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MG MGB Technical - carbs

hi all what su carbs should a 1978 b have.
bob taylor

2x HIF4

Dave O'Neill 2

hi dave thanks for your reply.
i thought it should be them. iv just bought a 78 b with hs4 carbs on with no heatshield..will these carbs cause any problems and do i need the heatshield for this type. also i think it has a 4 branch manifold. iv got to pick car up tomorrow so will know more then.

thanks bob

1970 bgt
1978 b
bob taylor

Whether there is a problem is dependent on whether the dizzy is also '78+ (45D or 45DE). The later HIF equipped cars, had the vacuum advance operated from a take off on the inlet manifold. Their modus operandi was different. Ideally if you stick with the HS4's you would be better with the earlier 25D4 Dizzy with vacuum tube on rear carb. If you have the later 45D or "E" dizzy, blank off the vacuum take off on the carb and put the advance tube on the inlet manifold. Definitely you need a heat shield in place.
Allan Reeling

hi all.
picked up car today got it running nice.does anyone know what these carbs and inlet manifold are from. they are 1 3/4, they sit very high on engine just missing bonnet. standard air filters will just about fit with slight mods i think. the carb flange that fits to inlet manifold has 4 bolt fittins on each carb and float chambers seperate as on hs4s. no tags on float chambers but on side of carb body is AUD9. any ideas.

many thanks bob.
bob taylor

I think the Austin/Morris 1800S had twin 1&3/4s, not sure about the angle of the manifold, though.
Dave O'Neill 2

Picture would be useful Bob!
Allan Reeling

Those are SU HS6 carbs and may be too much carburetor for your engine. RAY
rjm RAY

heres a picture lads.

bob taylor

rjm RAY

HS 6's were used on the Land Crab and various Leyland BMC commercials, all with the 1800 engine, hence the mating manifold. Seems a bit over the top to have 1& 3/4" carbs on such applications, but there was probably a good reason. Even the 1500 Maxi used them!
Your head and engine might have been worked on and the larger choke carbs put on to flow more air at high revs, the only practical reason for upping the carb size on a B.
Allan Reeling

Fatigue had set in last night, it was the 1750 Maxi, and the commercials only used a single, but an irrelevent piece of info anyway!!!!!! The only reason I can think of for using oversize carbs on the 1800s and Maxi, might be restricted air flow because of position or type of filter.
Since the air a carb is flowing is dependent on the butterfly position and hence the height of the piston above the bridge, you can't flow "too much"! As long as suitable needles are selected they should work fine. If clearance is an issue, the manifold of the 1800 was probably shorter and "kicked up" to clear the bulkhead, you could always machine down the "dash pot" and shorten the damper.
Allan Reeling

hi allan.
the dashpots clear the bonnet just about but the standard mg air filters dont. will probably have to use pancake filters or ram pipes.? for the price of either of these i dont know wheather to get a second hand pair of the right carbs.
bob taylor

Bob, the HS6 are a desirable tuning modification. I'm sure you'll find someone on this site who'll be more than willing to swap yours for the "correct" ones.

They'll have your hand off!

Have to differ on the vacuum advance. There is no difference in the distributor and no need to change the distributor or the carbs. UK cars changed from carb source to manifold source in September 76, but the distributor remained the same. Yes the way the two take-off points cause vacuum advance to work differently, but it is only at idle - carb source gives no advance at idle, manifold source gives near maximum advance at idle for lower emissions. As you start to open the throttle the two sources give the same signal. Carb source *is* preferable in my view (it makes for easier starting), but either combination shouldn't cause a problem.

But you will definitely need a heat-shield as has been stated.
PaulH Solihull

anyone got a heat shield for hs6 carbs for a resonable price. thanks bob.

bob taylor


The tops of the float bowls should be parallel with the ground surface. That's the first thing that needs to happen.

Otherwise, you'll be fighting the float level (and the fuel level inside the jets and hence the lean/rich balance) forever.

It can easily be done by changing the float chamber adaptors. I think the Burlen/SU part nos. are AUD2676 and AUD2677.

Greg Van Hook

Sorry Paul, the dissy's are so different. The maximum mechanical advance is greater on R/B 45's (12deg and later 15deg, thats 24 and 30 at crank!) 25's had 10deg, and the curve is different..............just look at the centrifugal springs!! I believe also that the vacuum unit gives more vacuum advance (source Mr P Burgess). The two work in "harmony" together with the different characteristics caused by the upstream/downstream change. I definitely wouldn't mix.
Allan Reeling

Yes 25Ds are different to 45Ds in most cases. And yes a change in distributor curve may well give better overall performance with today's fuels, and especially a modified engine. But the vacuum characteristics of the two sources are identical off idle, see
PaulH Solihull

This thread was discussed between 06/06/2011 and 11/06/2011

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