My old man has a Via Venezia and I recently depressurised the stock portafilter for him (removed the internal parts of the portafilter). In doing so I had to show him how to adjust his grind, dosing, tamping to ensure a decent extraction. For the standard filter basket, we found that to achieve his preferred shot, we dosed close to or level with the top of the basket (prior to tamping), adjusted the grind such that it achieved approximately 50 to 60ml (gross volume) in the cup in approximately 25 to 30 seconds.
If the extraction is quicker than this and you get more volume in less time, then adjust the grind finer and try again. You will know if you go too fine as the machine will be unable to extract through the coffee and you will only see a few drips or drops in the cup. If you choke the machine like this, then generally in my experience it only takes a relatively small adjustment (depending on grinder) to get the ideal.
Having said this, I'm assuming you are grinding your own coffee and not buying pre-ground. Also, of critical importance is freshness and quality of beans.
Persevere and you should get a good result. The via Venezia can produce a decent shot when you get it right, and far better than with the pressurised portafilter.