What is considered "normal" oil use for SU carbs?

[Otto Skorzeny]

I'm the current caretaker of a 1972 240Z that didn't get all its regular maintenance as it should have.

The oil in the SU carbs seems to get "used up" pretty rapidly. I check it every time I drive it and usually have to top it off. On drives of 30 to 40 miles if there is a lot of stop and go traffic I will need to fill up the reservoirs before returning home.

Are their seals or anything that wear out which cause excessive oil use in the dampers? I suspect that the car was driven for a long time without ever having the oil reservoirs checked. Could the piston have worn the cylinder such that more oil is lost than normal? On long trips on the freeway (100+ miles or more) where the driving is pretty steady the oil is usually ok when I get to where I'm going.

What is considered normal use for the oil in these carbs. The owners manual says to check it every month or two which seems logical but mine definitely needs to be checked before each drive and usually needs some added.

Thanks for your help.

[SteveJ]

Seals dry out, especially if they don't have the proper fluid around them. Z Therapy (Welcome to ZTherapy.com) can help you. Send a PM to Bruce Palmer.

Meanwhile, where in Atlanta are you residing? Find the Georgia Z Club on FB. You can find out about local events and meet some fellow Z people. There is a car show in Marietta on Saturday that I plan on attending as are some others.

[240260280z]

The oil only should be filled to ~ 1cm below the tube in the big aluminum piston. It just has to cover the plunger piston and everything below it. It will never stay filled to the top.

If you over fill the tube, the excess overflows into the bottom and gets sucked out through the holes in the base of the big piston.

[Captain Obvious]

There are no seals involved. Everything is metal on metal. Nothing that would dry out.

Here's a recent thread that dealt with the same question:

http://www.classiczcars.com/forums/carburetors-s30/46939-su-oil-consumption.html

[Otto Skorzeny]

Wow. That thread was entertaining and somewhat enlightening . My problem is identical to Jetaway's.

I fill the dampers to the proper level - not above. I drive it and the oil level drops below acceptable level. This results in sputtering during hard acceleration. When I add more oil, the engine smooths out and runs like it should.

It doesn't look like any reason was found for this in that case, though. Maybe I'll email Jetaway and see if he ever figured it out. I too use 20 weight oil as specified in the manual. I even tried gear oil to see if thicker oil would a.) work and b.) last longer. No such luck.

I should also point out that just like his, both my carbs lose oil at the same rate. Also, I used ATF for awhile and there's no trace of pink left in the reservoir so that stuff's going somewhere.

[240260280z]

Maybe some oil-pan sealant at the bottom of the tube is all that is required to stop weeping through the margins.

[Captain Obvious]

Since the time of that other thread, I have disassembled a round top suction piston and have verified that the round tops are machined from one piece of steel and don't have any seams at the bottom. So if you're running flat tops, they might leak out the bottom, but your round tops can't.

I'll snap some pics of the round top tube in disassembled state when I get the chance.

I know that won't help your diagnosis though... Only makes it more difficult.

[Stanley]

Not sure but I think my problem with disappearing oil was partly due to pushing the plunger down and pulling it up too fast - makes the oil slosh out. I think you're supposed to let the plunger sink on it's own, and lift it out super slow. In my case the disappearing oil syndrome seems to have more or less gone away on it's own, which makes me suspect my original observation. Still, it's a fact that you need to top off the oil from time to time.

There's the Heisenburg (spelling?) theory about that, measurement affecting item being measured. Maybe it applies to SU carbs.

[SteveJ]

There's the Heisenburg (spelling?) theory about that, measurement affecting item being measured. Maybe it applies to SU carbs.

Are you certain about that?

[Stanley]

Uncertainly not.

[Otto Skorzeny]

I admit it is pretty weird and since I couldn't figure it out myself I came here. The level only goes down during operation not when the car sits as far as I can tell.

I do not shove the rods with the pucks on the end in the reservoirs. I let them slide down on their own.

This might seem weird but I smelled each of the pucks today to see if I could detect any trace of the ATF which I had used in the past. I could not. I did detect a faint odor of gasoline, however.

Is that possible? Is that a clue? Was the fuel smell just the ambient odor of the under-hood area? I don't think so because I walked away and took another wiff.

'Tis a puzzlement.

I'm just going to keep checking it and add when its low or starts stumbling on acceleration. Seems like the easiest solution.

[Captain Obvious]

Tis a puzzlement to me as well, but I can tell you... You aren't the only owner with this issue.

I, on the other hand, ran SU's for quite some time and never had this problem. I don't ever remember having to add fluid. Unless I took it out on purpose, it never changed.

And I doubt the gas smell has anything to do with it. You pull a part out of your carb and it smells like gasoline? Color me surprised!!