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MG MGB Technical - Carburettor options

Hey guys,

I am looking for some advice about which carburettors would be best to use on my GT.

I have a choice between a pair of HS6 and a pair of HIF4 carbs.

The engine they are going on has been bored to 84.5mm with O-Series pistons (about 2100cc I think), Peter Burgess big valve fast road head, Piper 285 cam, Peco bigbore exhaust and K&N filters.

The car is driven hard at times but is only taken over 6000rpm occasionally. Mostly it is driven on motorways.

Currently it has the HS6's on with BAP needles but it doesnt feel that great.

Any advice would be greatly appreciated.


J Miller

Ordinarily I'd say go for the HSs. The differences on the HIFs are primarily to do with reducing emissions and those changes make life more difficult with things like access to floats and valves, enrichment valves with O-rings and extra passageways, poppet valves in the butterflies etc. Can't comment regarding the mods though.
PaulH Solihull

Unless the engine is seeing use in the 6,000 and over rpm range, the HS6 carburetors are just too large for overall street use. You'll find it difficult to find the limits of the HIF4 carburetors on all but the most modified engines. They flow more than you'd think. I'm running a Moss supercharger that came with an HIF44 carburetor. The supercharger only comes into operation about %5 of the time when I drive the car and the HIF44, while being only a few mm larger than the HIF4, has more than enough velocity for my needs. The HS6 is simply too large, IMHO, and will result in poor performance under 4,500 rpms. The HIF44 has none of the emission components found on the HIF4. If you could locate a pair of these, it would be the perfect solution. RAY
rjm RAY

HS6 are the same size as a HIF44, except the HIF has the under mounted float etc that is more annoying to service/open to problems. Having a pair of them is no different to having twin HS6s...except potentially more annoying to set up and maintain.

If a supercharged car can get away with a single 1.75"/44mm carb, why would your N.A. engine need 2 of them?

Like has been said, the twin '4s' will flow a lot of air and the velocity increase you get from the reduced venturi size in the lower revs of street use makes them way more preferable to having large carbs that need a screaming engine to make use of their larger venturi size.

Personally, i'd try and swap the HIF4s for a nice set of HS4s with none of the poppet valves or wax stats etc and keep things simple and easily tunable.

Don't forget to put in radiused stubs inside the K&N filters to get the best air flow into the carbs you can. Use deep filters also.

Thanks for the comments. I think I will go back to using the HIF4's. The reason I tried the HS6's was to see it the much bigger engine capacity would make use of the extra flow but obviously it doesn't.

I have modified the HIF's in the past so they now have solid butterfies and thinned throttle spindles. They have also had the carb bodies smoothed (not the bridge though)and been matched to the phenloic spacers and manifold so there are no worries about a smooth gas flow. I am using the original radiused filter back plates with K&N filters to help with better gas flow aswell. The only downside to them now is the thermal compensator but I might try and machine one out of steel rather than the bi-metallic factory one.
J Miller

84.5mm on std stroke is 1996cc.
With properly modified heads, we have found good improvement from around 3000 RPM upwards with the twin HS6's, with no bottom end loss. If the head is standard then there is very little gain changing carburation and the money is better spent on a head job.
With the combination you are proposing, I'd expect a very useful and useable torque and power increase with HS6's properly set up and ignition curve to match.

"If a supercharged car can get away with a single 1.75"/44mm carb, why would your N.A. engine need 2 of them?" Simply because a supercharger has a much higher pressure differential. Without the big pressure difference to drive the air into the engine, focus has to be on reducing resistance - hence head job and bigger/twin carbs.
Paul Walbran

You have the displacement to take advantage of the HS6, but as Paul pointed out, only if your engine can generate the flow.

Using the HS6 on smaller displacements works fine on a race engine where you don't care much about bottom end but do about a few more BHP up top, but normally sucks on a street engine where 95% of your time is spent right in the area where you'd be giving away torque (i.e. moving the band up)
Bill Spohn

Ok, now I am properly confused as to which carbs to go with.

The engine should flow enough to take advantage of the bigger carbs as I am using a piper 285 cam and a modded head from one of the uk's best head builders (head spec can be found here

The car currently drives OK but the performance isn't as sparkling as I would expect as it just doesn't feel any better than when I was using a slightly modded head, weaker cam and HIF4's before I started the restoration 5 years ago. This might be due to incorrect needle selection in the HS6's but I have no starting point to go from so am taking a guess that BAP needles are about right.

I might put the modified HIF's back on and see what it is like with them before spending a fortune on different needles for the HS6's.

J Miller

The beauty of the HIF44 is that it has none of the emission components used on the HIF4. It's a set it and forget it design. I ran a Weber DCOE, with a ported and polished head, for ten years before installing the supercharger in '02. I have a Kent 280 camshaft and the performance, while not under boost, is way better than a pair of stock HS4 carburetors. RAY
rjm RAY

Your car has a Burgess head, so I would phone Peter and tell him your tale and ask for advice. There's no-one more qualified to help when it comes to MGB engines.
Mike Howlett

You description of how the car goes doesn't match the results of equivalent spec engine we've prepared. As Mike says, talk to Peter - and I would say invest in a rolling road session. RR usually gives really good BHP/$ (sorry, no pounds on this keyboard) and is for a modified engine is usually minor in cost compared the cost of all the mods.

Having said that, other things that influence results are compression ratio and cam timing - these are the first two things I would look at if such a car was presented to me, just to ensure that the waywardness isn't there. Probably not, but there's no point in chasing a problem via ignition advance rate and mixture correction if they're not where the fault is.
Paul Walbran

I donīt agree, that HS6-carbs donīt make sense on a bored out and tuned engine. My engine is bored to 83.5mm (1.930cc) with a ported big valve-head (AFAIK 44 mm inlet and 37 mm outlet), higher comp, an oselli B271 cam, twin HS6 on an original HS6-manifold from the BMC Special tuning dept.. Exhaust is a freeflow-manifold with a 2"-exhaust with single rear silencer.
Thatīs how the engine was built in 1982 y Brown&Gammons.
Power at the rear wheels is 106 hp, taken on the rolling road of Cambrigde Motorsports. With a CAEH714-cam we had 112 hp at the rear wheels.
The engine idles at 1.000 rpm, has power from 2.500 rpm, goes really well between 3.000 and 6.500.
Ignition is set a about 35° @ 2,500 rpm, the carbs have RR-needles.

IMO the problem are not the HS6s, but the setup. You will never get max power without setting it up on a rolling road.

I run this engine since 1982 or 83 with no problems except the usual wear and tear, and would never again go back to smaller carbs. And the best: it does 28 to 30 mpg, which is not bad :-)


Thanks for all the comments. I had no idea this would be such a divisive subject. Anyway, I have the booked a rolling road session at Peter's for the beginning of next month so all should be sorted soon.

J Miller


I can agree to the posts of Ray and Joern.

On my engine I use twin HIF6 carbs with BDR needles and red springs. Otherwise the Setup is very similar to the one Joern runs but with different crankshaft and other con rods.

R.S. Ralph Siebenhaar

This thread was discussed between 10/04/2013 and 13/04/2013

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