Dash pot oil

[jocklow]

What proprietary oil can I get that is the correct viscosity for the dash pot dampers on a TR3A ?

[BlueTR3A-5EKT]

You know how to open a can of worms....

 

I use 20/50 usually in my HS6 carbs. Have also used the proprietory SU/Solex oil from Burlen Fuel Services which is SAE20 http://sucarb.co.uk/dampers-caps-oil/damper-oil.html

 

Not having any definitely changes the running.

 

Peter W

[Don H.]

SAE20 oil isn't that easy to find at one's FLAPS. I topped my carbs off the other week with 3-in-1 oil, which is pretty close to 20 weight (somewhere betweeen 15-20, as far as I can tell) and is everywhere, at least over here.

[Bobbie]

If you're feeling flush, then penrite do an oil specifically for dash pots which from memory is sae 20.

Obviously the thickness of the oil,, or indeed no oil has a direct effect on the speed of lift of the piston and therefore important for best engine performance.

In practice most of us put in a thin oil, 3 in 1 among these.

[Lebro]

ATF works well

 

Bob.

[Guest]

Back in the 60s, my dad used the oil from my mum's sewing machine for the SUs of his Austin. Later, he changed this for chain oil that he had for his motor bike. I think it's pretty close to 20 SAE.

 

Menno

[john.r.davies]

I've read people recommending oils from "Marvel's Magic Mystery Oil" (!) to power steering fluid.

There is nothing 'magic' about dashpot oil.

 

A "Constant Depression" carburettor - an SU or a Stromberg - is designed to maintain reduced pressure in the throat at a constant level, as revs rise and fall, and the flow through the throat varies.

This ensures that the amount of fuel drawn into the air stream is always correct to achieve the "stoichiometric" mixture that provides optimum power and fuel consumption.

The piston rises and falls to equalise the pressure in the throat and above the piston, carrying the needle that moves in the jet and controls how big is the orifice through which the fuel can be drawn into the air stream.

Without a dashpot, this would happen very quickly - the dashpot slows the movement of the piston.

 

On acceleration, the oil in the dashpot slows the piston's rise, so that the throat of the carburettor is narrower than ideal as the engine increases, the flow through it will be faster and pressure will be lower than usual drawing an excess of fuel out of the jet, richening the mixture, as is required for good performance.

 

The more viscous the oil, the more slowly the dashpot with the needle it carries rises and the richer will be the mixture, for longer as you accelerate.

 

So, the viscosity of the dashpot oil should be determined by your driving style and wish to balance performance and economy.

The best performance would be achieved by a thicker oil which would worsen fuel consumption.

But the potential for adjustment is limited, by the construction of the carburettor and the limited selection of oil viscosities, which may contribute to the bizarre list of oils that have been used.

Adjustments beyond the range quoted, 3-in-1 (20 or less) to engine oil (20-50) will not have any greater effect.

 

John

[RogerH]

Hi John,

thanks for that. I've always had it a*se about face thinking that a rapidly rising dash pot would put MORE fuel in.

But as you state the piston position dictates the air pressure in the throat and so the fuel flow.

 

Always learning

 

Roger

[Peter Cobbold]

And here's why the damper is needed, it does the same job as the throttle pump on Webers etc Its all to do with liquid fuel wetting the manifold walls. Bold is my emphasis:

Peter

=======

 

( pinched from a wiki:)

"

Here is my explanation of the role of the accelerator pump.

