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MG MGB Technical - Dual carbs?

whats the skinny on dual carbs? do you need a special head or what? can someone recommend a good set up?

Richie
mg?
richie

Dual SU carbs were stock on MGB's from its introduction up to around the 1976(?) U.S. model year when they were replaced by the single Zenith Stromberg.
No real trick the head is the same you will need the intake manifold, the stock exhaust manifold or a header, and the carbs.
william fox

I installed HIF4 carbs, intake manifold, exhaust manifold, heat shield, linkage, conical K&N filters. These were used, from a supplier in England, and they were simply bolted on. They made a noticeable improvement in the car's performance.
Randy Olson
1977 MGB
randy olson

What kind of carb do you have now? Dual SU's were stock in the states thru 1974. In 1975, a single ZS carb was used with a miserable intake/exhaust combination manifold that robbed a lot of power.

If you've got a Weber, you'll need the carbs, linkages, heatshield, and intake manifold. If you've got the ZS, you'll need to add a early exhaust manifold to the list. No need to change the head.

For day-to-day travel, the differences between the HS4 and HIF4 SUs are academic; either will do, and either will provide a significant HP gain over the ZS, and much better lower-end response over the Weber.

Do a search on past threads on this subject and you'll find reams of info and opinions. The debates on this subject border on the religeous...
Brian

The single carb appeared with the 1975 model, on cars manufactured after January 1, 1975. There was a relatively rare combination on cars made between Sept. and Dec. 1974. The U.S. emissions standard went into effect on 1/1/75. MG modified the cars in Sept. to include the "Rubber" (actually Urethane) bumpers and increased ride height made necessary by U.S. safety regs. Since the emissions standards didn't go into effect until January, there were a small number of cars made with rubber bumpers, but dual carbs. This variant is referred to as the 1974 1/2 model year. Note that cars outside of North America continued to get dual HIF-4 carbs until the end of production in 1980.

The 1974 1/2 year included the last of the North American MGB/GT's. After 1/1/75, the GT was withdrawn from the NA market.
Paul Noble

In my opinion.
The best setup for power for a stock motor is a set of HS4 carburetors. If you have a 1968 or later set, replace the butterfly plates with the ones without the springs. The HS4 were used on MG Bs from the beginning until somewhere around 1970-71.

The next best setup, but not my favourite would be the dual HIF setup that were used to replace the dual HS4 for emmissions reasons. These have float bowls on the bottom.

The best setup for people not wanting to do any tuning and just drive would be the downdraft webber grille, ah Carburetor. Since this is a single carb, tune it once and drive the car. As long as the car is not parked for 4 years in between use, you should not have to retune it or clean it out from excessive varnish buildup.

The carbs to stay away from is any aftermarket weber/munie side drafts. Most side drafts are FAR too much for US standard spec late model motors with low compression. There are numorous drivabilty issues that I have found with people adding these onto thier car for performance. It is a lack of understanding, to say the least, for someone to install a sidedraft Webber or other on a low compression motor with the camshaft specs of the late model motor.
Again, this is my opinion.
-BMC.
BMC Brian McCullough

Richie:
Pickup a copy of Peter Burgess' book, "How to Power Tune MGBs" (or something like that). There are several explanations there that may be of interest to you and help you answer your questions.
Bob Muenchausen

Been running a Mikuni side draft for about 15 years now. Once you get it jetted correctly (it does NOT come that regardless of what it says in the package), set the accelerator pump linkage to work correctly and set the float height by actually measuring the fuel level in the bowl and installing 38 mm venturis instead of the 34's it comes with, it's really pretty good and trouble free.

Mine said it was jetted and set up for an MGB but when you read the Mikuni book for the bin specs you will see that it is actually set up just how the factory sets every single one up when they make them.

They tend to run rich at high revs and you really have to fiddle with the main fuel and air jet combos to alleviate this.
Richard Smith

This thread was discussed between 19/03/2003 and 22/03/2003

MG MGB Technical index

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