAY

Mark. Consider setting up the system to operate off of manifold vacuum. That will give you the benefit of better fuel economy, due to more advance when in at steady throttle. The strictly mechanical advance is good in racing engines, but does not provide the best fuel economy. Some form of vacuum advance does provide better fuel economy than the straight mechanical advance only type of distributor.

I should note that quite a number of vehicles ran the 1 3/4" SU carbs with a vacuum advance off of the rear carb. Thus, there were various HS-6 carbs made which do have the proper fitting to work with ported vacuum. But, the difference between ported vacuum (from the carb) and manifold vacuum is relatively slight. Aldon should be able to set you up with a proper distributor to work from either vacuum source.

Les Bengtson

Many thanks Les and Ray, I do have a modified dizzy (mechanical) and wondered about the merits of modifying the vac one. Think the latter is best as engine is not being used for competition.
Mark Dollimore

I too have the same engine Mods as yourself, Engine bored out to 1950cc which breath through HS6 Carbs, Piper BP270 Cam, Distributor is electronic Lucas DM4 with vacuum capsule connected to port on Carb,which was rebuilt for me by Aldon Engineering,
Even though my engine is set up to as near best that I can get,because of the larger intake volume of these Carbs, You will no doubt suffer with higher fuel consumption,and so vacuum advance is in fact your friend with this Mod, You have already been given some very good advice,and I can,t agree further
After going to all the trouble and expence with these Mods, don,t spoil it by using a substandard, old worn dizzy,there are plenty to choose from,and as already been said you can get one built to match your engine specs,do not under estimate the importance of having the correct one, It will make a big differance to power and ecconomy and general smooth running of the engine, Don,t forget your Ignition Timing will be totaly different to original,so be prepared to play around with this as this is also important for power and ecconomy,
I found the old tried and tested adjustment method of Advancing the timing until engine starts pinks,pinging, and then retard from that point until the pinging just stops,I have found my engine runs best for ecconomy, power,drivability,etc if I set my Timing at 16 degrees BTDC Strobe, although all engines are different,

Hope this is usefull to you, Malc
M Brady

And as an addition to all the good advice. Have you considered the latest addition to the 123 electronic ignition range as now offered by Peter Burgess, so that you could design your own curve, albeit on a rolling road. As has been said you have spent enough already go the whole hog and get it running perfectly. Mike
J.M. Doust


go for a D43 or D23 without the vac can. Ask Aldon whether they can help you with a 'Special Tuning' specs dizzy.
I got mine there some years ago and it works great on the modified engine.
BTW, I used HIF6 carbs from an SD1 on my B. Needles are BDR with RED spings.


R.S. Ralph Siebenhaar

I had good results with a 43D on a 1930cc (83.5mm)-engine with B271 Oselli cam, ported head, big valves, extractor manifold and HS6-carbs with K&N-filters, combined with a Lumenition Optronic = 106 bhp @ rear wheels on the rolling road of Cambridge Motorsports in the late 1980ies.

The engine revs to 5.500 in 5th gear (I have an LT77 fitted).

This thread was discussed between 19/02/2012 and 24/02/2012

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