Double pumper?
 

Just bought a new car (GT40 replica) with a modded 302, (forged pistons, hot, roller cam, roller rockers, Dart heads, offenhauser manifold etc)
It's fitted with a Holley 650cfm double pumper carb. What's this like for road use? should I get a carb with vac secs for economy?
someone has worried me with tales of "washdown" of the cylinder walls causing problems (I'd never even heard of washdown and am still not sure what it means)
Thanks guys.

Simon

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 :)

Simon;

I ran a Holley 600 vac sec on my 350 hp 351-W for seven years and loved it,good gas mileage,good throttle responce.... Then I went to an open track or two and have decided to go "road racing" now,a buddy gave me a 15 year old 600 double pumper and I rebuilt it an d put it on my car,played with the jets to get it like I wanted,now that is it "right", I'll never look at a vac sec carb again....Been driving on the street and the track now almost a year with the double pumper and love it.... Do not worry about gas mileage,mine gets the same mileage on the highway with either carb.....I drive it to work (200 to 300 miles a day) about once a month or more in good weather and have checked the mileage with both carbs,same result.... I have never really checked my "city" driving mileage,but my guess is it is close to the same for both carbs.... In town driving I almost never have to give it enough pedal to open the secondaries,got enough low end grunt to scoot along without opening the rear butterflys....

On my car when I switched carbs to the double pumper,the throttle responce above 2000 rpms was incredible,the thing just shot out like a rocket and the tach goes very quickly from 2000 to 6000....Makes you feel like you got an extra 50 to 75 hp.....

David

My opinion...Double pumper is the way to go, especially if your car is a stick shift. "Fuel wash" would only occur if for some odd reason you continuously had the urge to full throttle the carb with the trans in high (or any) gear while turning very low rpms for an inordinate amount of time- OR if something is internally wrong with the carb. Vaccumm sec. carbs are excellent in relatively heavy vehicles that have slow rpm building capabilities.
Just speaking from my experiences.

Thanks for the advice guys.