Distributor Vacuum pipe to carb location

Broughton

Club Member
I have 280ZX running twin SU carbs, HFI 6 type, with standard 280ZX dizzy (E12-80 Module).

Need to know the best place at the carb/ manifold/ wherever to connect the dizzy vacuum pipe and have searched "tinternet" but now more confused than ever.

Some say the take off position can be upstream of the carb. butterfly or downstream dependant on whether the dizzy is vacuum advance or vacuum retard type ( I believe the 280ZX is vacuum advance type but please say if that's not so). It's also said the distance from the butterfly is important.

So, upstream or downstream the butterfly? And, just how far from the butterfly is best location for the pipe to sense the vacuum?

Presently the take off for the pipe in my set up is from top of the inlet manifold flange, about 1.5" downstream the butterfly spindle (1st photo).

I note both my carbs have a small hole in line with the lower edge of the butterfly (something to do with fuelling?) however one of the carbs (the rear one) has a much smaller additional hole alongside (2nd photo). This smaller hole seems to go through the carb body and is blanked off.
Could the smaller hole under the butterfly be an alternative location for the vacuum tube and if so is it better positioned here than the present position?

You will gather from this rambling that I'm quite bemused by the HFI 6 carb and can make little sense of the many holes I see in and around it.

Any help in locating the best take off point for the dizzy vacuum tube would be much appreciated.
 

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jonbills

Membership Secretary
Site Administrator
It wants ported vacuum, from just in front of the throttle. The carbs should have a nipple for it.
So that the distributor sees no vacuum at idle, then max vacuum on low throttle and progressively less as the throttle is opened.
 

Broughton

Club Member
It wants ported vacuum, from just in front of the throttle. The carbs should have a nipple for it.
So that the distributor sees no vacuum at idle, then max vacuum on low throttle and progressively less as the throttle is opened.
The carbs have no nipples. Only thing that might relate is the tiny hole visible to the left of a larger hole both in alignment with bottom edge of the butterfly (photo 2). I've no idea what the larger hole is for. Have you?
That tiny hole is currently blanked off but a vacuum connection could be made there. Then as the throttle opens the tiny hole would feel less and less vacuum (I think?).
Could a vacuum pipe connection to that tiny hole do the trick?
 

jonbills

Membership Secretary
Site Administrator
I've been reading around a bit and it seems HIF SUs don't have vacuum ports. People do drill and tap them as you suggest.
 

Broughton

Club Member
I've been reading around a bit and it seems HIF SUs don't have vacuum ports. People do drill and tap them as you suggest.
Thanks Jon. My HIF 6's have several orifices additional to the main jet in the bore of the body. I've researched for explanation of the various orifices but unsuccessfully. I suspect most/ some these orifices are to do with cleaner fuel emissions, which I'm only marginally interested in as I'm looking for a simplicity. If you come across an explanation for the detail operation of these carbs (rather than basic principles) I'd be grateful to know of it.
Cheers.
 
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