engine vacuum question

[juggernaught]

Will try and keep it simple. Got my newly rebuilt 360 running great with the Holley t/a 670 except for one thing...... If I mash the accel. to the floor or on a steep grade my vacuum gauge will drop to zero and the secondaries on the 4 barrel carb. will not open. This gives me a "flat" spot in my power curve. As long as I have at least 1Hg. of vacuum showing (lighter foot on the pedal) the secondaries stay open and the engine continues to build power.

I have nothing to compare my vacuum gauge readings while driving to. My vacuum reads 14Hg. at 850rpm. I could really use some ideas about this delima. I am already using a very light spring in the secondaries, although I could still go two springs lighter if necessary.

[juggernaught]

The answer to my question is................a heavier secondary spring in the diaphram. Seems to be moving in the right direction. Never mind.

[tufcj]

When I put the Edelbrock carb on my AMX, I had a bad bog just off idle. The manual suggested stiffer secondary springs, which sounded counter-intuitive. I tried it and it did work. I just didn't post because I didn't know if it worked the same with Holleys.

But thinking about it, the secondaries don't really work from manifold vacuum. They work from vacuum created by the airflow over the venturies. Low manifold vacuum at WOT has a high airflow thru the venturies, so it opens the secondaries. Figure a bog is usually a lean condition, giving it more air with lighter springs is going to compound the situation.

Glad you got it figured out.

Bob
tufcj

[juggernaught]

You got it....It is bad enough to have the primaries opening, but you don't want the secondaries opening to early(ie. under a load at WOT). I my still have to go one spring heavier. Trying to launch a jeep with 33" tires off the line creates a serious load and tends to open the secondaries early.

Once the jeep is rolling vacuum comes back up and it is not a problem.