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MGB SU Carb Conversions

SU conversion on late-model MGBLate-Model U.S.  MGB -  Stromberg to SU Conversion

For those of you who drive a late model US-spec MGB with a single Stromberg carburetor, by far the most significant change you can make to the car is to fit twin SU’s; as the factory did to the non-US models right up to the end of production in October 1980.  
When I first came to the US in 1984, I brought with me a UK-version 1978 MGB GT that we had bought from H.R. Owen in London when it was almost new in 1979. The car was fitted with twin HIF4’s, and, in line with the European standards at that time, had only the most basic emission controls.                                                                                                                                                                                                                                                                                                        

After we had lived here for a while I drove a friend’s US-version from 1976, and could not believe the difference; it felt as though it was being strangled, I simply could not believe it was the same car. The image above is my 1977 MGB, which I bought in California in 2000, and converted to twin HIF4's in 2005, after we had moved to North Carolina. Later, in 2011, I switched to the HS4's you see here. You might notice that they are the later UK-spec models with ball-bearing suction chambers. 

The problem was not just the single Stromberg. In order to comply with the EPA regulations, the car needed a catalytic converter, and the only way the factory could accommodate one was to redesign the exhaust manifold to allow the converter to fit to the top of the front pipe. This resulted in a manifold which sends the exhaust gases through an excruciatingly tight bend, that does absolutely nothing for the performance. During the conversion I also recommend that you remove the Smog Pump, and the Air Injection Rail, both of which absorb a fair amount of horsepower.

Note however that I do NOT suggest you remove the Evaporative Loss System when making this conversion. The system as installed by the factory does a great job of collecting all the fuel fumes from the fuel tank, and the carburetors, and instead of allowing them into the atmosphere, it recycles them back through the engine. So please, leave the entire system in place, and make sure it is functioning correctly.

So, what will you need to make the switch? First of all you need to check your local laws to make sure that this change is legal in your state. It would be very frustrating (not to mention illegal) to go to the trouble of making the conversion to discover that your car will not pass the annual inspection. I was able to make the change when we moved from California to North Carolina, but PLEASE, make sure it is legal in your state before you start.

As for the parts you will need, here’s a list of the basic components:

Other than the carbs, and possibly the inlet manifold, if I have one available, I DO NOT sell these items. They must be purchased from one of the normal retailers, of which Moss Motors, Brit-Tek, and Victoria British are three examples.

·    Intake manifold (be sure to find one with the check valve for the
     brake booster) + Insulation Blocks
.    Accelerator Cable (determined by the carbs. you fit)
·    Pair of either HS4, or HIF4 carburetors (I strongly recommend HS4's)
.    Gaskets: (6 for the inlet side, plus 2 for the Air Cleaners)
·    Heat shield (essential)
·    Exhaust manifold + Sealing Ring
·    New Exhaust Front Pipe (‘72-’74 works well)
·    Air Cleaners - as there is very little clearance between the carbs and the
     brake booster, K&N conical filters (P/N 56-9320) were specifically
     designed for this conversion, and are strongly recommended
.    Blanking Plate & Gasket to replace the Water Outlet for water choke
     (Moss P/N/s 052-431 & 295-040)
.    Manual Choke Cable (recommend Moss P/N 331-490 - "T" Handle type )

In addition to the above parts you will also need to consider the following:                                                                                                                                                           

- You will need to change the heater hoses, in the earlier models the heater return runs across the top of the rocker cover, so you will need to to run a pipe from the heater return to the bottom hose (I used a length of copper pipe), and you will no longer need the feed to the water-heated choke on the Stromberg, so use the Blanking Plate referred to above.
- You will need to consider the exhaust system; the new front pipe does not reach the front silencer box. I went one step further, and removed the front box (as BMC  did on all the factory racers, and rally cars) and had a local muffler shop make up a straight pipe from the front pipe to the rear silencer. If you do this (it is a little louder as the front box removes the lower frequencies - but you gain about 5hp) I suggest buying a length of flexible exhaust pipe and two clamps from your local auto parts store so you can legally drive to the muffler shop.

Removal of a Weber Conversion

We are often contacted by owners of vehicles that have been converted, either by them, or the previous owner, from the original SU carburetors, to a fixed-jet carburetor, usually a downdraught Weber.

We are also frequently asked if we will accept a Weber in trade against a set of SU's; which unfortunately, we must decline.

Here are some images of a recent conversion that we assisted a local North Carolina customer with.

Late-Model MGB with Weber carb, prior to SU carburetor conversion  This is a Weber DGV installed on a beautifully maintained 1973 MGB.

The owner was not at all happy with the conversion, despite the sophisticated inlet filtering system, and wanted to revert to the SU HIF4's that the car originally came with. We rebuilt the carbs for him, and assisted in the reversal of the conversion.

The newly-rebuilt SU's prior to re-installation          
Rebuilt SU HIF4 carbs awaiting installation on late-model MGB  

Here we have the SU's after the initial installation.

The owner (who had carried out the Weber conversion himself) had been wise in carefully storing the original carbs, in case he wanted to revert to the SU's at some point in the future. He now reports that he is pleased with his decision to revert to the SU's and is enjoying a "much better throttle response"
1973 MGB after rebuilt SU carb conversion

British Classic Motors - More than 40 years of SU Carburetor Rebuilding & Restoration experience..............


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