ORIGINAL: bla bla
I think this guy is so green the last thing he needs is a Perry thrown into the works!
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That could be true.
But, if it truly is a low muffler pressure problem, the VP-20 would be a simple solution and it requires no adjustment, or tapped pressure line like the VP-30. Unbolt the top two backplate screws, insert the VP-20, replace and retighten the backplate screws (capturing the VP-20), cut the fuel line at the appropriate point, fit to the appropriate fitting on the VP-20, and you are finished. Go fly.
All I can go by is what he says in his post. I have seen Pitts mufflers that actually provided a negative pressure pulse at the pressure fitting. No, I didn't believe it either until I saw it for myself. Removing the fuel line from the pressure fitting and letting it hang open was the solution. The carb was retuned and the engine ran just fine (Enya .40 baffle piston). That was back before engines came equipped with carbs so large that they truly did depend upon some assistance drawing fuel.