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My Zenith to HIF 4 Conversion

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the-tripwire Avatar
the-tripwire Luke L
Cazenovia, NY, USA   USA
So here is the before and after of my HIF 4 rebuild and tune. I purchased the set of Carbs on a manifold from ebay for $100, parts for rebuild and tuning ran about another $250 in total. All in all the carbs I received were complete but badly needing some TLC. After some de-greasing I started polishing and honing the mated areas and ultrasonic cleaned the carb bodies. If any one out there has questions about rebuilding the HIF4 feel free to shoot me a message I have a bunch of detailed pics showing the process.

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riley1489 Avatar
riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
Have you removed the brake booster from your car? confused smiley

B



Check your ego Amigo!

the-tripwire Avatar
the-tripwire Luke L
Cazenovia, NY, USA   USA
Nope, Still have a brake booster. LOL good eye though, these K&Ns were included and will be going on a different car or back to ebay. I personally like the factory cans for reducing intake heat, but for this car I'm using the K&N cone with an "Cold air Pipe"

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the-tripwire Avatar
the-tripwire Luke L
Cazenovia, NY, USA   USA
Anyone out there like those Flat Pancake style? Seem a little restrictive IMO.

Ahmed Avatar
Ahmed Silver Member Ahmed EL Abasiry
Chestermere, AB, Canada   CAN
1958 MG MGA
1972 MG MGB
1979 MG MGB
In reply to # 3715414 by the-tripwire Anyone out there like those Flat Pancake style? Seem a little restrictive IMO.


They are restrictive as you said.
These are the ones you need:

https://www.amazon.ca/gp/product/B00062Z11U/



"... the motor car, after woman, is the most fragile and capricious thing on earth." - London Daily Mail 1908

jimandcassdavis Avatar
jimandcassdavis Silver Member Jim Davis
Swanton, OH, USA   USA
1976 MG MGB
I plan to do a similar conversion. I bought a fabricated exhaust header, then after some posts recommending the cast manifold I bought a earlier (two outlet) cast one on Ebay. Not sure which I plan to use, since I would also need to buy a downpipe for the cast iron one. ($$)

The HIF's I bought were complete but I have to replace the heat shield and add a manual choke cable to the dash.

So, how do you plan to run the cold air pipe? Is it just for bringing cool air into the vicinity of the air filters or will it be connected? What size pipe do you plan to run?

Looking at the specs for 1973 (HIF) vs 1975 (ZS), it looks like an increase of 20HP. But are the cams different, do you know? And what about the distributor vac advance and curve?

barry s Avatar
barry s Silver Member Barry Stoll
Alexandria, VA, USA   USA
1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
That appears to be an HS4 heat shield. For proper attachment of gas and choke cables, you need the bracket that attaches to manifold and top of heat shield. There may be a way to attach to that heat shield, but I'm not familiar with that mod.

Using the term "honing" when discussing SU carb rebuild makes me a bit nervous.

Spend time finding and studying tuning procedure. While straightforward, attention to detail is required.

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barry s Avatar
barry s Silver Member Barry Stoll
Alexandria, VA, USA   USA
1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
Sorry but simply swapping carbs will not provide 20HP increase.

Defender405 Avatar
Defender405 Silver Member Chris B.
Des Moines, IA, USA   USA
1975 MG MGB
the pancake style filters are a little restrictive but performance seems to be OK, so I guess that they flow enough. I have a 75 so the bigger brake booster requires the thin pancake style filters, even the KN cones won't give the necessary clearance. With the hotter street cam and the higher compression pistons the twin HIF4s make a huge difference HP wise. I didn't install them on the engine before I had it rebuilt so I can't give a apples to apples comparison, my engine was tired anyway so the SUs may not have helped bunches over the old ZS. Sure looks much prettier though.



Chris AKA "Defender405"
1975 MGB
1979 Porsche 924
1999 Porsche 996
1987 Nissan 300ZX

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riley1489 Avatar
riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
Luke,
A suggestion to make your life that much easier. Replace the fast idle screw you show, with the OE type screw and nut, on both carburetors The inverted screw head 'always' aligns with the cam.
As well once set this fast idle is generally 'ever' adjusted. cool smiley

B



Check your ego Amigo!


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jimandcassdavis Avatar
jimandcassdavis Silver Member Jim Davis
Swanton, OH, USA   USA
1976 MG MGB
Understood, as this would be a lot to expect. I thought The "derate" in HP was the result of adding all of the smog stuff. Along with the changes from HIF's to the single Zenith, what other internal changes happened? Thus my question re: cam and distributor changes.

Or, was the published HP reduction overstated by MG?

I have removed all the smog, and plan to go to the HIF's. A cam change to a fast road may be something I will consider in the future, and of course this would also benefit from a change to the distributor advance curve.

danthefitman Avatar
danthefitman Dan H
Portland, OR, USA   USA
1979 MG MGB MkIII "Simply, A Great B!"
I have a HS4 carbs, and I use the older Weigand half moon filters. Just press in the rear filter corner in a little bit and the brake booster easily clears without any restriction on the filters and the air intake.



1st Place Winner ABFM Portland Oregon 2008, 2010, 2011, 2012, 2013, 2014, 2015 & 2016!
Email me for questions or needs, I'll respond promptly! dan@allpointsorganized.com
Life. Positively in order.



Edited 2 time(s). Last edit at 2018-04-12 01:29 PM by danthefitman.


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awjensen467 Avatar
awjensen467 Adam Jensen
Tremonton, UT, USA   USA
1977 MG MGB
Here is my conversion. I finished a few weeks ago. As you can see I went with the K&N filters. They are a bit pricey but I like them a lot. It's like driving a new car.


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the-tripwire Avatar
the-tripwire Luke L
Cazenovia, NY, USA   USA
As far as choke cable and throttle cable, I’m using a proper bracket just have it off for paint. As of now looking to paint the intake manifold and then it’s time to get back to paint prep

the-tripwire Avatar
the-tripwire Luke L
Cazenovia, NY, USA   USA
Nice weather has finally arrived here in NY. Been out and about running the new carbs and love the difference. For now I am running 15 Degree advance on the factory 1980 CEI distributor with vacuum advance disconnected, but I think i will be sending it out to Advanced Distributors to be tuned to my specific setup. Still have a bunch of work to get the car where I would like it to be but it just passed NYS inspection and Ive put about 300 miles on all readysmileys with beer


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