Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB GT V8 Factory Originals Technical - Choice of carb

I have a 3.5 Rover SD1 with an edelbrock carb. I was told by my garage that switching to twin SUs would improve acceleration maybe at the expense of top end speed. Any thoughts?
Chris Holmes

I would have expected the SU's to give perhaps slowers acceleration but generally better fuel economy.
The reason being the SU has no pump jets arrangement to add extra fuel when the pedal is pressed quickly hence the better economy but slightly slower pick up.
Peter Thomas

Peter, your reasoning is correct, but don't forget that any Edelbrock carb is normally way to big for a 3.5. If you have anything like a Weber/Edelbrock 500, you're really talking 500 cfm flow whereas your engine produces something like half that figure (prob around 280 cfm max). So there might well be a problem with pick-up. The air doesn't really speed up through the chokes. What state of tune is your engine in, Chris?

A normal 3.5 engine is very good with twin SU HIF 6'es. If you don't like the bonnet bulge that comes with the SD1 setup, and you don't like the original V8 setup, then there really are two ways to go: smaller carb (like Holley 390, which is still too big) or EFI. Smaller carb or EFI not only enhances engine pick-up, but also aid in keeping your engine relatively fuel-efficient, while at the same time retaining a good top-end bhp and nice mid-range torque. Big carbs do not make sense for a 3.5 touring car.

Frank de Groot

Peter Thomas wrote: "The reason being the SU has no pump jets arrangement to add extra fuel when the pedal is pressed quickly hence the better economy but slightly slower pick up."

Peter, this is not true, the SUs do indeed have such a feature: it's called a "dashpot." While it's not, strickly speaking, a "pump" jet, it serves the same function, adding that extra shot of fuel when pedal is pressed quickly.

The extra shot of fuel when the pedal is pressed quickly is not a just a feature of the carb - it's a requirement of the engine, any engine, and must be satisfied regardless of the type of carb used.

For more on this, see:
Dan Masters

The dashpot reduces the amount of air to the engine which, with the jet controlled by the throttle possition, richens the mixture when accelerating. No fuel being pumped in as with an accelerator pump. You get better economy at somewhat reduced performance, as Peter said.
George B.

George B.

Creating a richer mixture or pumping fuel in - what's the difference? If the dashpot doesn't add enough extra fuel to provide the necessary richer mixture, the engine will stumble; if the pump jet inserts more fuel than is needed, the engine will stumble. The amount of enrichment needed is a function of the engine, not of the carb. Either type of carb must provide just the right amount of fuel to ensure smooth acceleration. If the Edlebrock carb is dumping in too much fuel, then the accerator pump needs to be recalibrated for that particular engine. If the SUs aren't providing enough fuel, then they need to be recalibrated for that particular engine.

BTW, the jet is not controlled by the throttle position, at least not directly - it is controlled by engine vacuum. Unlike the accerator pump, there is no mechanical linkage between the dashpot and the throttle.

Nor does the dashpot reduce the airflow to the engine. Quite the contrary, it is the additional vacuum created by the extra airflow through the carb before the dashpot can rise that pulls the extra fuel through the jet to provide the fuel enrichment during acceleration. That's the whole principle of operation of "constant depression" carbs, such as the SUs.
Dan Masters


The additional vacuum is created by the throttle plate being open and the dashpot restricting the volume of air. The dashpot restricts the air volume until engine revs come up, governed mostly by the dashpot lag.

You are correct about the throttle having no direct connection to the jets. I guess I was thinking about the "choke", which enrichens by moving the jet down. The dashpot controls the needle.
George B.


I think we are both saying the same thing. At the instant the throttle plate is snapped open, the dahspot is not allowed to move up instantaneously by the damping action of the dashpot oil. Until the dashpot does move, there must be additional air flow to create the additional vacuum. The dashpot does reduce the air flow as compared to what it would be if the dashpot were allowed to move up freely.

