oil type to top su carb

[hf240z]

hi everyone,

1. what type of oil do you use on an SU carb for Datsun 240z?

thanks,

hf

[jcdozier]

Haynes manual says 20 weight. That's what I use and it seems to work well. You can go lighter or heavier, depending upon your driving demands, but don't use a multi-grade oil.

If you search the IZCC site you will find posts that offer all kinds of opinions on alternatives to the 20 weight. I've even heard that some folks use different weights for winter/summer driving. Here in S.C. the 20 weight seems to work well all year.

[Ken]

I have used automatic transmission fluid in my 72 240Z SU's here in the Northeast. Temperatures seen run from 105 down to the upper thirtys. Excellent performance

[Marty Rogan]

I have been using Marvel's Mystery oil for a little quicker throttle response.

Marty

[jcdozier]

If anyone is interested in more SU information, go to the IZCC site (www.zhome.com) and scroll down the left side to the "Index of Technical Articles, open it and go to the article on "SU carburettors for the early Z".

Here's a bit from the article by Scott Fisher:

"You can also modify your car's throttle response characteristics slightly by adjusting the viscosity of the oil in the dashpot damper. SUs are set up so that a thicker oil will resist the piston's attempt to rise in the dashpot for just long enough that the engine's increased load (when the throttle is opened) will pull more fuel across the bridge; this enriches the mixture and temporarily bumps power up to help the engine achieve higher speed more readily."

Happy Tuning.

[Royce]

By the way... MSA does list 20 weight oil in their catalog. I don't remember what I used last time, maybe I should go check the levels, but I am probably going to use ATF, if I have some laying around, or 20 weight. Also, make sure whatever you use is designed for the operating temperatures of a vehicle. For example, I believe you can find 20 weight sewing machine oil but I don't know if it will hold up in the engine compartment of a Z.

Have fun!

[guy_geo]

I may look like an idiot, but why do we have oil in these carbs?

And do we need to oil them regularly?

[Royce]

It's a SU thing, I cannot think of another type of carb that has oil in it.

Very basically the fuel jet neddle is installed in a suction piston. As engine speeds increase and vacuum increases and the suction piston is drawn upward (maybe pushed a little too??) into the dome of the carb. In the middle of the suction piston there is a well with the "carb oil" in it, kinda like a water well. There is a smaller piston, for lack of the technical word, that hangs in the well on a rod that is attached to the black plastic caps you see on the outside top of the dome. As the suction piston rises the oil in the well is forced past the smaller piston. If there is no oil the piston snaps up instantly, if there is heavy oil the piston moves up very slowly.

The key here is that as more air moves through the throat of the carb the suction piston rises and the attached tapered fuel jet needle is pulled out of the fuel nozzel and adds more volume of fuel into the air. Also, at the same time it allows more air volume into the carb.

I have heard of some racers running with no oil but it may be just another legend tail. I would think this might cause damage to the suction piston or the inside of the dome, suction chamber and would not be good for fuel economy. Plus it would probaby only work if you didn't have to stop at a stop light and idle for 2-3 minutes. But in the right carb setup lighter oil can improve throttle response. I suspect heavier oil would be good if you want better gas milage!?

Boy that was harder to explain than I thought it would be. I tried to keep it simple, but that is not necessarily one of my stronger characteristics. Does that make sense? Corrections & comments welcome!

Have fun!

[Royce]

That long response and I forgot about the "oil them regularly" question, I suppose it helps to reread the original question before pressing reply!

You should check the oil level frequently but the oil should not break down like motor oil. Depending on your driving style it could probably be good to change it as often as you change your break fluid, probably once a year if you are driving hard.

Check the fluid by removing the cap and looking at the rod. There are two lines on it like a dipstick. I believe the manual says to keep the fluid between the lines. There are a few other "trick" ideas about what the proper level is but using the factory marks is a good guideline. It also makes sense to fill both carbs equally, assuming all parts are worn equally in both carbs.

Okay, I am done!

[AxtellZ]

If i remember correctly, in the original SU video from Ztherapy, Scott says to fill the carb right up with oil, then just lift the piston/damper by hand all the way up. Any excess oil will flow over and burn up--

anyone feel free to correct me if i'm wrong! JA

[beandip]

The recomended weight is 20w . I have been using Mistery oil for a long time but have noticed a lag off the line if I dident have my foot in it. Just a slight stumble and couldent figure it out, but I recently tried some 20w and all is right with the world. The thicker oil allows the piston to rise slower thus giving a richer mixture into the engine before the slide rises. The difference is only a split secound but just enough to clear up the problem. If you are running with an open throttle , like when raceing , then it dosn't make much difference because you are always in the powerband and with elevated rpm. but for street use do your self a favor and try 20w. The only place that I have found it available is at the local motorcycle shop and it is sold as fork oil. As far as the level needed if you over fill the dampiner it just runs out and is burned with the fuel no problem.

[guy_geo]

Well, as a teacher I was saying to my student after a class:

You will be less dumb before going to bed tonight..

Now I feel like a studentLOL