I think your car has the HIF6 carbie. In my Haynes SU Carburettor book for the HIF Type it says:
Fill the piston rod with oil. On standard suction chambers (ie without a damper rod guide clip, or upper circlip on the piston rod), the level must be 0.5in (13mm) above the top of the hollow piston rod. On ball-bearing suction chambers with a damper rod guide clip, the level should be at the bottom of the guide clip. On ball-bearing suction chambers with a circlip on the top of the piston rod, the level should be 0.25in (6.5mm) below the top of the hollow piston rod.
You can buy specific dashpot oil for the SU carburettors,..Penrite make some.
Alternatively, standard 20W-50 engine oil is perfectly satisfactory with a standard engine.
As for how much, unscrew the piston rod. If when you put the piston back into the dashpot you feel resistance from the oil before the cap rests against the dashpot, then you have sufficient oil to do the job properly.
Basically I just top up the dashpots to about half way from the top. It makes no difference to how they will perform, but if you put too much in then as the piston lifts the oil will spray out the top and lob all over the air box...
Thanks for your help.Just to confirm,when i unscrewed the three srews,and lifted off each pot,there was oil just about to the top of the hollow tube inside?.Is the chamber pot supposed to fill with oil also??.
I took off the pots and cleaned all surfaces(there was quite a bit of oily resin etc),and re-filled with Halford's 20-50W,to the bottom of the threads of the screw rod.I noticed a big difference in mid range smoothness,as before there seemed to be a flat spot,and it seems to rev far easier too! .
My only problem now is the tickover is pretty rough,to a point of stalling,but i'm thinking i have maybe over-filled the dashpots just a little?.
Unscrew the caps and see if the engine ticks over better. Any surplus oil will soon drain away .Personally I use ATF as I think the engine runs better when cold
But it's probably a case of whatever suits your engine best
Not to hijack the thread, but how much of this oil disappears or gets used during normal running? I have kept topping up at irregular intervals with every P6 I've owned or come across, and have yet to see one run dry. Reading about improved throttle response I realise I have to learn about the characteristics of these carburetters as I thought the oil filled chamber had to to with internal mechanical lubrication...
Iam no expert obviously,but after cleaning the throttle slide,and inside of chamber,i certainly noticed a difference.I could not say if the type/oil amount in the dashpot's will be of any improvement to the running?.
With the inside of the bell chamber (dashpot) clean, the piston will certainly rise more freely.
The purpose of the oil is to slow down the speed with which the piston rises when you open the throttle. It enrichens the acceleration mixture by doing so, otherwise the piston would rise far to rapidly, the mixture would lean off excessively and the engine would most likely hesitate and ping!
Too light an oil results in a faster rising piston and a leaner acceleration mixture, too heavy and the piston rises more slowly and a richer acceleration mixture.
Thanks for that, guys. I've heard it said these carbs are of simple design, but there is quite a lot of stuff to contend with in dealing with them. My father's South African V8 auto has massive consumption issues (13 mpg urban and he doesn't hoof it) with no explanation so far...
There are a variety of problems which can contribute to poor fuel consumption. As an idea, worn timing chain and camshaft,..dirty air filters, idle mixture setting too rich, float levels more than specified, less ignition timing than specified and one of the biggest causes of poor fuel consumption,...a vacuum advance module that is no longer working. Usually the diaphragm within is perished, but sometimes the port on the carburettor and / or the hose linking the module to the port is blocked.
How many miles / kilometers has your Dad's Rover covered?
Can you explain how to check for the vacuume port problem, I have high fuel consumption issues and high tickover despite backing off the adjustment screws to the max, but do not know how to check the vacuume port.
Between a carb and the vacuum advance, on the dizzy, there is a thin s tiff pipe 2-3mm wide- normally shiny black. Tug it off the carb, and suck. You should feel resistance, and if you put your tongue over the hole you should feel suction. If there's no resistance, your pipe isn't airtight, or the diaphragm on the vacuum advance is perished.
Worth also, tugging the pipe off the vac advance and try sucking through, just in the rare chance it might be blocked. You should be able to suck through freely.