Opinion question: correct SU needle height

[zigzag240]

OK, There are two camps on setting the height of a 1970-72 SU carb needle: flush with bottom of piston and flush with bottom of the groove. The British method is to use the bottom of the piston, but I don’t think they have the extra groove in the bottom of the piston that Z carbs have.

 

It would seem to me that with the needle sitting lower, its shoulder would disrupt air flow at idle through the bottom piston groove, and require that the nozzle be adjusted too low. All the junkyard ZCAR carbs I’ve opened up had the needle shoulder flush with the bottom of the groove. Does this mean everyone's had had it wrong for 37 years?

 

Can anyone provide a legit reason why one method is the "correct" one...with technical reasons?

[katman]

FSM: Flush with the bottom of the piston, so yeah, the shoulder sticks out of the groove.

[zigzag240]

But the needle hole "is" the bottom of the piston...you can't get any lower. It isn't helped by the fact that the FSM drawing is hazy.

[Goose52]

Actually, there's yet another camp - the "ZTherapy" method.

 

Take off the dome from one carb and pull the piston. Run the jet all the way up until it stops. Loosen the needle lock-screw on the piston and pull the needle down with the shoulder a bit below the bottom of the pistion. Tighten the needle screw just enough to hold the needle but still loose enough that the needle can slide back into the piston. Then, carefully lower the piston into the carb body until it's sitting on the bottom of the carb. Then, carefully pull the piston back out and tighen the needle lock-screw. Reassemble carb and lower the jet the usual 2 1/2 turns as your initial tuning position. Repeat on other carb.

 

You end up with the needle at the perfect position where it's contacting the jet with the jet all the way up. It is a known position and will be the same starting point on both carbs.

 

Usual mixture adjustment after needle position is set...

[zigzag240]

Thanks, that makes perfect sense (!)....I was getting hung up on the FSM ritual for some reason...too much brake cleaner over the years I think.

[Goose52]

I always set the shoulder at the bottom of the piston ... until I read the ZTherapy method. As you say - makes perfect sense. Sometimes folks do find a better solution than the factory guys !