3 or 4 screw SU's

[madkaw]

CAn someone tell me the difference between the 3 and 4 screw SU's(besides the obvious)?I noticed a small chamber area on the bottom of the carb with passages from the intake, is this for coolant? Is there any difference in performance?Also the 3 screw carbs came attached to a e-88 intake, how does that compare to the e-36 intake?I bought the 3 screws off of e-bay and they are like new but I want to make sure I put together the best performing package for my car.

thanks for any info

[240ZX]

To the best of my knowledge, the 3 screw and 4 screw SUs are identical internally, so performance is equal. Only external differences exsist. The chambers you mentioned are for heating up the carbs......emition stuff.

[26th-Z]

This is the best site I know of for an explanation. http://www.ztherapy.com/

[madkaw]

so I guess the e-88 manifold is to match up with the emission carbs?I have heard the guys mention the prefered intake is the 36 but I don't see much of a difference.

[240ZX]

The 3 screw SU will mount up to either intake manifold. I've got the 3 screw SUs on a 1970 intake and they work perfectly....of course the water cavity on the SUs are a non-function item now.

[nwcubsman]

The 3 screw carbs have the ability to be heated with the cooling system as has been said. Good if you are in a cold climate. If you drive all year, you should at least consider it. I am running this way. Many people stop the water flow anyway, better in summer when fuel percolation can be an issue. The 4 screw carbs can also align the nozzle/needle from underneath, which the 3 screw cannot do. 3 screws are alittle different and I think there is a tool to help alignment. I could be wrong on that. The alignment doesn't need to be done often. But, there are many with much more experience with SUs that I.

[beandip]

I just learned recently that the three screw has alignment ''pins'' on top of the body that align the dome the 4 screw does not . I have a extra set of each . I am assuming because of the ability to adjust the alignment as nucubsman stated . One more thing it is ablolutly impairative NOT to mix up the domes from one carb to another , they were fitted at the factory . This is mostly for the four screw carbs. Over the years there has been passed around again and again that certain intakes will produce more HP " the 10 hp thing " . I have yet to see any actual numbers that came from a dyno testing or any other measureing device. I did know about the locating pins on the 3 screws but not the fact that they were non adjustable. I have extra sets of SUs of each . And am running 4 screws on my ZX flattop engine. Another thing of the plastic pins , be careful with carb cleaners , some will damage the pins or remove them altogether , if the needle starts touching he nozzle it will wear it to a ovel shape . Then you wont be able to adjust the mixture properly . Gary