All in all, a pretty clever design, don't you think?
Dan Masters

A nice discussion on design of carbs, and SU's are pretty clever. But what would you do on a 3.5? Edelbrock, SU HIF6? What would be best, given the initial question?

Frank de Groot

Thanks for the comments. Apart from the interesting digression to the particulars of SU's, can I conclude that my edelbrock is probably too big for the 3.5 (my car does have poor fuel economy) and that switching to SU HIF 6'es would provide better mid-range torque, better economy but maybe lose out in acceleration? My engine is just about to get a full tuning at the garage hence why this question about taking the opportunity to switch carbs came up.

Thanks, Chris.
Chris Holmes

You've drawn the right conclusion IMHO, Chris: get rid of the Edelbrock and switch to HIF6'es. Or EFI. Clive Weatley has the v-shaped inlet adapter to place the SU's in the back of the block.
The SD1 setup is very good, even on mildly tuned 3.5's. The B GT V8 setup with the adapter is also quite good, though you'll loose some top end bhp; mixture is getting "lazy" after all those bends in the inlet track. The K&N filter setup for B GT V8 will help regain some.

I promised some figures:
- Henry Hebels has a 3.5 with a Holley 600 cfm double pumper. Claimed 220 bhp but 4.5 km on a liter of unleaded. Another user who has a conversion with a Holley 390 reports slightly better results: around 6/6,5 km per liter.
- Mario te Molder has a 3.5 with original setup, but tuned engine. Estimate at around 220 bhp, pushes hard all the time, still has 8 km on a liter of unleaded.
- I have a 3.5 that is mildly tuned; around 170 bhp at the wheels, original setup, around 11,5 km per liter.
- full original b gt v8's regularly get 13 km to the liter.

These figures led me to believe that I should stay away from these big four barrels. Reported mileage on EFI is even better than the original twin SU setup. Please let us know the outcome.

Frank de Groot


Saving the planet with a V8?

If you are intending switching, it would interesting if you could back to back on a dyno and compare mpg's on both systems properly tuned.

I'm hoping to do this between my current Holley 390 and Edlebrock 500.

For Franks stats my 4.6 is7km per L.


It is indeed ingeneous, Dan. Part of the problem in explaining it is semantics.


It's quite possible that S.U.'s would give you better low end acceleration and better fuel economy at the expense of a lower top end, which you probably can't approach anyway. I would use the S.U.'s if I were in the U.K. In the U.S. I would look for a Carter 400 to get reasonable mileage by staying out of the secondaries and good performance when you wanted it.
George B.

It's not about saving the plannet, but about saving your wallet. Last week I was surprised at the height of your fuel prices. I think it is on par with the Dutch outrageous prices.
Paul, as you know a 4.6 does very well with a Holley 390. This carb can accurately fuel your engine size and might even be a tad too small. The right size of carb just makes the best of your engine. Your score points this out: for a 300 bhp 4.6 it is good. Especially considering your driving style. I just can't keep up.

Frank de Groot


The Twin 6's may flow nearly as much as the Eldebrock 500, so for a 3.5 the Holley or Carter 400 may be best bet. The Holley is thirsty which is perhaps why used for drag racing but I will be surprised if much diff in mpg between Eldebrock and SU's and Eldebrock has performance advantage.

For the 4.6 the Holley 390 may be too small and 600 too large so Eldebrock(sitting in garage) looks favourite.

For Wallet comparisons Gordon Brown Large motorist small, we are ripped off in UK.

World cup; well done USA and Ireland, come on England.



I have run my 3.5 (from a twin plenum Vitesse SD1) for two years with an Edelbrock suitably jetted and on a dual port Offenhauser inlet manifold. It has been splendid in terms of acceleration, flexibility and even with fairly spirited driving gets around 30 MPG. It's only problem is it does not always start as easily as I would like, but if used daily this is less of an issue. I originally had a Holley, but this was not as economical.
I also have recently purchased an RV8 and this is much less frugal with it's fuel consumption, and I,m not sure it's any faster either.However it starts wonderfully. The tickover is less smooth than my conversion also.
I am a big fan of the Edelbrock, and will never forget the feeling of admiration for them when I literally took it out of the box, bolted it on and it started and idled first time. I then got it tuned and they said it was in need of only minor adjustment.
The carb was supplied by RPI engineering.


