Spitfire Carburetors – What type of oil do I add to the dashpot?
Here’s a great explanation from John in the forum:
An SU or Stromberg carb is a “Constant velocity” device. It changes the diameter of the orifice at the fuel jet, so as to maintain a constant air/fuel mixture, by allowing the piston to rise and fall in response to the changing demands of the engine, moving the needle in the jet to allow more or less fuel to escape into the changing air flow. (NB do not confuse ‘flow’ with ‘velocity’.)
On acceleration, the carburettor piston rises as the engine draws in more air. But for acceleration a slightly richer mixture is desirable, so the damper/dashpot slows the rise. Until it reaches equilibrium, the orifice is narrower, the air velocity is greater and a slight excess of fuel is drawn into the air flow, to provide the richer mixture.
The carburettor is designed so that with engine oil in the dashpot this change is optimum for the majority of drivers. If, however, you want to achieve maximum fuel economy and drive with featherweight boots on, accelerating and braking seldom and then lightly, a thinner dashpot oil will produce less enrichment on acceleration and less fuel consumption. If you are a hot-shoe road burner then a heavier oil may suit you, but the only way is to try oils of different viscosities. There is no ‘right ‘ oil.
Personally, I use automatic transmission fluid (ATF).
At the end of this video, Jon Twist of University Motors suggests using 90 weight gear oil.