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Crane distributor
Madbill,
I ran into the same misfire situation with the local Cobra club cars. They ran overdrive trannys with too big a cam and carb. They were trying to cruise at 1800-2000 RPM with high manifold vacuum, lots of ignition advance and very light throttle opening. If they gently pushed on the throttle (still with pretty high manifold vacuum) the misfire would go away. When they disconnected the vacuum advance the problem would go away and the engine would no longer misfire. (Vacuum advance ~10-12* above 12" Hg)
I felt it was a situation where with the high manifold vacuum and lots of advance the throttle wasn't open very far and the depression across the boost venturi's wasn't high enough to start the fuel flowing. The engine was basicly running on the idle circuit and was going lean. When the vacuum advance was disconnected more throttle opening was required to maintain cruise power, and the boost venturi's would flow fuel, and the misfire would go away.
I called Holley's Tech Line and got someone that had seen this same situation several times and he agreed with my theory. I helped the situation by raising the float level so tip-over came in sooner.
I remember that David Vizard did some testing on carb venturi's and found that he could modify the boosters and drasticly increase the depression across the boosters. That might be the best way to cure this type of problem. Maybe David could publish the article again on this forum.
I suggested the Cobra owners could completely eliminate the problem by running a Rochester Q-jet carb. I bet you can guess how that suggestion was received.
Ron
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