Vacuum line question 280Z SU Carbs *with pictures

[jonathanrussell]

21 hours ago, rossiz said:

I’m still using the little bottle of oil I got from ZTherapy with my rebuild kit. No idea what weight it is. My rpm fluctuation is only at idle, about 1-2 seconds, pretty constant. More noticeable when the engine is hot. 

If you add a very small amount of choke does the idle stabilize?

[Johnny280]

I forgot: my carb oil is 3ml of 20W50.

Small amount of choke would be equal to adding a little bit of fast idle screw right? Since the choke is first moving the butterflies and later enriching the mixture. 

 

[jonathanrussell]

The choke test...it is a quick way to determine whether the mixture may be too lean at idle. I was thinking that Rossiz's situation might be a lean mixture at idle. These cars, in my experience, like to idle a bit rich. When you add choke, you are dropping the nozzles...which is the same thing you do when you turn the mixtures down, away from the carb body, making the mixture more rich. So, if he adds a bit of choke and the idle stabilizes, it probably means he can solve his problem by adjusting the mixture a bit....if he is okay with the trade-off of potentially running a bit more rich throughout the power band.

Regarding the dampening oil in the carbs...I have never paid too much attention to the quantity of oil. I start with one carb and add tiny amounts of oil until I the piston drops steady, all the way to the bottom, makes a slight thump, etc, but doesn't free fall and go clank. It is kind of a feel thing that you get used to seeing and hearing when you have it right. Important too is to have the domes aligned as you tighten down the dome screws so that the piston falls freely. Then, I make the other carb piston drop the same. Having the dampening set correctly ensures that the piston doesn't rise too fast (but rises and drops fully) during wide open throttle.....which would cause a sudden lean condition. Someone correct me please if I am describing this inaccurately. Regarding Oil weight...I think 20w50 would be fine. I have always used the oil ZTherapy provides but I don't remember the weight they use. Probably 20 weight but....

 

 

Edited April 9, 2019 by jonathanrussell

[Johnny280]

But at least for my carbs giving a little choke first moves the butterflies and much later in the lever travel drops the nozzles? I don't know if there are different throttle linkages out there?

[Mark Maras]

 The choke should move the nozzles first. The butterflies open a bit later in the choke actuation to increase the cold idle a bit.