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I maybe wrong, but wouldn't a lighter spring open the secondaries sooner, which could cause more of a bog. I believe the power valve moderates the fuel for the instant WOT, then the secondary jets...

READ YOUR PLUGS! Too lean will cause serious motor damage, jets are a lot cheaper.

Mine was doing the same thing, read the article above and checked the plugs,they showed lean. Re-jetted the carb to 68 primary, 76 secondary and instantly no bog!

I have my primary jets currently...