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MG MGA - SUs or Weber. What is best?

Nothing like a good debate during the Covid-19 lockdown! I reckon it is a matter of personal choice based on efficiency, looks and noise!

May be Webers have a better accelerator pump but lose out on complexity. SUs suffer from throttle shaft and housing wear that Webers don't because of roller bearings. SUs can also suffer from main jet/neddle wear when incorrectly assembled and are prone to leaks around the main jet seals (both easily fixed).
On competition cars I estimate the spread as 50:50 SU to Weber, on roads cars it is more like 95:5.

Mike Ellsmore

Mike-- In an effort to fill-in some boredom time being encounted by others and to stimulate some interest through contraversary- I make this submission-
--The answer is - Neither- carburettor- These are just old fashioned Jugs!
Thats all they are--- a container with with a leaking tap fitted--
Substitute the manifold ---and and the leaking-messy--running-on when hot--knob enriching-- flooding float valve sticking--slow to start in the frost--time wasting ---un balanced-- mythical pieces of hardware ----for an EFI System. Instant starting. No-Running ON-- Far better fuel economy--TOO. No maintainance needed.
It will however--take longer than a wet Saturday afternoon to build up, on your engine. But its a fascinating satisfying path to travel!!
I hear the purists who only know how to copy what was already there --- howling already! Good luck to them.
I suppose they would be able to work-out how to make the change.
While dreaming----Add Electric Power Steering Systems for good measure!! Wonderfull improvements to our special little cars.
I've just turned the computer OFF before the flack commences.
Cheers from down under.
I.W. Cowen.

Webers tend to work well at WOT, whereas SUs seem to gives better all around performance in normal driving conditions. Each carburetor has its advantages and disadvantages of course. A lot of guys coming into British cars from American stuff put the Webers on because it's what they know, without realizing the brilliantly simple and effective design of the SU.

Generally speaking, there is a lot to be said about the original design of the MGA as a whole. Other than a few minor gripes (like the cramped driver's seat) I've never owned a better designed car from the 50's. Heck, it's a better car than my big Healey that has 10 a year advantage.

I W Cowen, if you find the need for power steering on an MGA, then don't ever drive an American car of the era! ;)
Steve Simmons

Well Steve---,
I reckoned I would create some comments--

It happens that I ran A Ford Franchise in those times!!
I was quite familiar with what was available on new U.K. & U S models.
Those pnuematic Ford/G M / Chrysler Power Steering systems
leaked oil-- worst of all--they had NO road feel, at all. Awfull by comparison
The Jap. electric models are excellent. No plumbing--no drive belts
I.W. Cowen.

What my last sentence meant to convey was that the MGA has a very light steering feel, combined with a huge steering wheel. In the 1950's, most American cars lacked power steering and were an absolute bear to steer for women. I know a girl who finally got her dream car, a 1960's Mustang, and she had to sell it soon after because she lacked the strength to steer or shift. So just saying that if you find the need for power steering in an MGA, then you may want to steer clear of most old American cars. But we're drifting off topic...
Steve Simmons

I'm thinking Steve is talking little 2 barrel down draft DG series Weber

I've played with SU's and Webers all my life and I still can't answer your question
Sometimes you just happen accross a combination that works and it could be either
On my last midget which was 2300cc B series powered, i tried everything, and several camshafts as well
The best top end power ,by far was with 2" SUs on long manifolds but it was really dead down low, the mixture was good but it just wouldn't hook up under 4000rpm
Tried 1 3/4" SU which made it better low down but not much and then it wouldn't spin out properly
Tried a 45mm Weber, and played and played with it and gained some bottom end ,it would actually pull from 3000 with my best cam so that was an improvement, but it still wasn't as strong up top as with the 2" SUs
Next was a 48mm Dellorto, ran the same chokes sizes and jetting as with the 45Webber, and it went half ok ,Had good throttle response, still pulled from down low but hit a wall at 6800 where the 2"SUs would pull hard to 7ooo(limited)and would have went further
The next plan was a 50mm Weber with bigger chokes
but that didn't happen
William Revit

I reckon Dellorto's are better for road use compared to Weber
but having said that, the later Webers with 3 progression holes instead of the earlier 2 seem to be an improvement for light throttle driving
William Revit

I'm not surprised that those 2" SUs didn't deliver down low on that engine. The air velocity probably wasn't high enough to take advantage of them and keep the mixture right.

