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MG MGB GT V8 Factory Originals Technical - Factory V8 carburettors

A few years ago, when I had a 1.8 MG BGT, amongst other modifications I replaced the 'poppet valve' butterflies for plain ones in the SU carbs. This made a fair bit of difference to throttle response, and pull at the top end.

Does anyone know if the carbs on a '73 Factory V8 will also have poppet valves? I'm guessing they will, but I'm struggling a bit to see down the chambers! Has anyone else removed these?

Also, the car came with some K&N filters. I've not tried fitting them, as I've not got a pair of richer needles, but I'm guessing they make quite a difference. Are they really noisy, though? Are there any other problems with them (running in hot weather, for instance?)

I might wait until a few fingers grow back before attacking the carbs: I spent 6 hours (is this a record?) changing the gearbox mounts this weekend, and it was perhaps the most unpleasant job I've ever tackled on a car! I'm sure I did these on my old 1.8, but I don't remember it being quite so impossible...
Darren Ainsworth

Yup, mine have those decceleration valves

Cheers Jake!

Has anyone else removed them?
Darren Ainsworth

Darren - I threw the SUs away years ago but I am pretty sure the original factory HIF6s had plain butterflies.

The K&Ns should help a little with richer needles but both are marginal unless you have had your heads modified and a hotter cam fitted.

.....Sympathize over the gearbox mounts (worth doing) but wait until you have to replace the starter !



I have changed my butterly valves to plain ones. You can solder the poppet valves closed or get another plain pair.

Replacing the starter is 100 times easier with tubular headers!

Ian Buckley

mh never noticed them before, they might be there.. I have to remove front crossmember to get at engine mounts ... (it took a "specialist" 4 hours to put the engine mounting bolts back on)

Anyway this is child play
rover 2000 inboard rear discs
you need swivel joint fingers to work on them
one caliper leaked, took it off 4 hours - stipped it and fitted a rebuild kit - complicated caliper or what! 2 hours back on - yep still leaking - waiting (since start of Aug) for exchange units - cant wait... al least all the wings unbolt...
Derek Watson

From the factory service manual, the carbs came with the valve. You can swap them out with any SU 1 3/4" carb throttle plates ie. TR3,4, MGC, etc.

My 75 has them and were sticking open when warming up from below zero, giving an 'idle' of 2500 rpm. I soldered them shut being the quickest and simplest way to cure the problem and without introducing others of butterfly seating etc.

For the most difficult job on an MGB changing the roadster windscreen glass gets my vote, and the GT is bad enough.
Paul Hunt

My 75 V8 has them

Oh and my manifold and carbs are off of a '73 so I know they started life with the valves. If yours are missing, its the work of a PO.

Many thanks, I shall order some new butterflies!

The biggest problem as I saw it was that the poppet valves are a huge obstruction to the airflow at wide-open throttle. I haven't based this on any tests, but it seems like common sense when you see the size of the things.

It would be interesting to carry out a before and after dyno test, but that would be a bit expensive. I certainly noticed a difference with my 1.8.
Darren Ainsworth

The poppets can be broken off and removed then the openings soldered shut by laying the poppet against a piece of stainless or aluminum and soldering from the other side to get a smooth surface.
Jim Blackwood

This thread was discussed between 07/09/2004 and 14/09/2004

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