Booba,
I tried your smoke test and was not able to get the smoke to come out of anywhere, although it did go through the exhaust when I started it. Also when you stated that the coupler is designed to fail, it is not designed to fail to keep the shaft from being damaged. It is there for misalignment between the input of the blower and the shaft. If the couplet fails then you have the pins hitting each other and that will damage the blower beyond repair. The coupler is the same type you would see in any other industrial application where you have 2 shafts meet. They are designed to take up for slight angular and parallel misalignment. Look up Lovejoy coupling they do the same thing.
I am still not getting any boost. Sometimes it will go up to 1-2 psi if I ease into it, then it drops back to atmospheric pressure "0".
Sometimes the engine feels weak then it comes alive, but still no boost.
Jeff