• Fuel enters the throat as atomised droplets. As the charge flows towards the cylinders the droplets vapourise to a large extent. At low manifold pressures (eg idle throttle position) the low pressure aids in vaporisation of the droplets. At high manifold pressures (eg high power) the higher pressure retards vapourisation of the droplets and the interior of the throat and inlet manifold is wet with liquid fuel. When the throttle is opened quickly there is a quick rise in manifold pressure which causes an increase in the amount of liquid fuel wetting the interior of the throat and inlet manifold. As a result of a net increase in the amount of liquid fuel wetting the interior there would be an increase in the air-fuel ratio (leaner mixture) of the charge reaching the cylinders. So when the driver calls for an increase in engine speed or power the result would be a leaner mixture that delays the increase. This can be avoided by use of an accelerator pump. When the throttle is opened quickly the pump injects extra fuel into the throat so the mixture in the throat is enriched. However, by the time this charge reaches the cylinders a little of the fuel has been lost to the walls of the throat and inlet manifold so the air-fuel ratio has fallen to approximately the best value for accelerating the engine.

When the throttle is closed quickly there is a rapid reduction in manifold pressure and a net reduction in the amount of fuel wetting the walls of the throat and inlet manifold. This delivers an extra-rich mixture to the cylinders. The extra-rich mixture isn’t a problem when the throttle is being closed because the driver is calling for a reduction in engine speed or power.

Edited May 10, 2015 by Peter Cobbold

[BlueTR3A-5EKT]

If you look at your damper piston the sleeve retained on the end has some up and down free play. There is a documented mod to increase this free play, by reducing the length of the sleeve. The free play occurs before the oil damping comes into action as the piston and needle assy rise or fall.

 

I will try to seek out why the mod was introduced. It was MG orientated if I recall correctly.

 

Peter W

 

PS I have used ep 90 oil in my dash pots during the winter to reduce the cold weather stutter. It saves having to richen the carbs by a flat for smoother winter use.

Edited May 10, 2015 by BlueTR3A-5EKT

[BlueTR3A-5EKT]

If you look at your damper piston the sleeve retained on the end has some up and down free play. There is a documented mod to increase this free play, by reducing the length of the sleeve. The free play occurs before the oil damping comes into action as the piston and needle assy rise or fall.

 

I will try to seek out why the mod was introduced. It was MG orientated if I recall correctly.

 

Peter W

 

PS I have used ep 90 oil in my dash pots during the winter to reduce the cold weather stutter. It saves having to richen the carbs by a flat for smoother winter use.

Here is the pdf from MGAGuru.com

Interestingly TR4A with HS6 carbs in the UK used AUC8103 dampers but the USA spec car used AUC8114 dampers in its carbs - Any comments?

 

 

Peter W

Change of carburetor damper samller.pdf

Edited May 15, 2015 by BlueTR3A-5EKT

[Peter Cobbold]

Here is the pdf from MGAGuru.com

Interestingly TR4A with HS6 carbs in the UK used AUC8103 dampers but the USA spec car used AUC8114 dampers in its carbs - Any comments?

 

 

Peter W

The later, shorter damper piston will give more rapid rise of the 'air valve' in response to sudden throttle opening, so less duration of the rich pulse. But how the old damper could restrict max speed - ...dunno. Maybe if the SU piston flutters with inlet valve openings, less dampening could have an effect and allow piston to rise higher...just guessing.

Peter

[BlueTR3A-5EKT]

The later, shorter damper piston will give more rapid rise of the 'air valve' in response to sudden throttle opening, so less duration of the rich pulse. But how the old damper could restrict max speed - ...dunno. Maybe if the SU piston flutters with inlet valve openings, less dampening could have an effect and allow piston to rise higher...just guessing.

Peter

I have pairs of both types and have tried them in my TR without messing with jet setting, found no difference in the driving.

Peter W

[Peter Cobbold]

I have pairs of both types and have tried them in my TR without messing with jet setting, found no difference in the driving.

Peter W

I suspect SUs used used flow benches and trial carbs kitted out with manometers and pitot tubes. Perhaps that's how they arrived at the damper mod. AFAICS the interplay of depression sources at the jet seems to me to be too tricky to calculate in the pre-computer age. Would be good to know how they did it, but I've never seen any mention of SU archives.

Peter

Edited May 15, 2015 by Peter Cobbold

[stuart]

I suspect the damper differences were to do with emission control probably for California, it possible gave slightly less CO.

Stuart.