Simon,
Washdown just means dumping to much fuel into the cylinders and some of it doesn't burn and will go down alongside the pistons and cause the rings to scrub any lubricant off the cylinder walls. Not good. As for the double pumper I have used them on my cars for years with no problems, but for all around daily driving, I think a vacuum type would probably be better. The double pumpers just dump in huge amounts of fuel when you push the pedal far enough down for the cam to open the secondaries and that can be done at very slow speed or even just setting. A vacuum secondary won't open until you get whatever amount of vacuum you have it set for. Either carb, you are just running off the primaries until you get on it.
Ron
