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MG MGB Technical - Distributor Vac with HIF4 Carbs

Hi,
I took off my HS4 carbs to install a set of HIF4 carbs on the 18V engine in my MGA. The little vac tube from the distributor used to run to a port on the rear HS4 carb. Where is the best place to connect the vac tube now?

thanks,
Tyler

Tyler Irwin

Which distributor are you using? The two B dissy's were different. The later 45D used manifold vacuum, the earlier 25D used carb vacuum. The delivery of the vacuum to the diaphragm is slightly different, not sure how interchangeable they are. Your manifold would need a take off in the blanked (bolted) hole just above the end of the tube.
Allan Reeling

thanks Allen,
It' a 45D that's been recurved by Alden. I tapped a barbed hose fitting into the intake manifold. Just wanted to make sure I didn't miss something.
thanks,
Tyler
Tyler Irwin

Tyler,
If you look on Paul Hunt's site, I seem to remember he has a section on the vacuum take offs and differences therein!
Allan Reeling

Vacuum take-off point did not depend on distributor but on year. North American spec changed in 1971 when they were still using 25D4s. The UK didn't change until September 76 for the 77 model year and by that time they had been using 45D4s since rubber bumpers.

It was an emissions measure, by connecting to the manifold instead of the carb you get near full vacuum advance at idle. The effect of that is to increase the idle revs for a given throttle opening, and when the idle screws are turned down to compensate for that you are effectively using less fuel and hence emitting less emissions.

If the HIFs don't have a vacuum port (usually underneath, unlike HSs) then short of drilling a hole for a port which needs great care you will have to fit a port to the inlet manifold *(which you seem to have done). You could also do without vacuum advance altogether, but as it improves cruising economy and part throttle acceleration it is worth having.

paulh4

It will be very difficult to drill a port because it must enter the throat such that it is not exposed to vacuum (throttle plate edge upstream of the port) when the engine is at idle but exposed to vacuum when off idle (throttle plate edge downstream of the port). The trick for the machinist will be to locate the exact point in the throat at which the port comes out to meet this requirement.
Richard Smith 1

This thread was discussed between 15/05/2016 and 01/06/2016

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