10W/40 According to the Haynes Manual 

I've always used automatic transmission fluid and it works well.

I think "foot valve" means "float valve"  and "Se one" means "Second" start.
Either auto-correct caught him or he types like me!

Oh, and when Ian says "blow into the fuel inlet" he means with your mouth not an air compressor. The inlet valve can only withstand 4psi at most.

How it works:

When you "step on the gas", you open the throttle plate, which provides a sudden increase in vacuum between the throttle plate and the "bridge". The bridge is where the main jet allows fuel into. the air stream. When the engine is off, the dashpot is down touching the bridge and the long needle is all the way into the main jet. At idle it is just about the same condition. There is minimal air flow across the bridge and wiht the jet nearly closed, very little fuel is released. At open throttle, vacuum has pulled the dashpot up and with it the needle. So, there is ample air and fuel supply.

When you open the throttle, the vacuum will lift the dashpot and needle quickly, which results in a momentary lean condition because the fuel flow does not match the sudden inrush of air. The dashpot damper overcomes this by slowing the rate of rise of the dashpot. This keeps the amount of vacuum between the throttle and the bridge higher and restricts the flow of air. Because the vacuum is high, it can suck more fuel from the jet providing an accelerator pump effect. Eventually (in a few seconds) the dashpot rises to an appropriate level for the amount of throttle, providing the right air and fuel supplies.

If your dashpot damper has low or no oil, and the dashpot rises too quickly, the engine will stumble because of a momentary lean condition. This can be modulated by adding a bit of choke, which richens the mixture to make up for the problem. Too thin an oil will give similar results.

If your dashpot damper oil is too thick, the dashpot rises too slowly, resulting in an excess of fuel (too much "accelerator pump" action), and the engine stumbles or lags. Addition of some choke as a test would make the situation worse.

Note I wasn't suggesting to run with some choke all the time - just to test if it makes the stumble change.

1963SV3 mentions that "If it runs better with some choke then you have the wrong needle". although true, more likely the "idle" mixture may be set wrong. It it set at idle but affects mixture throughout the range of engine speed. On both the HS and The HIF types, adjusting the mixture pulls the jet up or down exposing a different needle diameter at the top of the jet.

On a HS type, the mixture is adjusted by turning a nut to move the jet up or down. The "choke" mechanism uses a lever to pull the jet farther down for better starting.

ON the HIF type, there is an adjustment screw that operates an internal lever to move the jet up or down. It is a real pain to get right because the Lever has a bi-metal action which bends it as the engine warms up, leaning out the mixture for supposedly better economy and less pollution. For the home mechanic trying to adjust it, the heat from the exhaust manifold warms the bottom of the carb and the bi-metallic lever, causing it to move. So, as you try to adjust, the lever continues to compensate. You need a BIG fan to simulate the airflow of the car moving down the road.

The attached picture shows the open fuel bowl of a HIF carb, It is upside down. The orange piece under my thumb is the float. On the left side, to the right of the float pivot is a brass tab that regulates the fuel inlet valve (needle and jets. The white thing is the bottom end of the main jet.  The silvery thing behind the jet is the bi-metallic lever. The big screw in it is the pivot screw, not the adjustment screw. It comes in from the side of the bowl behind the lever. When you turn that screw in, it pushes the lever which pulls the jet down. So "IN" = RICHER. Counter-intuitive!

There are a variety of "official" opinions as to what the damper oil should be. My 63 Cooper Drivers Handbook says use 10W30, the 68 Cooper S Drivers Handbook says 30wt, my 60s SU manual says 30wt and the (mid 60s) factory workshop manual says "top up with the recommended oil" (but can't find any recommendation in the manual??). I use 10W60 synthetic. All this advice refers to cars with HS carbs ..but I would expect HIFs would be the same.

Working out what you have is quite simple. Does your carb have a float chamber sticking out the side? If so, it's an HS .. or maybe an H. HIFs have their float chamber underneath. All my comments are applicable to the HS type so won't be useful if you have an HIF..

However, I agree with Dan and 805 - the best stuff is what works. My engine runs quite happily with no oil (oops) ... but it's a b^&&%er to start. It's worth noting that its damper oil ...and only comes into play during initial acceleration. If you have a bit of a stumble or flat spot as you initial hit the accelerator then maybe the oil is an issue. It won't affect the general richness of the mixture. I would disagree with Dan in that, IMHO, the choke has nothing to do with it. If it runs better with some choke then you have the wrong (mixture) needle..... changing the oil won't fix it.

Testing the needle (float needle that is) and seat is also quite easy. Hold the carb upside down (or just remove the float chamber lid and up end that). Blow into the fuel inlet ... you shouldn't be able to get any air past the seat.

BTW, what's the "foot valve" you refer to in your post? Maybe an HIF thing?? Couldn't interpret "....it warms up on the Se one start..." either??

Cheers, Ian

I think I have an HIF, I can't figure out how to attach a photo

+1 to what 805minis wrote. Add to that ambient temperature (i.e. summer vs winter).

Before driving yourself crazy testing oils, make sure your carb is functioning properly in every other way.
What type of carb do you have - HS of HIF?
Make sure your choke mechanism is working properly.
If you have a HS type, with the fuel bowl on the side, you can remove the cover and check for dirt and varnish. Difficult to do with the HIF type.
The bowl should be not quite full with the float in place. If the float valve is not sealing or the float isn't floating properly, too much fuel will come in and would flow out the overflow.

Assuming your fuel bowl is functioning properly, the next thing to check is that the dashpot  is rising and falling smoothly. YHis is best done with the oil damper piston removed. Some carbs have a little pin on the underside to lift the dashpot. If not, you remove the air cleaner and use a finger to lift it. When you let go, the dashpot should drop with a definite click. If it doesn't, the long needle is sticking in the main jet and needs to be fixed.

I'd start with 10W30 to see if that makes an improvement. 20W50 is probably too heavy. You will be getting close if it runs right but application of a little choke makes the problem return. If it runs better with a little choke, you need a thicker oil. If it runs worse, you need a thinner oil.

I'm sure there will be a lot of differing opinions on this one. I've heard of people running everything from gear oil to brake fluid. From personal experience, I have not found any single answer to this question. I've worked on cars with very similar engines that preferred very different dashpot oils. Everything from the cam, weight of the flywheel, your altitude, and even how restrictive your air filter is will make a difference as to what viscosity of oil your carb prefers. The only way I've found to get it right is by changing it out and putting it through its paces. 

I have always used ATF (automatic transmission fluid), I've used it in all of my SU carbs since my first Mini in 1969.

What type of oil does everyone use in their carb? I’ve been having issues where my engine is running rich after it warms up on the Se one start and wants to stall after applying gas. After talking to a local carb repairman, he said it’s either that the oil I put in my carb is too heavy (20w-50), my needle and seat valve has gone bad or I just have a bad foot valve. I’d be curious what everyone’s thoughts are