Morris Minor Owners Club

I have been to view many cars and one of the things I look for which gives a good indication of how it has been looked after is the oil level in the carb dashpot.

SU's guzzle this, and it should be checked every 200 miles and replenished with FRESH 20/50w ENGINE OIL. Otherwise you will get piston flutter, poor fuel consumption and reduced carburettor life.

Unscrew the small brass or black plastic cap from the carb, and the oil level should be about half an inch above the easily visible inner lip.

Any fool can do this but it is one of the most overlooked, and easiest, jobs.

Do not use cheap oil or you will disappear in a cloud of blue smoke!!!!

Runs much better with 3 in 1 oil - and it doesn't need to be visible, in fact it pretty much sets it's own level depending on how high the piston ever lifts. A hard run engine where the piston lifts right up will show little oil in the dashpot. Fill it up -and will just overflow the excess oil next time it lifts right up! Especially if it is 'thick' oil that doesn't slide past the piston..... Where else can the oil go? It's a sealed well in the dashpot ........ They only 'guzzle' the oil if overfilled in the first place !

If it comes out of the vent hole when the piston rises, you've overfilled it!!

Oil is good for God's sake!!

Use clean, fresh engine oil.

Never used 3 in 1 and never will, but if you recommend it.... but it's good on bonnet catches and door hinges anyway

Always use 3 in 1 oil. I imagine engine oil would be too thick and restrict damper movement, I also very rarely have to top mine up although I check it regularly. You only need enough to "damp" the movement.....

It states quite clearly in the owners' manual use engine oil. Back in the day this would have been straight 30, now 20/50. It works fine. I thought 3 in 1 was a thin machine oil. Certainly it is more expensive than using engine oil for the amount of times you have to top the dashpot up.

Whatever oil you decide to use - 3 in 1 or engine oil - the point is this advice to owners to help keep their engine running in good order. Either way don't mix oils, you must drain it before filling with another type.

3 in 1 gives a better response if the carb is set up correctly. Mine uses ~ 1 teaspoon per annum......... Don't see why not to mix oils if that happens to suit? Out of interest - BMC Works used EP90 in the dashpots of the rally Minis, to stop the pistons flying up and down uncontrollably as the car bounced along on rough stages! But they also specified a special damper piston assembly with increased free movement on the little piston. There's a LOT more to setting up an SU than just filling it up with engine oil...... The weight of the piston assembly can be reduced - the strength of the spring can be reduced (or increased!) and the viscosity of the oil varied - as well as the previously mentioned free play movement. And of course - the flow passage can be streamlined by bell mouthing the carb body, rounding the incoming edge of the piston assembly, smoothing the walls of the carb where the airflow passage intersects the vertical passage where the piston moves, knife edging and thinning the throttle plate, removing the protruding excess length of the fixing screws for the throttle plate, thinning down the throttle plate shaft, and even modifying the jet bridge, although this last can make the carb 'difficult' in road conditions. Interestingly - many moons ago my track Mini when running in a 'single carb' class at a Sprint event (two laps of Ingliston) ran best of all with NO damper (or oil) in the carb. It gasped horribly and would have been impossible on the road - but as long as due allowance was made for the gasp when flooring the throttle - it ran much better round that track!

i use 3 in 1 good enough for me ,and cheaper than the su damper oil.

I have always occasionally topped up the dash pot with 3 in 1. Only occasionally as I have to remove the air silencer to get to the carb. As long as there is resistance when pushing the piston up with the finger then the oil level is ok

mike.perry wrote:As long as there is resistance when pushing the piston up with the finger then the oil level is ok

Quite right.

This is an interesting discussion, and something that I have explored in quite a bit of detail on my Rover.
In my experience, there is often quite a bit of disagreement over the level of SU piston damping required, and this seems to come from a misunderstanding as to the exact purpose of the damping in the first place.

