Weak smoke "force" ?

The impeller fan could be loose on the motor shaft. The motor spins up to the correct speed but the impeller might be slipping on the shaft. I've seen this a few times, I'd remove the impeller, clean the shaft from any contaminants (smoke fluid), and pinch the opening with needle nose pliers to increase the grip the impeller has on the motor shaft.

It could simply be the motor.  Also, the 5V power source on the smoke unit may be dying, top suspect is the capacitor or regulator.  Finally, the impeller could be slipping on the shaft.

The smoke PCB isn't available, so if the regulator circuit is dying, you'll have to entertain the option of repairing it.  An easy way to check it's health is with the locomotive powered and the smoke unit running, measure the power supply voltage.  Measure the DC voltage using the ground reference, if you don't get 5VDC or very close out of the regulator, that's a problem.

Hi John Checked voltage was around 4.0 - 4.5 bounced a bit. Is this OK ? What parts would I have to replace?  Also would the super chuffer work here ?  Thanks

If the voltage is low, I'd look closely at the capacitor, it's probably either bad or loose from the board.

The Super-Chuffer replaces all the logic on the semi-smart smoke unit, so clearly that would solve that problem.  Obviously, it's more expensive than repairing the smoke unit board, but it also does a lot more.

Note: the capacitor is on the bottom of the board.

Thanks John