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MG MGB Technical - Which SU Carbs for 80B

Several people keep suggesting I do a winter project of swapping out my Stromberg-Zenith carb for a pair of SUs.

I've begun the process of removing emission equipment from my 80B, but that is it. I don't really believe in just swapping out parts for other parts just because they may look better or give some barely noticeable performance improvement. I tend to go for reliability and mileage, not going faster, quicker.

But in the back of my foggy recollection, I have this recollection that one cannot simply exchange one carb system for another. There are other compatibility issues, like carbs being too big or too small (internally), like in too much carb or not enough carb.

I imagine carbs off of an earlier A may not work, but the most recent recommendations is a set of SUs that look just like what I have on my 72B
- the external float style SUs.

As far as I know at present, the S-Z is working just fine, and if it ain't broke don't fix it - comes to mind.

Thoughts.
R.W Anderson

The late UK spec Bs have HIF4 carbs with integral float chambers but any 1.5 inch SU will work - even those off an MGA if they are correctly jetted/needled.
Chris at Octarine Services

RW-
You'll have to use SU HIF4 carburetors as the remote float bowl of the rear SU HS4 carburetor will conflict with the booster unit on the master cylinder.
Stephen Strange

I believe the pair being suggested are HS4 carbs.

R.W Anderson

Ahh, now that sounds like a good reason to simply leave things alone.

Thanks for the heads up on the rear float - booster interference.
R.W Anderson

I've got HS4 carbs on my ex-U.S. l/h drive car, with a servo (booster). The only problem you might come across is the air cleaner on the rear carb. I've overcome this problem ok.

Let me know if you want a picture.
John Bilham

I have a pair of original HS4's on my 77B. They work very well and I have no intension to change them. However if I wanted to replace the SZ (which I don't like) at this time. I would go for an older single 13/4 SU from a Volvo. They will just bolt up to the intake manifold like the SZ because the bolt pattern is the same. A bit of work on the choke and throttle cables and your done. Performance is great too. You can get into changing needles etc if you want to change performance but they work well with the Volvo needle. A bonus is now you tune a single rather than twin SU's
Barc Cunningham

RW-
In some cars the remote float bowl of an SU HS4 carburetor will just barely clear the brake booster of the master cylinder. However, you'll find that there'll be a problem with getting the air cleaner to clear it. This is usually solved by using a set of K&N truncated conical air filters. The problem with the truncated conical air filters is that their shallowness creates induction pulse problems above 3,500 RPM, their small internal volume that will not allow the fitting of a set of velocity stacks, and their small surface area that offers insufficient airflow for an enhanced-performance engine.

Retrofitting the earlier non-boosted brake master cylinder is the common solution, but this is not a bolt-on affair as its mounting flange is turned 90 degrees, so the mounting holes of the pedal box will not line up, and the appropriate earlier pedal box assembly is radically different, even having a different mounting hole pattern at its base that requires drilling a new pattern of holes into the body of the car. This is just one of the reasons that it is unusual to see a Rubber Bumper model with a seriously uprated B Series engine: It is much more work. When somebody wants to go for really dramatic power increases, he swiftly comes to think that he will need to retrofit the earlier brake master cylinder and pedal box assembly so that he can mount air filters that have a decent airflow capacity onto the carburetors like the Chrome Bumper model owners do. “After all,” he reasons, “it is not really that difficult, it just requires some persistence and time, plus another brake master cylinder and the earlier pedal box assembly. If my brake boosting servo and brake master cylinder are in good shape, then I can always sell them as a unit to help cover the cost of the earlier brake master cylinder and pedal box assembly because the servo unit is getting harder and harder to find.” To the conventional, orthodox thinker, this reasoning holds true. However, read on-

Fabrication of a plenum chamber to go on the carburetors and running a large-diameter breather duct hose (flexible pipe) to a remote air filter housing would enable the retention of the existing boosted brake system. From the air filter housing the intake hose (flexible pipe) can be run to the air passages neatly provided beneath the bumper in the vented front valance of the 1972 through 1974½ models of the Chrome Bumper cars. You will need to do some scavenging in the junkyards to find an appropriate air filter housing (more work) and then figure out a mounting system for it (still more work), but the larger, more commodious engine compartment of the later Rubber Bumper models should make it a relatively easy task. To equal the airflow capacity of a pair of 6” diameter 3 1/4” deep round air filters you will need an air filter housing with an air filter that has an area of about 122 square inches (11” X 11”).
Stephen Strange

Here's mine


John Bilham

John B...

I can almost see light between the brake booster and the air cleaner. Which way is engine torque again !!!!

I just got outbid on the HS4s on Ebay, so I bought myself some more time on thinking about the carb change.

Once I get other items attended to I will get a better idea of my gas mileage too and see if I really want to change anything.

Thanks for the comments.
R.W Anderson

"Which way is engine torque again !!!!"

Rev it and see!!!!
Paul Hunt

This thread was discussed between 14/07/2013 and 17/07/2013

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