A904 Hard 1-2 Shift and Reverse

Also check the vacuum advance diaphragm. At Part throttle, light throttle, it should start advancing at about 1500 to 1700 rpm, or at least before stall speed. If it does not, and your idle timing is still at 5*BTDC, the power will suck.
The V-can adds it's timing to the base timing. IDK what your V-can is tuned for but 13/14 is typical. By the time you hit stall rpm, the mechanical might be up to 10* .. So adding those up at light to medium throttle, it might total 28 to 30 degrees, making for a smooth and relatively powerful take off. Without the Vcan, all you get is the 5*+10=15. and if the mechanical is frozen, then just 5* will be super lazy. I don't think the engine would be shaky or rough tho just from lack of timing.
However, if the mechanical advance mechanism is frozen at full advance, and this occurred after the base-timing was set, that engine will for sure shake with the application of throttle. because now it is getting perhaps 35* at idle, plus the Vcan, as you roll in the throttle, maybe 50*. No 318 I know of will like that. and shaking is it's way of complaining about it. It will idle just fine on 25*, your only clues are a high idle; and a stumble on throttle tip-in, if you slow the idle down. It might even like more than 25* at idle; it just won't complain about it until it is required to pull.