As we all know, the throttle opens, there is pressure imbalance across the bridge - vacuum on one side, ambient pressure on the other - so the piston rises a set amount for a set vacuum, exposing a set station on the needle. All easy stuff. The damper is the misunderstood bit. Its purpose is to resist the immediate rising of the piston, holding it down nearer to the bridge for a short period. This has the effect of restricting air flow, and therefore accelerating the charge (speed) of air across the bridge during the early stages of acceleration. The piston then begins gradually rising more readily, increasing the area, improve breathing, slowing the charge speed and, of course, increasing fuel delivery. If you plot the rate of the piston rise on a graph, it's a curve. The fall-time should be a straight line. When behaving correctly in this manner, the piston achieves the same effect as an accelerator pump in a fuel injected engine, and is the main reason why properly set-up Constant Depression (and particularly SU) carburettors are as close to EFI fuelling you can get.
So having the correct viscosity oil is massively important.....

But..... anybody who has a twin carb setup will know that no matter how clean you get the castings, the 'dry' piston fall times will never be exactly the same without professional polishing. In this respect, age, wear and tear, or just insufficiently tight assembly tolerances mean that every carb is subtly different. So there's one variable. The other variable is the source of the vacuum itself - the engine. Leaky valves, stem seals, or piston blow-by is going to have a dramatic effect on the level of vacuum applied to the carb, and therefore to all parameters of volumetric flow.

I'm a great advocator of experimentation, and found 15w40 to work best in my Rover's carbs. In my experience 3in1 is too thin to provide any resistance to piston-lifting, and causes them to 'jump' at the slightest blip of the throttle, negating its purpose. However, all this depends on the size of the billets on the damper stem, which are manufactured to fine tolerances, but not consistent between AUD numbers. If the tolerances are very tight, 3in1 may be optimum. There are just too many variables to go off the workshop manual.

If the oil is too thin, there will be a delay in acceleration after the pedal is pushed due to the lack of increased charge speed.
If it's too thick, acceleration may ultimately be strangled, and the slow fall-rate of the piston will result in high emissions on the overrun and generally richer running under throttle-on/throttle-off urban driving.

A further consideration is the fluctuation in oil viscosity due to temperature change. 20w40 will become thicker as the engine warms up. ATF has the benefit of maintaining a consistent viscosity (SAE20) irrespective of temperature.

Whether any of this would make a discernible difference on a Minor is down to the individual engine.
When all's said and done, I must concur with this sound advice:

JOWETTJAVELIN wrote:the point is this advice to owners to help keep their engine running in good order

Swapping dashpot oil is going to have precious little impact if everything else is off-kilter.

Sorry my tuppence-worth turned into quite a lengthy essay, but I thought it might be useful for those interested in extracting the last 1% from their engine!!!

Michael

I am afraid there are several very fundamental errors in Michael's analysis. In the context of a messageboard, it would be unreasonable to attempt a basic course in fluid mechanics, so I'll just mention two misconceptions.

Inlet manifold vacuum is not what lifts the carburettor piston - there's a throttle in the way. The piston adjusts its position so that the force exerted by the "suck" under the carb piston, that is roughly at the position of the jet, exactly balances the weight of the piston plus any contribution from the piston spring. This suck is caused partly by the flow resistance of the air cleaner, but mainly by the air accelerating as it passes under the carb piston. The needle then allows the flow of fuel to be matched to the flow of air - necessary because spark-ignition engines only run with the air-fuel ratio in a quite narrow range.

Open the throttle, more air flows in, so the carb piston rises. If its rise is delayed by the damper, there is briefly more suck under it, and this draws more fuel through the jet, giving a briefly richer mixture.

