Sounds like an FP or LA plain bearing engine (with the air bleed carb). What size prop? Are you running muffler pressure to the tank? Is the clunk in the tank clear of the back of the tank? Where is the tank centerline compared to the needlevalve? How far away from the engine is the tank? What size fuel tubing are you using? Is there excessive wear in the plain front bushing that would allow air to be sucked in? When you level off from a climb does the engine run ok or stay running lean. When you start it up do you hold the plane nose high before putting it on the ground to see if the lean condition happens? Your trusted friend should, and hopefully did, check all of these before condeming the engine to the rubbish pile.

Sincerely,

Richard