:confused: Last weekend I swapped out a 750cfm vac. sec to a 650cfm vac. sec. Holley. When crusing in 2nd gear @ about 2500 rpm the motor felt tight and had a surge. I went from #66 to #68 primary jets. And from a #28 to a#31 squirter. The problem went away:)

Today I'm out for a test drive enjoying the nice weather. When I go thru the gears at a leisurely pace the motor now starts to vibrate @ 4500 in 2 and 3rd gear. I was shifting up @ 5000. Engine temp is normal. Ambient temp today was 56 degrees.

Could this still be a lean condition? Or do you think I have something more serious happening in the bottom end?

The motor is a stock short block 350, 9 to 1 compression, Edelbrock performer cam, headers, MSD dist. total advance 34 degrees all in by 2800rpm (20 degrees in dist.)(14 degrees initial @ idle) (vac. advance plugged)

I'm into the throttle so smoothly that I'm not sure the secondaries are opening.

btt

Do you have a vacuum gauge Fred? Is the vac steady or shaky?

Very possible the secondaries are lean too.
Is this "vibration" new since the carb swap?

Yes. The vibration was just noticed yesterday 1/19/08 Yesterday was the first time I was able to get the car out so that I could go thru the gears on an open country road.

Holley listed the primary jets as #67 but I found #66's when I pulled off the float bowl. So I went up to #68's. Holley lists the secondary jet as #73. I wonder what's really in there?

It's a new carb #80783c

btt

Do you have a vacuum gauge Fred? Is the vac steady or shaky? vac?

34º with a 9:1 motor and stock heads? The SBC is an advance hog with old style chambers. In the ancient times 44º total was common.

vibration,misfire,fouled plug?A stick shift you'll feel more of a vibration when misfiring...

Idle vacuum is steady @ 18-19 lbs. idle 850rpm. initial timing 14 degrees at idle.