Pool heater - un-burned gas smell

[marksylvester]

Background:
Hayward H250FD venting un-combusted natural gas.
Local HVAC company tested everything they could think of. All measurements (electrical, gas and water) are as expected.
They checked the heat exchanger, manifold, orifices and burners. All looked in decent shape (other than some rust from exchanger leak 2 yrs ago).
They suspected that the blower was not sucking in enough air to keep up with the fuel mixture. When they removed the blower restrictor plate, the issue seemed to go away. Is it possible that the restrictor plate is the incorrect one or should have been installed in the first place? any other thoughts? (maybe fuel mixture sensor of some kind or one of the IC cards?)
The smell of un-burned gas is new this spring.

Thank you