Progressive mechanical carb linkage vs. vacuume op

[Howard Butler]

I have a '69 427/400 3x2 setup with 4 speed. The previous owner had installed new cam (L-71) with solid lifters and converted the carburators to a progressive mechanical linkage. The vacuume diaphragms on the outboard carbs are good as I had them checked when the carbs were rebuilt, but are not hooked up to the vacuum port of the center carb.

The car has excellent acceleration especially above 3000 RPM.

At what RPM range should I adjust the linkage for the outboard carbs to kick in?

Thanks,

Howard
#38700

[Gerry Proctor]

Re: Progressive mechanical carb linkage vs. vacuum

It's not based upon rpm, Howard. It's based upon load, which is why the vacuum outboards are such a well-engineered set up. The problem with converting a vacuum secondary carburetion system to a mechanical set up is that there is no enrichment circuit like the accellerator pump on your inboard carb. This means that if you stomp the pedal, your car is likely to suffer a lean backfire. In order to make the system somewhat tolerable, the secondary actuation has to come in at a fairly high rpm so that you have sufficient airflow for a good pull on the boosters.

I would not consider this scenario to be better in any way to a properly tuned vacuum secondary. In my opinion, if you have the capability to return the outboard carbs to vacuum operation, do so.

[Howard Butler]

Re: Progressive mechanical carb linkage vs. vacuum

Hi Gerry,

Thanks for the data. I do plan on reverting back to vacuume controlled secondary carbs. All I need is to get the return/closing linkage. Looks like both Corvette America and Paragon offer these parts.

Thanks,

Howard

[Dave Stafford]

Re: Progressive mechanical carb linkage vs. vacuum

Howard, try putting the "standard" or non-colored springs in the secondaries, and see if they open smoothly. If you have a low (3.70/4.11) rear-end, then maybe try the Purple ones. I use a .025 squirter in the center carb as well.

There are some real good archives in here for setting the return mechanism up properly also.

Good Luck

Dave Stafford