The cut valve is normally open. The vacuum line from the carb up to the valve is manifold vacuum. If the rubber diaphragm is not seated correctly, the small o ring cannot seal the two halves, and you will have a vacuum leak to atmosphere.
I found this out yesterday when I reinstalled my '78's. DOH!
If this is the case, it's easy to diagnose. The #1 carb has the vacuum line. It is clearly visible (on a naked wing). Pinch the line closed with a pair of needle nose pliers and see if the noise goes away. The rpm's changing would be a good indication as well.
I found this out yesterday when I reinstalled my '78's. DOH!
If this is the case, it's easy to diagnose. The #1 carb has the vacuum line. It is clearly visible (on a naked wing). Pinch the line closed with a pair of needle nose pliers and see if the noise goes away. The rpm's changing would be a good indication as well.
