Changed a belt, doesn't sound right now

I changed the broken belt on a JD X330. I don't think it matters, but the old blades were chipped, broken and they were dullest blades I have ever seen. (not my mower, helping a friend).
The old blades and belt were original and lasted 3 years.
In the past when the blades were engaged there was a quick shutter and a sound change. Now there is a heavy shutter a slap sound (I think the slap is from the tension on the pullies when the blades are engaged)
I have seen videos of people saying you should engage blades at half throttle, then move the throttle up to full power for cutting.
I made sure the belt was oriented correctly, not twisted and the V in the V chamber.

I did have to undo pulleys to get the belt guide loose. If I over tighten the pully bolts will pulleys not spin correctly? same question with the spindle?

Why is it slapping now, but not before?

a bad bearing may have caused the belt to break. Make sure all the pulleys and blades all spin freely with no extra free play sieways or up and down than design, or grittiness.

Dropped the deck again today. double checked it was the right belt, hand spun all of the pulleys. watched videos. everything seems like it operates correctly.
I did see a video of the one handed mower guy starting a similar one and his did give a good shake before the blades engaged.
I'm getting one of the spring loaded pulley pulling hard and releasing hard making a slapping metallic sound.

The engine will bog down if I try to engage the blades on turtle. If that normal?

When you initially start the blades they have mass , mass has inertia so they resist turning
This causes the tension arm to swing in and out and the deck jolting
Just the same as would happen in a manual car if you dropped the clutch at 1/2 full revs .
It leaps off the ground violently then settles down to a nice smooth running

greasing all the Zerks seemed to get it to settle down. I guess I got the spindles to spin a little easier and lowered the inertia.