While it's completely true that the Edelbrock (500cfm) and Carter (400cfm) are bigger than
necessary, it's only fair to point out that their "secondaries" (i.e., the back two barrels) are
vacuum-operated and "progressive" (i.e. they open gradually and on-demand). So it's not
like you actually use all that barrel area all the time. You may never get the secondaries all
the way open. While not optimized for the job, they seem to run pretty well on these engines.
If you do need to tune them, it's pretty easy. For example, you can change the accelerator pump's
stroke length... (Wouldn't it be cool if you could spec the barrel diameters, front and rear?)

I've never played with the little 390 Holley. Do they have linkage-operated secondaries?

I agree with Simon Denney about the smooth running Carter. I have it installed for 5 years now on my 3.5 liter engine and never had any problems. Fuel economy is around 27 MPG. Curtis is right, the carb only opens on demand of the engine. The secundaries might never open all the way unless you push the pedal to the metal but than speed goes up so quickly that the "blue light special" makes a short end on that!
Werner Van Clapdurp

Another consideration - not often mentioned - is that SUs will perform well when driving up and down mountains (e.g. in the Alps), but fixed jets like Weber & Eidelbrock will richen their mixtures as you gain altitude - to the point where the engine loses power and won't climb anymore!
Chris Betson

If anybody is interested, I was offered a full hotwire system for around 200 pounds by a friend. I already have one, so perhaps there is somebody else who could put it to use.

Frank de Groot

Chris in Essex, you live in beautiful country but flat as a pancake compared to our Colorado! The SU's on my '70 B require considerable leaning out in the drive from Denver (elevation 5280 ft.) to Leadville (elevation 10,400 ft.)if one is to get any sort of performance at altitude. I like SU's but intend to use a Carter 400 on my '67 B conversion.
Chris Jones

The Edelbrock is very easily tuned deliver performance and economy. One must change the jets, rods, and main spring to match the engine needs. Downsize in the cases of Buick and Rover swaps. Edelbrock offers a wide variety of parts(in the states). Taking a carb out of the box and expecting perfection rarely works wether using SU's for 4V's.

I too have the Edelbrock 500. Installed it right out of the box. It is a little hard to report my fuel consumption as my right angled gear drive for the tranny-speedo is broken (the manufacturer has shipped me another one). I use my Rover 3.5 for drag racing twice a year and I am wondering if I should re-jet? How would I be able to work this out. And if so, is it possible to re-jet for cruising around and then put bigger jets in for drag racing, would this then require the carb to be re-adjusted?


if the carb you got from RPI is a webber/carter 5oo I think you will find it does not have vacuum operated secondaries.

I've just been reading an article on the RPI engineering website (taken from MG World magazine) regarding the Holley vs Weber. Both this article and RPI suggest that a Weber 500 is the best carb to use on any size Rover V8, and that the ease of re-jetting should mean that it can be adapted easily to the engine. Any thoughts in relation to twin SU's?

I'm also thinking about changing my camshaft and tappets - I have a v.high hydrocarbons reading and I'm thinking this could be due, in part, to a worn camshaft. Another thread on this BBS suggested that a cam from a Rover 3.9 would be a cheap way to increase BHP. RPI Engineering offer a range of Piper cams - has anyone used these - are the claimed increases of 10-15bhp realistic?


Chris Holmes


The comparison was between a well used Holley vs Weber and showed no measurable diff between them.

Real Steel sell their own cams for the V8 and sell both carbs.


This thread was discussed between 07/06/2002 and 18/06/2002

MG MGB GT V8 Factory Originals Technical index

This thread is from the archive. The Live MG MGB GT V8 Factory Originals Technical BBS is active now.