FWIW the factory inline dual setups mounted on the 542991 and 9770859 intakes had 3/8" spacers on them. I do not know the thickness of any spacers the factory put on bathtubs but found some 1/4" spacers that I've put on bathtubs sets I've restored for people. The next time you have the tops off of your carbs pull the rear boosters out and then the secondary air valves and weigh the valves. The problem may be that you need lighter secondary valves and I may/probably have a lighter set I can send you to try if you wish.
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