what kind of oil for pre 73 SU carbs?

[mattymatmat]

This might sound like a really elementary question, but what kind of oil do i put in the SU carbs? and how do I know when I've put enough? Do I just fill it till the dip stick thingy on top is covered in oil?

[MrMoonshineMan]

well, Precision Z, a total Z garage near me, told me to put in tranny fluid, so that it covers the big ball on the tip. i'd love to hear what some of the z-heads on this board have found works well.

[Dave Sommers]

I do believe that the carb dashpots call for 20 wgt oil, so ATF might be very close in viscosity to that. You can get 20wgt at some motorcycle shops as it is also used in some forks. Too heavy a oil or not enough will effect the motion of the sliding piston.

If you take a close look at the dashpot damper rods you may see they are marked just like a dipstick. Put enough oil in to fall between the marks. If no marks, enough to cover the end parts and just a touch more.

[mattymatmat]

Sounds good, tranny fluid or 20 weight, I've got a few Z books but either I didn't look good enough or they didn't mention that little tid bit of information. Thanks for feedback!

[jcdozier]

There was a long string posted within the past few weeks on this subject - you might be able to find it with a search. Summarized, it said ATF or 20 weight are good, multi-grade or Brake Fluid are bad. Without a poll, I suspect most of the SU's out there have ATF in them, because it's more readily available than 20 weight oil.

As to fill level, overfilling isn't a problem since any "overflow" will seep down the side of the piston and mix with the fuel coming in through the carbs. Don't underfill.

[Ken]

To avoid overfilling I use a medicine dropper to add ATF until the fluid level is between the two indicators just above the plunger. Less messy and who wants ATF running into the carbs and bell housings which can gum and cause sticking or slowing of the piston.

[Phacade]

On my SU carbs I had before I had to sell my 240Z [single tear], I always used 20 weight oil. Tulsa being the oil capitol of the world helps just a little in finding it. I have heard some using Marvel Mystery Oil, however have never tried it. I talked to Scott at Ztherapy about this. He said to use 20 weight oil if you can find it or ATF (Mercon) if you cannot find any 20 weight oil. He said if you add extra don't worry because it will burn off or equalize itself. MSA sells it in little packages but IMO bigger is better and costs about the same. I have an unopened quart of 20 weight oil that I now no longer need if anyone is interested.