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MG MGB Technical - SU carbs

somewhere I remember reading about different carbs (su/hs4/whatnot) and if I recall there were two different carbs that came on the mg. one that was worthless and one that wasn't. I'm just curious about which is which and why one's so much better
Chris

The MGB used three different carburetors during its 18 year production run.

1962-1971 Dual SU HS-4 This is a 1.5 inch throat side-draft carb, with the float bowl on the side. Easy (relatively to tune, external float bowl can starve in hard cornering)

1972-1974 Dual SU HIF-4 (Horizontal, Integrated Float). Also a 1.5 inch throat, but with the float bowl integrated into the body of the carb. Said by some to be easier to tune, it does not suffer as much from fuel starvation in cornering.

1975-1980 Single Zenith Stromberg (North America only). Automatic choke. This carb is widely blamed for the drastic lowering of HP ratings in the RB cars. In reality, it is probably the incredibly restrictive combined intake/exhaust manifold that caused most of the loss. This carb is often replaced by a single Weber DGV downdraft carb, with a seperate, less restrictive manifold, making it more powerful and easier to tune.

Our British friends, in addition to being able to get GT's after 1974.5, also continued with the dual SU HIF-4's until the end of production in 1980.
Paul Noble

There's no such thing as a federal rubber-bumper GT?

Now there's a thing. Any idea why?
--
Olly
Oliver Stephenson

Actually there are rubber bumper GTs in the US. They were only around for the 74.5 model year. We have one in town, and I don't really feel like we missed out on much. As I've heard the reason the GT was dropped from the US market is that the heavier weight put it in a different emissions class than the roadster. It aparently could not meet the stricter standards and thus was dropped from the market completely. Don't know for sure if this is true but it makes as much sense as anything thing else I can imagine.

-Jared
Jared Snider

And the GT would have competed directly against the TR7.
Dave
Dave Wellings

Dave:
As would the MGB V-8, which apparently passed its emissions testing for the US, but the factory chose to scrap its importation at about the same time, and likely, for the same reason you cited for the GT. British Leyland was not much for internal competition amongst its brands, and certainly not when it came to MG vs. Triumph. David Knowles probably knows better than anyone what the reasons were.
Bob Muenchausen

Personally, I like the rubber-bumper GT. For some reason, while I don't particularly care for the RB roadsters, but I really like the RB GT's. Go figure.

I believe that around 1,200 RB GT's were imported before they stopped.

The 1974.5 MGB's (both roadster and GT) had twin SU HIF-4's, rubber bumpers and the raised ride height required for US safety standards. GT importation stopped with cars built on or after January 1, 1975. Also on that date, US-bound cars were equipped with the infamous single Zenith-Stromberg carb.
Paul Noble

Jared,
You may be correct but the story I heard was that the GTs could not meet the new roll-over standards coming in at that time.

Paul,
I have a RB GT and I prefer it's looks to the CB but there is no accounting for taste.
Leland Bradley

In my opinion it was only two things that stopped the rubber bumper GT's. First, the single carb models were very slow. Second, the GT's didn't sell very well and the rubber bumper models would sell even less. See point number one.

Jim
Jim Lema

The Z-S carb had plenty of problems - the auto choke being problematic, the plastic fuel line to it being prone to splitting after a decade or two, and a general inelegant look to the whole set up. Also, the two carb set up is much better for subsequent upgrades.

Most of the performance shortfall, though, I believe was due to poor emissions timing settings from the factory when the new carb system was introduced in 1975 (with the old points & condenser ignition). By the time the second generation electronic system was fitted to the B's ignition system (the third ignition on the RB) it's stock straight line performance was identical to the last chrome bumper. (source: R&T roadtests)

The Z-S had "manifold" problems in many ways, but it could do a decent job in moving the car along, if provided a proper ignition system.

(About the GT - it's always hard to get a straight answer. I'd have to say that making room for the TR7 was probably the #1 reason. Making room for Triumph was probably also the main reason that the MGC was pulled and the V8 never came over, either. Going up a weight class did mean a new test, but that's not a big deal, it just took money. The rollover may have been more of an issue in that the A and B posts look flimsy and may have needed a redesign - and that would take REAL money.