As for choice of oil: 20W40 does not become thicker as temperature rises: it simply doesn't thin out as quickly as a monograde oil would. I can't find any data for 20W40, but 20W50 typically has a viscosity of 169 cSt at 40C, but only 19cSt at 100C. A typical Dexron IIID ATF is 38cSt at 40C and 7.9 at 100C. It's the actual kinematic viscosity, expressed in cSt, that affects the drag on an object, like a dashpot damper piston, moving through it. For ordinary road use, there are so many other variables that I find fussing about precise grades is a bit precious: I use whatever's in the oilcan.

The damper operates within a closed-end steel tube. As long as the oil is below the top of this tube, the damper damps. If it's above, the top of the oil does nothing, and it will get sucked down the small clearance between the outside of this tube and the carb, and thence into the engine. The advice to fill it to just below the top of the tube is because you're usually filling it with the damper piston out: when you put the cap back on, the piston will displace a bit of oil and the level will be correct.

Kevin

The other problem is that the piston is a huge barrier to air flow until it pulls up clear of the flow. And so if it is 'held down' by viscous oil - the airflow in to the engine is blocked and acceleration sluggish. This is where (say) the Weber DCOE carb gains popularity - when you open the throttle - air rushes cleanly into the engine - no obstructing piston in the way! And of course the DCOE richens up the mixture at this point with a huge squirt of raw petrol through the acceleration pump jets. As detailed above - the SU has to do the same job by briefly holding down the piston so the air flow through the restricted gap rises -and sucks out much more fuel, until the piston staggers upwards - allowing more air to flow but also revealing an ever larger annulus for the petrol to flow into the airstream as the tapered needle is pulled further out of the jet. So it's a matter of balance - restrict the inflow of air to gain fuel richness, or perhaps run the carb on the rich side of optimum so the piston can be allowed to rise quickly without causing the mixture to go too weak. Each to their own - but I find 3 in 1 works well enough in my twin HS6 carbs (TR7 engine) - although I do have to run the 'choke' out for the first mile or so, or the spluttering and backfiring is quite dramatic. And also on my single HIF38 on the Minor -again, a little extra richness for the first mile or so -then it's fine.

I accept that some of those points are in indeed errors.
Regarding the choice of oil, I had it on the authority of an automotive engineer of some reckoning that fluctuation in ATF viscosity was very little between normal operating temperatures and ambient 'cold', therefore making it a far more 'stable' choice for a carburettor than mineral multigrade. With the peak operating temperature being much lower in a carb than in the engine oilways anyway, the fluctuation was effectively nil. However, if the data proves otherwise then I accept this to be erroneous. I can't say it will stop him swearing by it, mind!

As regards vacuum lifting the piston, perhaps this was a case of poor expression. I was describing the point just as the throttle butterfly opens, thereby 'sucking' air from the outside across the bridge. Of course manifold vacuum is in the manifold, but with the throttle open the pressure imbalance causes air of atmospheric pressure on the filter side to be forced over the bridge, through the butterfly and into the manifold. That air contained within the dashpot is also forced out of the hole, decreasing the internal pressure and causing the piston to lift accordingly.

If by 'basic misconception in fluid mechanics' you are referring to the curve of the rate in which the piston rises, then I accept that the resistance of the damper is directly proportional to the force placed against, and not progressively less resistive over a greater distance. But would argue that the inertia of the piston in the lowered position is sufficient to cause a hesitancy in its upward movement.
Again, this is based on discussions with others. I am more than willing to take conflicting evidence back to them. My philosophy with these things is to learn through discussion and refine a point, not to see who can shout the louder.

For risk of being seen to make a mountain out of a molehill, I will reiterate my point from earlier - faffing with dashpot oil will have precious little effect if tolerances and settings elsewhere are not A1. Compression, valve timing, lift, duration, ign timing, etc are all going to yield far more if correctly set.

Michael

I agree - it's a very small part of the engine management systems.

I also use 3 in 1 oil with good results on my cars.

Damn, just filled mine up with 20w50-thats what my MOT man said. (ps he is about 60 so has worked on proper cars before!)

Never trust what these youngsters say......