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MG MGA - 1/8 in vac nipple

I need to drill a small hole in my SU HIF44 carb. ( On a Judson ) and attach a 1/8in ( 3mm) hose nipple as vacuum advance connection for the distributor . Can anyone recommend a fitting for this?
Neil Ferguson

The real fitting is just a piece of 1/8 brass tube pressed in. You could use a 10-32 screw stub with end turned smooth.
It is critical that the very small through hole be precisely placed relative to the throttle plate; I will not attempt to describe this - find a carb that has it and copy.
It is easier and probably more reasonable to use manifold vacuum. Either case requires a suitable mech advance curve and vacuum capsule to be figured out, or you are wasting your time or worse.
You might want to search the MGB archive for the endless discussions about what each method of vac advance means and does. I think Paul Hunt covers it pretty well on his site.
Until you understand it thoroughly, just run without.

FR Millmore

FRM....will progress the way you describe but it is most likely in the end I will put a nipple on the carb. and my diet used a similar location on the standard 1 1/4in carb... I noted on the Moss Supercharger installation pics that the nipple had a hose spur on the end so was wondering where i could get such . I also note that the HIF44 body ,in the vicinity of where it will go ( just upstream of the butterfly and offset rom the top ), is fairly thin wall so not much material to press an interference fit through...
Neil Ferguson

Yes some have the pecker head, but many are straight tube, like the later distributors are.

I've never seen an HIF44, nothing came with them here, but usually carbs have a boss somewhere for the vac tapping. Some H*6/8 have a flat section, which allowed the through hole to be drilled, and an adaptor plate screwed on with two screws, which had a channel that offset the vac passage. The HIF44 appears to be a very short body, perhaps the vac tapping goes in the mounting flange, possibly at an angle. A closeup of that area could help me decide.

"standard 1 1/4in carb" On what, not MGA?? MGA H4 uses a bottom vac tap, on the rear carb, Functionally the same, but causes fuel vapours/droplets to migrate to the dist diaphragm, hence the little separator can in the pipe.

FR Millmore


On some HIF carbs, the vac take-off located just after the butterfly - that's between the butterfly and the carb to manifold mounting flange - has been eliminated.

Instead, the engines these carbs are fitted to use a vac take off in the manifold. The reason for this is again wholly idle emissions orientated. The higher vacuum that exists in the manifold area can be used to pull more ignition advance. Theoretically this will reduce emissions at idle. Unfortunately it's exposed to induction pulsing. In the A-series, this is UGLY. The pulsing and high manifold vacuum causes rough idling. Particularly where anything 'sportier' (ANYTHING) than a bog-stock-standard cam profile is used.

Classic example - the MG Metro. Its adventurously sporty cam makes for a slightly lumpy idle. This increases idle speed emissions, so Rover use manifold-sourced vacuum to increase idle speed ignition timing via the vac advance. We're talking 28 degrees of ignition timing here! Doesn't work because the pulsing created by the cam causes erratic ignition timing, culminating in running on. So an anti-run-on valve was fitted as a modification. Vicious circle. After messing about trying to solve the emission problems this caused, it was found that changing the originally quoted valve clearances to 0.014"inlet /0.017" exhaust (effectively reducing the cam over-lap) helped dramatically. And, like I said earlier, the sportier the profile, the worse it becomes. Even worse - when running, this system can cause ignition advances in the order of 50 degrees on lift-off and on again - an absolute sure-fire recipe for savage detonation.

The solution is easy - remove the vac take-off from the manifold and fit it back to just in front of the butterfly where it should be. If you need to drill a port to accept the vac pipe take-off it needs to be sited 9/16" back from the carb to manifold mounting face, offset slightly to one side. I.e. NOT at the dead centre, or 'peak' of the butterfly bore. And usually set off away from the crankcase breather port. Usually the vac pipe take-offs fit a 1/8" diameter hole - but measure whatever you're going to use before drilling. You're looking for an interference/air tight fit.

I can send you a close up photo of the mounting tonight

dominic clancy

FRM...I was amused by your pecker head description...the expression in UK/Oz was usually 'bobby's helmet'. The Hif44 is the euorpe equiv of the HIF6 I recollect.

Dominic....Thanks for advice and would appreciate the pic....
Neil Ferguson

Funny how everybody knows exactly what is meant, despite linguistic variables! We here cannot use "Bobby's" for anything, not having any similar headgear, excepting on motorbike coppers, themselves pretty scarce.

"The Hif44 is the euorpe equiv of the HIF6 I recollect."
Yes, but it is not the same, said he, not having seen an HIF6 either, but taking it to be more like the HIF4, well known here. I always wondered why there were HIF4/6, and HIF38/44, if they were the same.

FR Millmore

HIF 4 & HIF 38 are the same 1.5 inch carb, the 4 has imperial threads, the 38 metric.

The HIF6 and HIF44 are a larger version of the carb at 1.75 inch, again with imperial and metric threads.

There appear to be versions of the carbs with different throttle and choke assemblies, but the same core. The two HIF44s I have are the same in the body but show completely different arrangements for these assemblies.

I have scanned the workshop manual for the HIF44 if anyone needs the file - diagrams, parts list and setup details for single and dual setups
dominic clancy

This thread was discussed on 27/04/2012

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