MG Car Repair: MGB Needle Guide Position, emissions test, needle guide

Question
Hi Howard,
I have a 74 MG B and am rebuilding the carburetor (AUD 550's)... and have a couple of questions:
1  I'm a bit confused about which surface of the SU piston, the bottom of the metering guide is supposed to be aligned with.  The manual says "to ensure the needle guide is flush with the underside of the piston" Do they mean
A) the bottom of the piston or
B) the bottom of the slot, that runs front to back that the guide sits in the bottom of the piston. The older SU's didn't have a slot and aligned on the bottom of the piston...
2  The second question is about over run valves or lack of them on the throttle disc. My B's carburetor don't have them. (no holes either)... did some earlier owner swap the throttle discs? or did some AUD 550's ship without them, and does it matter?  My 74 B GT has them, and has always been easier to tune and get to pass emissions test...
Thanks again for your expertise,
Jim


Answer
Hi Jim,
The needle shoulder is suppose to be flush with the bottom of what ever surface there is that the needle is mounted in. If you have a slot running front to rear where the needle is mounted than that is what the shoulder of the needle is to align with. It is so that only the needle and it's taper is exposed to the air stream. Never the shoulder.
The spring loaded valve in the butterfly was a method they used to delay a quick throttle closing. These were very troublesome as the springs would get weak and make tuning very difficult so most mechanics soldered them closed or switched them with plain throttle plates. Unless your state has L-1 emission testing or some form of quick throttle movement testing, these spring loaded valves will have no effect on emission testing.
Howard