Really, the challenges of the seventies meant that we needed a new MG, but given what they designed for their favorite - Triumph - it may have been blessing in deep disguise.)
John Z

Going up a weight class would, indeed, have cost more money to achieve, and would have made the engines in the roadster and GT different. It would probably (given the technology of the time) have resulted in an even more emasculated engine.

OTOH, they could have followed through with the O-series project. I have heard that they had a fuel-injected O-series four cylinder engine that produced something like 150 hp, in US Federal emissions trim. That would have been more than enough power to make up for the slightly heavier GT. Several were sent to the US for dealer evaluation. Unfortunately, the project was scrapped.

I believe that, while no part of BL was making much money, what little they had went to Triumph. Leyland had owned Triumph for quite a while and when the shotgun wedding using wedding cars tonbridge between Leyland and BMC created BL, the management was dominated by Leyland and Triumph executives.

To give them a little benefit of the doubt, the MGB in 1975 was 13 years old and based on even older technology. By about that time, the MGB was the only car still using the B-series engine. The TR7 had new styling and somewhat newer engineering. The Abingdon plant still pushed cars from one work station to the next by hand. Ironically, the TR7 "wedge" was based on an MG styling excercise. Given the age of the MGB design and the Abingdon plant, it was easy for the executives to convince themselves that Triumph was a more viable marque.

The MGC was pulled before the TR7 came along. It just never sold in sufficient numbers. Even John Thornley once said that, if he were driving from London to Johannesburg, he would take the C, but for the twisty roads around Abingdon, he preferred the B. The V8 came along just as the Arab oil embargo hit the US and Europe. This scared off potential buyers. The MGB was 11 years old when the V8 was introduced, which made it too little, too late. Even then, it was considered a little dated. If they had built it immediately after the MGC was dropped and before the oil embargo, it might have succeeded. It might even have been sent over here. Then too, they claimed that they could never get enough engines from Rover. I will concede that, had Triumph people not been in charge, perhaps MG could have fought and kept the V8 and gotten the O-series four. Maybe they could even have gotten the money for a completely new car and you and I would be driving brand-new mgtf's right now.
Paul Noble

Paul - I mentioned the MGC and GT V8 as part of Leyland's pattern of behavior. While there always were "reasons" given for not doing this or that, the common thread seemed to be to leave room for Triumph, the C being a six cylinder competitor to the TR6, the GT crowding the then-new TR7 (initially only with a roof), and the V8 being competition for the Stag (in 1973). For example, the reason behind dropping the C was given that its 6 cylinder engine ended up being unique to that model as the sedan it was supposed to share it with was less of a sales success than the 'C. BUT if you had just spent all that money on what was really an all-but new engine, why wouldn't you try to amortize your investment by building ten or so thousand a year? It could very well have lasted until the V8 would have been ready.

The point about the single carb car was that its Achilles heel was not the carb and exhaust, per se, but rather the ignition timing and system. Yes, the first Z-S equipped rubber bumpers got 0-60 in over 18 seconds. But the last in 1980 did the same trick in 13.5 - the last chrome bumper (pre-Sabrinas) were timed at 13.4. The only real change during that period was the distributor.

I read about the O-series in David Knowles's books and it was intriguing, indeed. In my mind's eye, I could see an MG line up of V8 (priced suitably to be profitable), O-series B, and perhaps even replacing the midget with a decontented MGB and pushrod B-series motor going from the mid-seventies on to a suitable replacement in the early eighties.

Alas, it is easy to dream
John Z

John Z. While people often talk of chrome bumper vs rubber bumper, it would be wise to remember that, with the introduction of the 18V engines into the US, all engines sent to NA were "low compression" engines. BMC had not done a great deal of defining work with the LC engines beyond having a different distributor. By the time the 18V was introduced BL was in charge and money was not being spent to fund an effective product development program for existing cars. It is interesting to note that much of the improvement of the later RB cars came about after the BL emissions testing station had been built at Abingdon and staffed with long time MG workers. One suspects that some under the table development was also taking place which lead to the results you describe. I find my RB cars, with free flowing exhaust systems and better intake systems/air filters, give my CB car a run for its money. Like you, I am not sure that horsepower is the exact method we should be using when making our decisions. It is an old, well known rating system, but I once saw a car with an engine having over 100 HP get stuck in a muddy road. It was pulled out of the ruts by a team of two horses. Thus, by looking at only a single factor, a horsepower rating, we tend to ignore handling, braking, gearing, etc. all of which are factors in a well balanced car. Les
Les Bengtson