I feel the downdraft DGV aren't very good at all. Some people like them though. The Webers I have the most experience with are side draft. I use triple DCOE45 on my MGC, which is over 3 litres. They don't like putting around town nearly as much as they like WOT on a mountain road.
Steve Simmons

This reminds me of the debates I see continually with my other vice of old Triumph motorcycles. There the argument is rebuilding or the new Amal monobloc, or concentric. But wait what about Mikuni or JRC flatsides as a replacement.
Bill Haglan

EFI - fine when it works, but injectors are expensive bits of kit and can foul, the electronics can also fail and high pressure fuel is fairly dangerous.

Worth it on a classic - I doubt it.

Webers, downdraught certainly not desireable, sidedraughts, pretty but not practical for climbing mountains, use fuel prodigiously due to accelerator pumps.

SUs, more fuelling points than the most sophisticated EFI, compensates automatically for altitude.

Bigger is not better - a pair of 1 1/2 inch or 38 mm SU carbs will flow all you need for street cars - going bigger just adds a small amount at wide open throttle but loses low end torque.

See for a comparison of carb setups.
Chris at Octarine Services

Steve--My apologies to you on my comment--

"I'm thinking Steve is talking little 2 barrel down draft DG series Weber "

It was based on the fact that a lot of US import MGBs that come here have DGE or DGV carbs on them

It wasn't till later (an oh sh&t moment)that I remembered your collection of cars but it was too late to go back and change what I'd said

Apologies if I have offended you

William Revit

Not at all, Willy! You do see a lot of DGVs on MGBs over here, and it's almost always the guys who don't know MGBs very well. But they do know about Webers, as do most who are into American cars. So they just assume the DGV, coming in kit form for MGB, will be better.
Steve Simmons

Here's an interesting comparison with both on a stock MGB and a Midget race engine, complete with dyno results. Bottom line, almost identical with a slight edge for the webers:
KR Doris

When I got my new 1950 cc engine from Cameron Gilmour in Scotland four years ago and, intially, I was a little concerned when he recommended fitting a Weber 45 DCOE instead of a pair of SU H6 carbs.

I was planning to do a few trips over the Alps and the Pyrenees with the new engine and I had read that the Weber DCOE can run rich at altitude which may affect the running of the engine. Apparently, the SU carb is less affected and compensates for the pressure drop at altitude more effectively.

In the end I went with the Weber DCOE because it isn't easy to go against your engine tuner and I honestly don't regret that decision. The car really flies with it, I know the accelerator pump on the DCOE wastes fuel, but it really does give instant throttle response and the carb gives incredible top end power.

The engine performance at altitudes of up to 10,000 ft seems to be pretty similar to that at sea level with no rough running and it ran just fine with no obvious signs of the mixture being over rich.

The only downside is that the mpg has gone down from around 28 mpg with the 1840 cc 3-bearing engine and twin H4 SUs, to around 23/24 mpg with the 5-brg 1950 cc engine and the Weber 45 DCOE.

So, the trade-off for me has been an almost 20 % increase in fuel consumption in return for an almost 40% increase in power and I have decided that I can live with that! :^)

(Plus, the induction noise from the Weber is amazing.)


PS Great link Ken, I hadn't seen that article before, Cheers

Colyn Firth

Talking fuel injection, it's not without it's own set of problems young Ian
My son just splashed out on a new supposedly matched set of multi hole performance 440g injectors for his project car--expensive purchase
Not being one to trust anybody's word, decided to give them a run on the flowbench before he fitted them up-So, gave them a little swim in the sonic bath just for no reason at all then into the rack
Well, 2 ran at 110 ,1 at 105, and 1 at 100
So much for a matched set from a reputable supplier
2 more on their way but had to buy them as the supplier suggested within 10% was matched----maybe in his world not mine--I did ask if he'd be happy with 10% missing off the end of his watsit but he didn't see the funny side of that

William Revit

"--I did ask if he'd be happy with 10% missing off the end of his watsit but he didn't see the funny side of that"

LOL. Brings every man back to earth.

