You need to check the vacuum while you're driving. Watch how low it gets while you cruise around without getting on it. Then see how low it goes when you start to get on it, but don't actually floor it. You want your power valve to open at a vacuum below the lowest cruising vacuum, but above the reading from when you just start to get on it. In other words, if your lowest cruising vacuum is 10 inches, and it's around 7 inches when you start to get on it, you should get an 8.5 power valve. If your numbers are lower, adapt accordingly. Odds are a 6.5 (65) would be just fine.
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