double pumper question

[fia#09]

My local machine shop guy says that double pumpers pump excess gas in the engine during normal driving if the roads are bumpy, washing the cylinders and causing oil dilution, etc,etc. I took off the double pumper and went to vacum secondaries, but lost a ton of instant response and midrange (went from 650 to 750). Was wondering how double pumpers are working for other guys in Cobras that are stiffly sprung--I think he's full of baloney. Am going to rejet the double pumper and try it,as it was way too rich before. Holly tech man recommends a 650 double pumper with mechanical secondaries. Obviously both carbs could be dialed in, but, are double pumpers as a category bad on less than perfect roads? Thanks, fantastic forum.

[Cobra20646]

#09,
That's a new one on me The double pumper is not all that different from a single pumper when it comes to how the thing squirts gas into the manifold.
The primary accelerator pump and the secondary accelerator pump (on a double pumper) only "pump" gas when the accelerator is moved from idle all the way through full throttle. The only way I can understand the relationship of bumpy roads and excessive gas is if your foot in jumping around on the accelerator with every bump....... not too likely I imagine.
- Jim -

[Mike I]

fia#09,

He's half right (your local machine shop guy), double pumpers do have the potential to cause more of a cylinder wall wash-down effect than vacuum secondary carbs, but it has nothing to do with bumpy roads. I doubt the roads in Endwell, NY are that bad or you'd be driving a 4x4 instead of a Cobra!! It depends on a number of factors such as how well or how poorly it's been set-up and how active your right foot is.

Actually, a vacuum secondary carb is just as potentially damaging if it hasn't been set-up correctly and is running too rich.

Initially, I installed an 850 DP (big block spread bore 4165) carb on my engine and it was dumping in way too much fuel. My engine was dyno'd with a 750 DP so I didn't think an 850 was too out of the question. I was wrong, so I switched over to a 600 cfm vacuum secondary that I had laying around and other than the fact that it has a very slight off idle lag, it performs flawlessly all the way up to redline (my engine really roars to life when I drop the hammer!!!). I switched to the 600 as a temporary measure with the intent of putting on a 650 DP eventually, but truthfully, for a mostly street driven car it isn't necessary and my car runs great with the 600. The only changes I made to it were to change the vacuum spring to the lightest one available and I installed a new pump diaphragm and an agressive pump cam.

Interestingly, I have a complete Weber IDF system that I'm planning to install, but the car is running so well as it is, I'm hesitant to mess with it right now.....

Mike

[dalola]

#09,

There is no relationship between the surface quality of the road, and a Holley DP carb. I love the home brewed theories people toss around!

It's all in the tuning. Lots of people over carburate for lack of tuning! Here's a simple formula that will get you in the ball park size-wise, for picking a carburetor.

CID X RPM X VE, divide this by 3456.

Example: a 460 with a volumetric effeciency of 90%, turning 6000 RPM would require 718 cfm. Most performance street engines have a VE of 80-90%.

Once you settle on a size and brand, then tune, tune, tune!

Good luck

Dave

[Tim Votaw]

There is a recommended adjustment that can be made to your accelerator pump arm to neutralize any vibration dribbling. I live on a 1.5 mi. gravel road that is usually severely washboarded, and have experienced this problem with my 750DP.

You can find the adjustment procedure on the Holley website under the Carb Technical section.

Briefly, you adjust the preload of the pump arm off the cam to about .010 freeplay by feeler guage before it begins to activate the accel pump lever. That's what the spring and nut assembly is for.

I found I had to tweak it a few times to get the best compromise of pump stroke to freeplay. Like anything else on a Holley - adjustment, trial & error.

Tim