R Taylor


With the Motec EFI system you can adjust the mixture for each cylinder individually. This will compensate for variation in injectors.
You really need an exhaust manifold with a separate exit for each cylinder so you can move your Wide Band Lambda sensor to each cylinder in turn. We can do that with our Triumph race cars.

M F Anderson

Thanks Mick
I am aware of the Motec(and others) feature but we're running rechipped Mazda ecu just for the sake of semi originality- The car's a MX5 SP which are becoming quite collectable as they only built 100 of them so trying to keep visible mods to a minimum
There are ways around it but just peed off when they were sold as a matched set
Easiest fix is a proper matched set
Funnily, on the drive cycle test they aren't too bad, it's only on 100% duty they fall short, but that's where it matters most-----------


Sorry about the slight thread drift Mike
William Revit

Mick, reminds me of big diesels that have a temperature gauge on each cylinders exhaust (Mirrlees K8 for example). Easy to pick up something odd with these.
Mike Ellsmore

The big block Chev speedboat guys here run temp sensors in each port
It's interesting to watch how the fuel load jams up to one end of the engine under big boost pressures if the combo of hi/lo injectors isn't right

Wouldn't happen if they used SUs----lol
William Revit

"The big block Chev speedboat guys here run temp sensors in each port"

Well I think I would much rather be a sailor than one of your Chevy speedboat guys Willy!

Apparently sailors lead a much more interesting life because I have heard somewhere that they have a girl in every port! :^)

Colyn Firth

No no no Colyn, you've got it all wrong

sailing's ok but you can't beat the ride in a big block supercharged chev in a boat
I rode shotgun in Gringo when we pulled the first bloke in Aust at 100mph on skis--fairly exciting sitting there backwards seeing this poor fella on a rope out there getting hammered with spray
then had to do it again back the other direction for the record, poor fella couldn't stand up at the end

Your comment that you have heard somewhere that they have a girl in every port!
So you've only heard about it then------lol

William Revit

I must be getting old, I completely missed the girl in each port joke
Found one------lol

William Revit

Wow, you Tasmanians are obviously more fortunate than us Brits, all I have ever seen in in my ports is carbon! :^(

Colyn Firth

Good Day Roger---Willy--Mick--Mike--Colyn--
I had hoped I would stirr something-up.
I'm enjoying your contributione---and yes Mick, My EFI system has
great circuits in it--- We have over come the English 5 Port Cylinder Head bogey---one , THREE , four TWO. Combustion in- balance.
Its checked by lambders, fitted into bungs welded into the exhaust manifould-- Registering on a lap-top, as we drive.
Now I have raised your interest in why--the inbalance with EFI-!!

Young Ian---
I.W. Cowen.

Thinking about your 5 port comment Ian(young) it's just a matter of getting injector timing right
5 port would need double bangs, like -
bang-bang----------bang-bang and trim the injectors to suit
instead of bang-bang-bang-bang as a 4 inlet port head would be
Or go for high mount injectors and let the manifold sort it, but probably not quite as good as bang-bang
It'd be interesting to run lambda sensors with a pair of them SU replica style injectors and see how they suck
William Revit

Or fit double injectors to each inlet port aimed at the valves, then it would be just bang - bang - bang -bang...
Chris at Octarine Services

Talking about big diesels - we ran several ships with V16 Pielsticks, each unit had its own temperature gauge and people would insist on adjusting the racks to balance the temps, trouble was when you put a peak pressure indicator on the power cards where all over the place, talking to the rep we found out the exhaust system made the temp gauges practically redundant and all settings should be done on peak pressures.
David Brenchley

When I owned my 3A Sprite I went from the original SU carbs to a 45DCOE Weber. It was a long protracted process to get the engine to run smoothly throughout its rev range and many hours and $s spent on a dyno.

Weber was choked to 38mm, 1/4 cam and extractors fitted to get the engine to breath effectively.

The end result was the car ran on the smell of an oily rag, on a continuous run obeying the speed limits I was getting 45+ miles to the gallon driving from the Blue Mountains to Wagga. Roads were not like they are now that journey used to take 6 hours.

The quality of build in that carbie was a delight to behold.

Look back and realise why I was always broke.

G Evans

This thread was discussed between 16/08/2020 and 07/11/2020

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