Pneumatic tool oil in SUs

[ninjazombiemaster]

I'm still trying to get my mix right. I've tried ATF, 5w-20 (bad choices), 10w-40 mixed with 5w-20 which made it more drivable. My local motorcycle shop gave me some bottles of various weight fork oil to find my perfect mix but I haven't gotten into it yet.

From what I've seen so far 15wt anything should mostly help prevent backfiring and leaning out at WOT, factory 20wt is probably still ideal.

Heavier than that and the oil thickens when cold enough to behave as an accelerator pump effect which could be good for performance, but too heavy and you'll just waste gas.

Too light and the piston will rise rapidly at WOT and create a lean condition when your engine needs the fuel the most. This can cause a lean pop and cause the engine to stall at worst, and at best will damage performance.

Modern engine oils with multi-viscosity can cause the oil to be less viscous when the engine in cold. Good for engines, not necessarily for SUs. This can cause backfires and poor performance when cold, that smooths out when the engine warms up. That's what I experience with my car, and what I'm trying to curb with the fork oils.

[Captain Obvious]

The dampers are not just for WOT operation. They are for any time the throttle valve is transitioning from a more closed position to a more open position. All that damper does is slow down the upward movement of the piston.

The thicker the oil you use, the more it will slow the piston rise.

The more you slow the piston rise, the richer you will run on throttle transition. The important detail is transition. Not WOT.

It is supposed to act like an accelerator pump... It's purpose is to temporarily richen the mixture on throttle transition from more closed to more open.

Modern engine oils with multi-viscosity can cause the oil to be less viscous when the engine in cold.

And this isn't the way the multi-vis oils work. They are not thinner when cold. They are thicker when cold, just like any other oil. The trick to the multi-viscosity stuff is that they don't get as thick as they would get if the modifiers weren't added.

For example, think of 10W-30 like this:

"When cold - It will be as thick as a 10 weight would be when cold"

"When hot - It will be as thick as a 30 weight would be when hot"

That cold 10 weight is still way thicker than a warm 30 weight.

[Captain Obvious]

I've put 15w40 in it now, but feel it is very heavy to lift the pistons up. The car revs and pulls fine however. I think it's best to put a lighter oil like sae30 that says in the fsm ?

It's supposed to be hard to lift the pistons.

15W40 is higher than the manual recommends (and higher than I've had success with) but if it works for you, then go for it?

Are you blowing black clouds on acceleration?

Are you fouling the plugs?

Is your gas mileage really low, say below 17 mpg?

If your answer is "no" to all of the above, then are you looking for something more than that?

[ninjazombiemaster]

And this isn't the way the multi-vis oils work. They are not thinner when cold. They are thicker when cold, just like any other oil. The trick to the multi-viscosity stuff is that they don't get as thick as they would get if the modifiers weren't added.

You are correct, I misspoke (typed?). The polymers reduce the thickening of the oil in cold temps. But that thickening can be good for cold SUs, is the point I'm getting at. You're second point is also correct. It doesn't effect only WOT, but all transitions. However, the benefit remains as long as you don't go overboard. That could also depend on the needles you use, and other factors as you know.

This is all in my rather limited experience, of course.

[bartsscooterservice]

It's supposed to be hard to lift the pistons.

15W40 is higher than the manual recommends (and higher than I've had success with) but if it works for you, then go for it?

Are you blowing black clouds on acceleration?

Are you fouling the plugs?

Is your gas mileage really low, say below 17 mpg?

If your answer is "no" to all of the above, then are you looking for something more than that?

I got b7es plugs in it now, the core is dark white, and the metal edge of the plug is a dark/a bit sooted. which indicates rich idle but good condition on half to full throttle. But I will get the b6es plugs to match the stock heat range. I didn't have any other plugs in storage now. It stumbles a bit on cold start, but after 20 seconds it idles fine, and no smoke once warm. I don't need choke to start it, and it's pretty cold now !

I do notice the engine has trouble when above 5000 rpm, could also be bad ignition though..

Edited March 29, 2013 by bartsscooterservice

[John Coffey]

The more you slow the piston rise, the richer you will run on throttle transition.

Not really true. Remember, the piston controls where the needle is in the jet. So, if the rise of the needle is delayed while the engine sucks more air (which it can between the piston and the bridge) you can easily get a transient lean condition until the needle get into the thinner part of its taper. If you want to richen the transition run a lighter weigh SU oil. Marvel Mystery Oil is what I've run for decades.

[ninjazombiemaster]

Blue once got into detail on that "paradox".. there are so many dynamics at work that yes, its possible. It really depends on the needle you run, how much air you pull, etc. Also, the lighter the oil the more subject to bumps in the road and driving conditions. What works for one's SU setup easily wont on another's. The way mine is set up will go very lean with a light oil.

[Captain Obvious]

Not really true.

Counterintuitive as it may be, it really is true. There are lots of sources to cite, but here's two that should carry the most weight around these parts:

First is from Skinner's Union's themselves: SU Carburetters The S.U. Carburetter

Most S.U. carburetters incorporate a piston damper, the function of which is to restrict the speed of lift of the piston on snap throttle openings, and to allow the piston to fall at its normal speed on throttle closure. This one way damping is obtained by means of a non-return valve situated at the base of the damper.

When the speed of piston lift is retarded an additional air depression is put on the fuel in the jet resulting in an increase in the quantity of fuel discharged. A richer mixture is thus obtained until the piston resumes its position of equilibrium. This enrichment is necessary to provide satisfactory pick-up. The piston damper also improves cold starting and driveability from cold.

Second is a snippet from the Datsun factory service manual (1973 pg EF-9):

[Captain Obvious]

Remember, the piston controls where the needle is in the jet. So, if the rise of the needle is delayed while the engine sucks more air (which it can between the piston and the bridge) you can easily get a transient lean condition until the needle get into the thinner part of its taper. If you want to richen the transition run a lighter weigh SU oil.

And at the risk of typing a bunch of stuff you knew already... There are two things that control how much fuel will be pulled in. First is the gap around the needle, and the second is the speed of the air through the venturi.

The bigger the gap, the more fuel. Simple.

The faster the air through the venturi, the more fuel. Not as simple.

The trick to the damper is that when you punch it, the engine sucks a lot more air, but the piston will not rise instantly because of the damper. Because of the slowed damper rise, that "lot more" air has to funnel through a smaller opening and Bernoulli says that will cause a lower pressure. That lower pressure pulls more fuel in even through the smaller neeedle gap.

Once you have reached steady state, the gap is the only thing controlling mixture, but in transition if you slow the piston rise enough, you can actually create a transient rich condition EVEN RICHER than the steady state will be. That's the simplistic beauty of the SU design!!

So in short, a thicker oil will slow the damper more which results in a richer transient mixture.

[bartsscooterservice]

The Original manual says to use 10w30 weight oil and recommends not going heavier

[siteunseen]

Old thread but I'll play! I use 20W 3in1 with the blue label.

[SeafireXV]

Any 20w oil will work. That's what all cars equipped with SU's used from the factory. There is even a specific brand of SU dashpot oil still available in the UK. I tried 30w oil in my carbs for a while and it made them run too rich. Terrible gas mileage and lots of exhaust staining, although the car ran fine otherwise.