My rear carb spills petrol at a scary rate from the little vent by the fuel line, do any of you know the problem? could it be the fuel pump? i ask that cos the petrol was still pumping out for a few seconds AFTER the engine stopped running, any help guys??

Oh yeh i put my finger over the hole and the motor cut out!

 

The 122's are way before my time but sounds like it might be a sticking float making the fuel flow to much.

 

You make me feel old. The hole is the breather, which in your case releases the excess fuel in the bowl. Your float is not set correctly, has a pin hole and is full or the needle is sticking. In either case remove the screws holding the bowl onto the main body to check. By the way how old is the petrol? Old gas alway seems to effect the back carb most. Are you dealing with the SU HS6 twins?

 

twin SU's new fuel - rebuilt carbs - good floats - new jets

 

I am presuming that you have dual SU carbs. What you describe is a classic float chamber issue. Get a rebuild kit and have at it. Might as well rejet the carbs while you're at it.

Ooops. Just noticed some details. If it's leaking it may be just the float adjustment. It's been a very long time and I can't remember the spec, but the 122S Haynes manual will have it.