You’ve got questions. We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re diagnosing rough engine operation at highway speeds.
H.R. Marshfield, WI
Q: I have a 1967 Corvette with a fresh 460-cubic-inch big block. It has square port aluminum heads, a Weiand manifold with an 800 cfm Holley double pumper, Chevy L-71 solid lifter cam, and TRW 10.4:1 forged pistons.
The engine idles well, runs fine at slow speed and from 75 miles-per-hour and up. At legal speeds, the engine burbles and has a perceptible surge. I’ve jetted the carburetor down to a #69 jet (way too lean) and up to #76, which seems too rich. I also changed the power valve from a 6.5 to a 3.5. No matter what I do, the engine just will not run smoothly at legal highway speeds.
A friend told me to deactivate the vacuum advance on the distributor, which was custom-curved to the street specifications in the book How to Hot Rod Big Block Chevys.
A: First, get a vacuum gauge and determine what kind of vacuum you are pulling at the speeds in question. You could have a power valve dropping in and out, or the vacuum advance could be moving on you. To check this, put the distributor on a distributor machine and duplicate your vacuum and rpm readings. Check the operation of the vacuum advance and the degree of advance by slightly varying vacuum and rpm. If that doesn’t pinpoint the problem, you could try a rear-end gear ratio change to take the engine out of that particular rpm band.