MGB & GT Forum

Hi All,

I am considering drilling a ported vacuum port into the rear HIF on a '72. Currently the distributor, already rebuilt by Jeff, is hooked to the original manifold vacuum port. I suppose size and location would be the important things to consider. Am I correct in that some SU's came with ported vacuum ports?

TIA,
Joe

Yes, at least some HS-4's had vac port.

Yep, the early model HS4s did, you want the vaccum port hole at the point in the carb where butterfly closes, on the HIF4 the throttle palte (butterlfy) is in the close postion opposite of the HS4, meaning the wider angle is at the bottom off the carb, and at the very bottom of the HIF at lets say the 12 o'clock position is already a drilliugn there, so it would need to be away from it a bit. The hole leading into the carb for the port needs to be pretty small say 1/16" of inch should be fine. I made one for a HIF and use the smallest section brake line I could find, drill my actual port size hole, then a shelf hole for the nipple, if you will to insert the tube into, then put a double flare on the end of the tube to make a hose barb. I'll try to post a picture of it, if I can find in 1000 or so pics I've got in the computer.

Now the one I pictured was done well back of where the butterfly closes, and at the time several forum members made mention of this, but in reality I don't think it matter much as long as it is at the butterfly closing position or behind the butterfly, as when you need the extra vaccum advance is needed is when the rpm rise (say by 3000 rpm all the advance will be in) and then the butterfly would be in open postion and vaccum would be created at that point. I don't know why the factory put it at butterfly closing position, some say so when the trottle isn't open, it's blocked off, but the butterfly is really never completely closed, it adjusted slightly open for idle.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com



Edited 1 time(s). Last edit at 2010-04-16 01:38 PM by Speedracer.

Another angle.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

The hole is already on the HIFs. Look at the bottom of the carb just below the butterfly. The brass plug can be carefully driven into the card throat and replaced with a short piece of metal brake line to hook a vac hose to. Save the piece you drove out in casy you want to go back to original setup later

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Lord, please give me patience because if you give me strength I will have to ask for bail money, too

if Jeff did the dist with manifold vacuum in mind, changing to ported vac will make the engine "run funny"...ALL vac advance will be in at idle and then decrease as you open the throttle instead of the other way around.

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1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed

That port leads out to a cavity cast into the carb body at the mounting flange area, for what reason I'm not quite sure of, it could also have another drilling into looking at the carb body that leads into the float chamber as well, if you look closely at HIF, you see what I mean. This may also be worht mentioning I see aftermarket spacers for Minis that have ports on the spacer itself.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

What's the advantage? Quicker advance response? More "accurate"?
Just curious.

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Rob.


"Now the problem you have is that when you have the unerring certainty of machinery, it is a machine. When something has foibles, it won’t handle properly, that gives it a particularly human quality because it makes mistakes. And that’s how you can build a relationship with a car that other people won’t get."
Jeremy Clarkson

i believe gerry is correct. if i remember correctly on hs4 carbs the hole is on the top side of the carb whereas on the hif's it needs to be on the bottom side.

EDIT: mac is also correct.



Edited 1 time(s). Last edit at 2010-04-16 03:57 PM by gooser.

I'm looking at this scenario for somebody elses car but here is how ours is set up. Since we are running a Weber on our car, which has ported vacuum, that is the way I had our distributor set up. I drilled and tapped a port into the manifold for a pcv, which I later deleted, and put a nipple there to see how the distributor worked with manifold vacuum instead. I asked Jeff about changing it around and he said that he set them both up the same way so it really didn't matter in this instance although ported is better.

I think you've got it flipped around, Mac. With manifold vacuum all the advance is in at an idle, high manifold vacuum, and drops off when the throttle is opened. With ported vacuum there is no advance at idle as there is no vacuum signal, but it advances as the throttle is opened as the increased air flow across the opening of the port creates a vacuum. (More airflow = more vacuum)

Joe

After driving out the brass "plug" (as Gerry suggested) on a spare HIF carb, I discovered the port does not lead in fact to the flange area, but the other direction, along a passage that leads to an opening next to the jet. Looking closer at the figures in my Haynes manual ( see page 80, fig. 3.21) I discovered that this is part of a "bypass idle system", presumably feeding fuel when the throttle disc is nearly closed. Ironically, Fig 3.23 on the same page of the manual shows what first appears to be a vaccuum connection at this same location, but is labeled "Auto Ignition Connection". If this brass fixture was to be replaced as a vacuum port, you would have the system active on only 1 of the two carbs. If I was to try Gerry's method, I fgure I would have to block off the bypass idle system on the other carb.

So Hap, your location seems to be the best location to place the ported vacuum. I assume if it didn't work, you would say so! Wish me luck!

Cheers,

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Greg Johnson
New Brunswick, CANADA
72 MGB

The other day, I was rebuilding a set of HIF4s, it had been awhile since anyone had sent me any HIF, HS4 and HS2 out ying-yang, but had not seen any HIF4s in awhile. So I looked hard at the area Gerry has talked about discovered the same thing you did, and then wondered about this very thread. It one of those deals where they had to have a couple places/drillings in the housing to get a galley to where it needed to be, a union of the two drillings if you were, you see that sort of thing throughout the engine block as well.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

I have tried numerous times to install a ported vacumn source on a HIF carb without sucess. Now I just use the mechanical advance in the distributor and don't bother with the vacumn.

The one I did worked fine.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

Jim: I did the same thing until I had my 45D rebuilt by Jeff last winter. He put a manifold advance on it, which was easy to hook into the intake manifold. Although the car seemed to work fine without the advance hooked up I find that having it makes a big difference when accelerating.