John Z.,

I agree with you that the fundamental problem with the BMC/Leyland merger and the basic reason behind the demise of MG, was the Leyland/Triumph mindset. Everything else flowed from that; money was made available to Triumph to develop the TR7, while no such funding was given to MG; Triumph was to be the "Corporate Sports Car", while MG was allowed to whither on the vine; Abingdon was considered an old, inefficient, obsolete factory, even though their build quality was near the best in the conglomerate; the MGB was denied a new engine (the O-series) and then its reliance on the B-series was used as a reason for killing the car.

If the management team had not been so Triumph-centered, they might have seen things differently and distributed the wealth differently.

Many of their "reasons" were, IMHO, in fact rationalizations for decisions made because they wanted the answer to all questions to be "Triumph" and not "MG".

The best that can be said of that period is that it was a chaotic time in the auto industry, particularly in the UK. It may have made sense for BL to consolidate their sports cars into a single marque. They just chose the wrong one. MG had problems, but so did Triumph. Their Triumph bias blinded them to TR's shortcomings and emphasized MG's.

And, Les, you're probably right. The folks at Abingdon seemed to have done a lot of things without telling the corporate bean-counters, if we are to believe some of the stories that have been told.
Paul Noble

Paul - yep, can you imagine what kind of egg we'd have on our faces if the TR7 was instead called the "MGD"? Although David Knowles did have one picture of a TR7 derived MG - Lynx? Magna? I forget - without the swoooosh on the side that really did look handsome. I think it was in the "Untold Story".

Les - yes, I am trying to sort out what EXACTLY permitted the last rubber MGB's to be just as swift as the last chrome B's, despite their single carbs and strangled intake and exhaust manifold. I am thinking, like you, it was that "Abingdon touch". Just what the devil was it?
John Z

Paul. Back in my military days we understood what was wanted vs. what was specifically permitted. The old saying was, "Better to ask forgiveness than to ask permission". From reading the MG history, and I have a fairly extensive collection, it would seem that many in the factory followed this example.

John Z. I suspect that you have hit the nail on the head (an old shooting term from the time when "nail driving" was a form of target shooting) when you mention the ignition component. I purchased my 79 MGB in November of 79. (From Capitol Motors in Bitburg, Germany. I was told it was the last 79 imported into Germany and was to "Personal Export Delivery" specification.) The Opus distributor began to act up within the first four months of use. I was driving Helga Garcia, the wife of one of my senior director technicians (I was the Senior Director) down from Pruem to Bitburg where her husband, Steve Garcia, was in the hospital. When the car was taken in to the factory repair station, the conversation was in German, of which I spoke a little. The German BL rep told the repair station that the "verteiler is scheiss" and to replace it under warranty. I suspect, but did not know at that time, that they replaced it with the 1980 CEI system. In any event, the car ran wonderfully for the rest of the time I owned it with no problems with the ignition system (in point of fact, other than having the thermocouple for the fan replaced twice and the "bolt" for the water elbow/thermostat housing replaced once, I never had a single problem with the car in 62K miles). I find it interesting that the Midget group will sometimes use the SU HIF-44 carb as a replacement for the Z-S or twin SUs because it gives "better mid-range perormance". The Z-S carb was a mistake. The stangled air cleaner and intake/exhaust systems was a mistake. I am currently running a Weber DGV 32/36 with a Peco exhaust system (with an aftermarket cat) which meets all of the Arizona emissions standards with a great deal of room to spare. I very much suspect I could pass emissions testing with twin SUs (but that is not allowed here because the 75 and onwards MGs had "a single carb"). The performance of the US and Jap cars was degraded during this time also. The big difference is that the US and Jap car companies decided they would meet, and in some cases (mostly Japanese) surpase the requirements. Unfortunately, the Brit cars did not take this attitude. Les
Les Bengtson

This thread was discussed between 10/03/2003 and 12/03/2003

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