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MG MGB Technical - 1 3/4' SU Carbs (HS6)

Hi,

I have some 1 3/4" SU (HS6) carbs off some Triumph Dolomite Sprints [actually six carbs in all in two differing styles].

If I wanted to fit them onto my '72 BGT, what would I need?

1. A new inlet manifold housing - do I need new ones or can I adapt the HS4 types.

2. The throttle and choke set up seems wider than HS4's.

3. Anything else?

Ta

Iwan
Iwan Jones

Iwan-
You'll need a different intake manifold and air filter backplate, at a minimum. However, the real question is: Why do you want to do this? Due to its greater flow capacity (204 Cubic Feet per Minute), the bigger 1 3/4" SU
coupled with a stub stack (APT Part# SS-52) might make for a bit more power in a small bore engine (1868cc and smaller) at high engine speeds (above 6,000 RPM), but unless you're mounting them to meet the demands of either a Big Bore 1950cc engine with ported heads or a smaller bore engine with a Piper 285 camshaft and ported heads, due to their lower port velocity resulting from their larger venturis, you'll get it at the price of less power at the lower engine speeds below 2,500 RPM (which is where a street engine spends much of its operating life), a lumpy, vibrating idle, and difficult cold weather starting. On a standard displacement engine they will sacrifice as much power below 4,000 RPM as they will gain above that point.
Steve S.

While I cannot (an do not wish to dispute) Steve's expertise I can relate that on my HS6 carbs seem to work just fine on my engine. It's .60 over engine, with a Kent 717 cam, MSD 6al box and a Aldon Vac assist dizzy. The head has been worked and polished.

The engine starts even in the coldest weather (3 degrees F) idles smoothly at 850 rpm. Acceleration is smooth and powerful from 2000 rpm to redline (limited at 6k).

I am using 60wt oil in the dampers and today traveled over 100 miles on just over 3 gallons of premium fuel (93 octane).

The carbs have AUD 505 markings and were purchased new for just over $100 with the manifold - I couldn't resist.

rn
RN Lipow

RN-
No contradiction at all. Read it again- "unless you're mounting them to meet the demands of .... a smaller bore engine with a Piper 285 camshaft and ported heads" +.060 is an 1868cc engine, just big enough. The Aldon-modified distributor is just the thing.
Steve S.

RN- What other specs to your engine? CR, valves, etc. No doubt the large volume of 1868 and flowed head allow you to take advantage of the large carbs. I seem to recall you drove it for some time with HS-4's. What difference is there with this new set-up?
Tom
Tom L

Hi,

The block is bored to + 0.060 and has a fast road cam. The head is in the process of being ported & larger valves used.

What I really want to know is the difference in the linkages between the two carbs. What needles to use compared to the Triumph Dolomite Sprint set ups that I have. I don't suppose that you had that car in the States.

Thanks for your help.

Iwan
Iwan Jones

HS6 fixed std KP with KN TE
HS6 biased std BBW with KN BDR
source : how to power tune MGB 4 cyl eng Peter Burgess
michel

I'm with Steve: If you compare the ratio of engine size to flow capacity of the stock carbs with, the big hi performance 4 barrel carb on a hi performance contemporary 327 cu in chevy v8, you'll see that the stock MGB has large carbs for its displacement.
In racing where you are running a modified engine at top rpms the extra capacity can help at top end. But in a street car the 2500-4500 rpm band is where all the action is. The larger carbs will probably hurt rather than help. The SU variable venturi design gives amazing flexibility and allows use of an oversize carb, but it has its limits.
Barry
Barry Parkinson

Iwan-
The Original Equipment exhaust valve borders on being overlarge. This was a deliberate design feature made for the benefit of the factory race team so that there wouldn't be any problems with the homologation rules. With an 1868cc engine there's no point in going for an intake valve larger than the 1.625" version of the early 18V heads. Which camshaft have you elected to use?
Steve S.

Tom,

These are the first SUs...I had a DGV 32/36 - ugh carb.

the CR is 9.75 (or so): 1

The engine is extremely responsive - and this may be due to the engine work as much as the 9lb aluminium flywheel

rn
RN Lipow

Iwan,
With your current setup there is nothing to be gained from using HS6's, unless your are continually above 4000 rpm, at which point (approx) their increased flow rate will be of some benefit. What will happen however is that you will loose some of the magic flexibility that the B engine has at lower revs. I've done it and got the T shirt and am now back to HS4's (1868cc,BP270 cam, alloy inserts, 3 angle seats, etc). The car goes really well and has plenty of grunt from 1500 up, right where you need it, unless of course you are circuit racing.
Graham
Graham Cherry

Iwan,

My biggest concern was setting up the linkage, but once I had the cabs mounted to the engine I didn't have much of a problem troubleshooting it. My heat shield had a bracket built in for the throttle cable, you may have to fabricate or modify an existing one to line up with the throttle shaft linkage. My big problem was the choke- the linkage is set closer to the carb body than my previous carbs- I ran the cable through the hole in the front carb body but I have a slight interference problem with the air cleaner bolts (mine have to be through-bolted as someone reamed out the threads) but I've managed to solve most of that. I was replacing H4 carbs on an MGA, not HS4's so I can't give you a direct comparison. I do not know exactly which needles I am using. They came with the car and tune out fine, so I haven't bothered to check. I have an HS4 manifold bored out to match the carbs and of course velocity stacks and air cleaners to match the bores.

My engine (1800) is bored +.030, has a 9.5:1 compression ratio, and retains the standard cam and the valves have not been modified (other than being a lead-free head). I would say that I have noticed a minor drop in torque, but only in off-the-line acceleration. Acceleration through the gears is powerful and steady, and there is a marked improvement at high RPMs.

Hope that helps.
Mark
Mark

Whoops, I should clarify that the needles I'm using came with the CARBS, not with the CAR. Don't want to confuse things.

Mark
Mark

I have been running hs6's on my B,s for years and love em! I have built several motors with hs-4 and hs-6 carbs and there is a big diff in the 3,000 and up performance.
I am a bit more radical in my current setup than most would do for a street car, cross drilled and nitrated crank, corello rods, cosworth .040 pistons, Factory racing 300 camshaft with lightweight lifters, extensive head work with 1.5 roller rockers, with an aluminum flywheel, cintered bronze clutch disc and 4.5 gears out back.
Cory

This thread was discussed between 12/03/2004 and 18/03/2004

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