The SU works by maintaining a “Constant Depression” in the Venturi. What this means is that there is a constant vacuum reading in the area where the needle sits in the jet…at full throttle the piston is high up in the chamber to allow a lot of air through the Venturi…but the vacuum reading where the needle sits in the jet is the same. If you do a series of tests then you can taper the needle to allow a certain amount of fuel through the jet at each height and at that height there is a certain amount of air flowing through the Venturi so you have a ratio of fuel to air.
The dash pot prevents the piston raising instantaneously to the right height and I think this temporarily INCREASES the vacuum in the Venturi so that more petrol than usual flows through the jet at that height…which temporarily enriches the mixture.
The piston is temporarily lower than it is designed to be in the Venturi and the mixture is rich.
I think the weight on the piston would mimic the same condition for the purposes of a test…and is a clever trial!!!
I wonder why the spring is used in these carburettors…because it would seem to me that the weight of the whole piston assembly would provide the “Constant Depression”…but that’s a different matter…
I’ve been doing all the reading about it. The idea is that the vacuum / airflow traveling under the piston creates suction between the jet and the needle, and pulls fuel from the bowl. When the piston is lower but the engine vacuum is the same, the air has to move faster, therefore more suction, more fuel. When the piston rises too fast, the air can go more slowly, creating less suction. So resisting the pistons movement / slowing it down creates richness under load. Ideally, the piston lands in the same position under steady throttle, therefore the cruising mix is still the same, just richer during acceleration. I’ll try the big washer this weekend!
Apparently the early SU’s had heavy pistons, but they went to aluminum pistons for some reason, then added the spring and fluid to slow the rise and achieve proper mix. Not sure the reasoning but it works.
Yes…but the “piston” is actually two “pistons”…one that sits in the Venturi and the second larger one that lives in the bulbous shape on top of the carburettor. The larger piston is affected by the amount of manifold vacuum which is transferred to the top of the bulbous shape and essentially pulls the “piston” up into the bulbous shape. …this is a nice “mechanical” method to connect a fuelling system to manifold vacuum…which is used to determine fuel and air flow.
The “dashpot” is actually a damper that prevents the piston moving as fast as it wants to…which creates a temporary rich mixture. Interesting that the spring was added to save weight.
They are a clever and quite simple device…compared to all the complexities of a fixed jet carburettor…
They did: they were bronze.
Likely changed to aluminum and springs for cost.
OK, one washer in the carb piston made no difference… but two washers did! Now the rear carb A/F stays at or around 13.5-14 under heavy acceleration, which I’m happy with vs. 14.5-15+. No affect at idle or cruise. So I’ll leave them in for now and try to figure out what heavier spring I need.
Glad to hear that it worked. I have suggested it before but you are the first to take the leap of faith! Before shopping for a heavier spring there is one more trick that you can try. Get a spare spring and give it a bit of a stretch which will increase the spring pressure. Pretty crude method but if you do it incrementally you just might get lucky. Don’t try it with your present spring as you don’t want to mess with success.
Both of Geoff’s suggestions can work.
A coil spring works in shear action on the metal in the spring. Stretching the spring will reduce the shearing force component of the axial force compressing the spring, effectively a stronger spring, just longer. (A coil spring compresses in total axial length but the spring material is not compressed, rather it is bent by the shear force).