3 screw vs 4 screw SU carbs

[JimmyZ1971]

Hi All

I got a really good deal on a set of Z Therapy SU carbs for my 1971 240 BUT they are 3 screw and I have 4 screw on her now. I don't want to remove the existing carbs until I know there is not any compatiblity issues. I have compared them as closely as I can and cannot see any design differences...BUT....I could be missing something !!! Any ideas out there??? Also what oil/fluid do you recommend for the float chambers?? Thanks for any help.

James

[hls30.com]

They are interchangeable-depending on the needles. We had a thread on damper oil recently-I don't remember the details other than two proper and easy to find oils were mentioned.

http://www.classiczcars.com/forums/showthread.php?t=35474&highlight=oil

WIll

Edited August 19, 2009 by hls30.com

[Bruce Palmer]

Three screw has provision for water into the carb base: 4 screws don't.

[JimmyZ1971]

Thanks for the information...looks like Sunday will be spent swapping carbs which really sounds like a perfect way to spend the day. :D

[26th-Z]

Learn someth'n new every day...

[mayitin]

Back in the old days every body used auto-matic oil I even heard 3 in 1, A few years ago there was a good article on one of the site ( I can not remeber what site it was ) but they stated that the best oil is 20W, I live in the outh and I can not get the 20W so I use auto-matic oil. My personal experience with the 3 bolt is that if you want to change the needle seat or jet is not as easy as the 4 bolt also the disk on the bottom is to thin for my fingers. I was just educated by B Palmer about water on the base, there is so much to know. By the way Z theraphy carbs are great.

[DeesZ]

Damper oil -

My FSM dated 1972 recommends "Mobile Oil SAE 10W-30 for damper oil. Do not use SAE #30 or higher weight oils."

If you wish to check the reference, this is found in the book under Engine, Fuel System, SU-Type Twin Carbs, Page EF-26. There may be other references in the book as well, but I came across this last night while researching something else. I hope it helps. (I have also successfully used SAE 5W-30, Marvel Mystery Oil, and 3-in-1 oil.)