20HP intek shakes and knocks when engage blade

[bronzemaxell]

my lawn tractor is 2006 craftsman lt3000, 20hp intek engine, 917.276813, manual transmission 6 speed.
i just cleaned carburetor, fuel filter and replaced head gasket, adjusted the valve lash, intake 0.03in and exhaust 0.05inch, starts and runs fine, no engine shaking, also rebuilt the mower deck with new genuine oem spindles, pulleys. re-used the deck belt and (1 season) blades
but as soon as i engage the blade, full throttle, with the hood open, i can see the engine shaking and vibrating, also felt through steering wheel and seats, and there is a little knocking sound. and as soon as i disengage the blades, the shaking, knocking noise stopped.

i would think if the connecting rod knocking, i would hear it all the time.
i suspect maybe loose engine mount bolts, but would like to hear from any of the experts here that may have other suggestion or opinions.

 

[Scrubcadet10]

Loose engine mount bolts is my guess as well.
Most are screws that self tap into the mounting holes cast on the engine block.

 

[bertsmobile1]

Lump missing from blade belt
Deck pulley broken or frozen.
To add to the list above

 

[ILENGINE]

Loose motor mount bolts will normally show up when starting and stopping the engine. just the slow turning off balance clunk clunk sound. If the only time it does it is when the deck is engaged then I would agree with Bert about a deck issue.