V8 SU carb dashpots do not retain oil

[ricky8664]

Ever since I acquired the car in the early 1980s every time I checked the carb dashpots they were empty of oil. I never used to top them up as a matter of routine unless going on a longer journey when, invariably, anything over about 25 miles would see both dashpots empty by the time I got to my destination. Now I have the engine running again I'm wondering if there is a seal that needs replacing. There is nothing I can see on the underside of the black cap holding the rod with the piston, so not sure if there should be and they're missing. Or if there is a seal somewhere else.

So how long would you expect the oil in the dashpots to last? And I have tried lots of different oils to see if the thickness made any difference It didn't. Is this just normal to have to top up the dashpots every day or does it last longer for others? 

[enigmas]

AFAIK only later versions of the Zenith Stromberg CD carburettors (as fitted to early model Range Rover V8s) have an O ring within the dashpot to minimise oil loss within this mechanism.

Unless it really affects the driveability of the car I'd learn to live with it. Oil also lubricates the shaft and the bore it works within. So add oil on occasion.

Causes: A previous owner having minimal knowledge as to the working function of these carburettors and cleaning the parts with a mild abrasive cleaner or wet'n'dry abrasive paper.
Similarly, do not attempt to polish the interior surfaces of the SUs dome as these are manufactured to precise tolerances.


* The Reason oil is added to the Dashpot.
Oil is added to the dashpot to add resistance to the SUs airslide under instances of sudden acceleration. The oil slows the lift of the airslide thus the needle out of the jet which causes the mixture to enrichen momentarily. This mirrors the effect of an accelerator pump on more traditional style downdraught carburettors.

Fit a slightly heavier damper spring to add a bit more resistance to the piston lifting.



* Not all CD Carbs have Dashpots
Mikuni carburettors as used on various motorcycle engines (Harley Davidson, BMW, Suzuki) don't use a dashpot.

PS.
SU are a carburation design that harks back to the days of vintage cars. They are still a very effective and fuel efficient method of metering fuel and air to an ICE given the simplicity of the design. Generally cars prior to the 1960s would always have the engine oil checked either weekly if regularly used or with every tank of fuel. If you recall all service stations in the day had racks of oil bottles beside the petrol pumps. Adding a half pint of oil per each tank fill was a common occurance.

[ricky8664]

 I never really found any driveability issues with an empty dashpot, presumably as I did not have a heavy right foot, so the damping effect of the dashpot oil on the piston was not required much.

I'll just have to put up with it or top it up regularly. Shouldn't be a problem as the rover was my daily driver when last on the road in 1992 but when it finally gets back on the road it won't be, so topping it up once a week during the summer won't be that much of a hassle!