Thread: 3.5 V8 Carby Dashpot Oil

The oil in teh dashpots or more correctly Dampers acts as an accelerator pump for constant depression carbies.
As you put your foot down the piston raises to keep a constant depression, the oil slows down the raising of the piston so that the mixture is richer than it would otherwise be.
Old carbies will be so worn in the main jet and needle that it probably hardly matters as mosty old strombergs will be very rich, but it should be engine oil in them.
The inner tube of the dashpot has an O ring in the bottom , held in by a star clip type thing. If you can get the clip out without damaging the tube , you can replace the o ring.
Regards Philip A

I think the oil should be straight 20W (a multigrade will get thicher as the carby gets hot). Somewhere at home I have some 'Stromberg Oil' but I'm not sure if it lists the actual viscosity.

A quick search throws up conflicting information, here they recommend engine oil as per PhilipA :-
zenith stromberg tech tips

http://www.penriteoil.com.au/pis_pdf...%20Dashpot.pdf


Colin

Hi Garry
What oil to use in the dampers will depend on a number of factors. It can go from 0W - heavy gear oil (not recomemded). Is there much wear in the dash pots. Does the motor flood under very hard throttle usage. ETC
I have had some experience in tuning triple SU's on a 186 race motor. Did a lot of fiddling with oil weights in the dash pots.
If your engine floods when you jam the throttle down move up to a heavyer oil . This will slow the rate at which the piston in the carby rises. Letting less fuel in.
Probably start with an ATF oil . IIRC I ended up with about a 60W+ oil but this was because needles were wrong profile. Engine was fine on the dyno but on the track was flooding on take off at 4000 rpm. Mixture right at 7000rpm

Gary

Thanks for the responses - I will change to standard engine oil once the machine oil goes.

I take it from Phils comments re accelerator pump that the lack of oil could have been the problem when cold starting - not enriching enough.

Cheers

Garry

What oil to use in the dampers will depend on a number of factors. It can go from 0W - heavy gear oil (not recomemded). Is there much wear in the dash pots. Does the motor flood under very hard throttle usage. ETC

Gary, i think your memory has played tricks, in that the thicker the oil the richer the "pump" , the thinner the leaner, as The speed of raising of the piston will be less with thicker oil and this inhibits AIR entering and thus richens the mixture.
See below for a good tech description.
Moss Motors, Ltd - Zenith-Stromberg Carburetors 1
BTW if you google Zenith Stromberg carburetor ther eis a good description on how to replace the O ring on the later models with self centering needles IE without 2 Brass nuts on the bottom.
Regards Philip A

Originally Posted by PhilipA

The oil in teh dashpots or more correctly Dampers acts as an accelerator pump for constant depression carbies.
As you put your foot down the piston raises to keep a constant depression, the oil slows down the raising of the piston so that the mixture is richer than it would otherwise be.

Regards Philip A

Originally Posted by PhilipA

Gary, i think your memory has played tricks, in that the thicker the oil the richer the "pump"
Regards Philip A

Thanks Philip - was mainly using your top comment - machine oil is a lot thicker than no oil but I take your point that enriching when running may not also apply to the choke - starting when cold - though it just started when first turn of the key on petrol and it has not done that in over a year. Normally need to start on gas using the gas squirter and in winter when the gas is frozen up via aerostart direct into the inlet manifold via some special piping I have from the cabin.

Any way - I will read up on the references and work out what weight oil needs to go in - in my jag which has four of the same carbs it uses the same ATF that goes in the gearbox.

Cheers

Garry

If no oil in dash pot this may allow the piston & needle to sit too low in the seat. Effectivally blocking fuel flow, oil would act as a cushion. Raising needle JUST. Depending on whicn moble carby needles can the moved up & down in piston.
Do you have a choke. This will raise piston & needle to richen mixture to start. On our race motors rarely needed choke to cold start and the was on methanol.
As the piston rinses this not only allows more air in , Also depends on throttle butterfly position, but allows more fuel to flow. As it raises the neddle out of the needle jet. This is not to be confused with the needle & seat, this is a constant.
Have a chat to these guys

Home | Midel Australia

Gary

Google Burlen UK. They do SU's and Strombergs. Their website says sae20. I have used ATF in my Austins and Triumphs for the last 30 years. Where does the oil go if the seals leak? Into the engine intake manifold, then it gets burnt.