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MG Midget and Sprite Technical - MKIII 1275 vaccuum advance

Its a MKIII 1275 RWA from 1971. It was imported in the 90's from the USA to Europe. All the emmision stuff is removed. It run's wit 2 AUD502 Carbs wich have 2 pipes in the middle wich attach to Y piece for crankshaft ventilation. I have a vacuum pipe on the intake manifold and this runs to the advance unit on the Dissy. Is this the right place?, doesn't the vacuum line has to be connected to the carb's?.
When running idle i already have a lot of advance from the vacuum, seem to do very little when i "hit" the trottle. Can someone explain this to/for me?
Niek Lammerts

Gez this is getting to be a popular topic... Exact same issue for a couple of other threads on another forum

It connects to a little christmas tree on the front carb on the far left side as your looking down... Very close to the black spacer blocks

Ill post a pic in a bit
Prop and the Blackhole Midget

Later 1275s had a 'ported' vacuum take-off from the carb body, approx in the plane of the throttle shaft. When the trhottle is closed, the port hole is positioned on the atmospheric side of the butterfly - so no vaccum to the carb.

As the throttle is opened, the butterfly sweeps past the port, so as the inlet vacuum lowers, it becomes available to the port.

Finally, at WOT, there is very little vacuum in the manifold / carb; but the speed of the air across the port will ensure a residual small vacuum.

Closing the throttle at this pointwill cause maximum vac in the inlet manifold, but none in the vacuum tube.

HTH

A
Anthony Cutler

Niek-
US cars at that time used manifold vacuum; earlier and other market cars used carb ported vacuum. Later UK cars changed to manifold at some point, but I know not when.

There is not a whole lot of difference between the two, but tuning, distributor mach and vac advance characteristics, and possibly carb mixtures will be somewhat different. If you have the OE distributor, the manifold vac should be OK if not optimal.

Much info on the MGB tech board/archive about this, and especially from Paul Hunt's "Pages of Bee and Vee" site.

FRM
FR Millmore

Isn't it a vacuum retard for emission control? Or was that not applied in MG's? Volvo has it I know.
Alex G Matla

Alex -
No vac retard on this car, nor any MG I think. Only possibility would be some 1500 Spridget, but I don't think so. Triumph and others did use it; some had both advance and retard.

FRM
FR Millmore

Ok, thanks F
Alex G Matla

This thread was discussed between 28/05/2012 and 29/05/2012

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