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MG MGB Technical - SU carbs -- what nozzle is what?

I picked up a reasonably cheap pair of SU HS4 carbs that I'd like to put on my '79 B to replace the Weber that was on there before. Aside from the respective merits of the one system over the other, I'm having some difficulty figuring out what hose goes where. On my Weber, it was easy: the fuel hose went to the only nozzle on the carb. My SUs have a bunch more:

One nozzle, slightly larger than the rest (maybe 5/16"?) at the base of each dashpot.

Atop each float chamber, there is a nozzle pointing towards the engine.

Atop the front float chamber, two nozzles point away from the engine.

Atop the rear float chamber, one additional nozzle points towards the engine; it is the mirror image of one of the front carb float chamber nozzles.

All I've been able to figure out so far is that the "extra" nozzle on the front is the overflow. Other than that, I know that one nozzle per carb is devoted to fuel and one to the linkage, but I'm not sure which is which, and I'm not sure what the other nozzle on each carb is for. Can someone help me figure out how to hook these things up?

Thanks!
Chris St. Pierre

Chris-
"One nozzle, slightly larger than the rest (maybe 5/16"?) at the base of each dashpot." -These are crankcase vents, go through a T to the pipe from the front tappet sidecover.

"Atop each float chamber, there is a nozzle pointing towards the engine." These (front and rear) are for the interconnecting fuel line that feeds the rear carb from the front.

"Atop the front float chamber, two nozzles point away from the engine" The top one is fuel in from the main line from pump. The lower is the vent/overflow.

"Atop the rear float chamber, one additional nozzle points towards the engine; it is the mirror image of one of the front carb float chamber nozzles." This is a lid from an earlier car or other car, it should have a second lower pipe as the front does for the vent. Since it does not, there should be a small aluminum plate around the inlet pipe, with a dimple in it, covering a hole below the pipe - this is the vent with no provision for attaching a tube to keep fuel from dripping down the engine.
FRM
FR Millmore

Thanks!

The front tappet side cover was vented directly to the air filter on my Weber, and took a significantly larger hose than the 5/16" at the base of each dashpot. Will there be any problems with putting a reducer above the T joint to get the correct sizing?

Also, I was planning on connecting the crank case vent to the air filter, as with the Weber. Since I won't be doing that, I assume I should plug the holes in the rear of my filters. Correct?
Chris St. Pierre

The T for the vent -actually a Y - is 1/2 on the crankcase side, and whatever the carbs stubs are, like 9mm, 5/16 is too small and 3/8 too large, but 3/8 works with a spring type fuel line clamp.
This method is preferable to the air filter hookup, as it doesn't get the filters oily. There is also a slight mixture, hence jet needle, difference between the early HS without these vent points and the later HS or HIF with them. If your engine is otherwise stock, then use the appropriate needles from vented carb HS4 cars as a starting point. Vented carbs should use spring biased needles and earlier cars have fixed needles. Also use the correct oil filler cap, either the non vented one if your evap control system is intact - recommended - or the restricted vented cap as the pre evap cars had.
FRM
FR Millmore

You might find this helpful:-

http://www.chicagolandmgclub.com/photos/carbswap_zs2su/index.html

Derek Nicholson

This thread was discussed on 30/08/2006

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