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I just installed a Dellorto PHF 36 kits on my 1990 750 Sport and the bike is running terrible. Starts up and idles fine, but hesitates, stumbles, lacks power, backfires and eventually the RPMs drop and the engine dies.

I've removed the fuel pump and am letting the carbs be feed fuel by gravity from the tank. The pump just overflowed the bowls everytime. The fuel is getting to the carbs though.

I don't understand this. Can anyone help?

I just fitted a Mikuni set.
I kept the returnline intact
So I:
- kept the fuel pump
- kept the T-split in the fuel line
- 1 direction to the carburettors
- 1 direction to the Fueltank (returnline)

Engine starts good, Idels good, runs good.
Used a special needle and Jet Kit of Dynojet for a 750 Ducati.

With the 750 Sport, you will need to retain the service of your fuel pump, as the fuel pickup is pretty far up in the tank, and you will 'run out of gas' with about 1.5 gallons remaining.
The Sport does have a fuel return line in the tank, and you will want to run a T in your fuel lines to feed back up to it. The fuel pump will be too strong for the float valves on the Dellortos if you don't use the return line.

Basically, I used two brass T's. The first T is the junction between the fuel feed and the fuel return, and I positioned it in between the carbs and as low as I could. The second T splits between the two carbs and connects to the banjo fittings. With this setup, you will always have fuel in the lines, but it won't be under much pressure.
Initially, I ran the fuel line in series between the two carbs using a pair of twin barbed banjo fittings, then back up to the tank. This produced too much pressure at the float valve I think, and it ran too rich. The newer setup works much better all the way through the range.

E-mail me off line and I'll send you photos of how I replaced the whole petcock assembly with a banjo fitting. The hole in the tank is a 10mmx1 straight thread, same as a Brembo banjo bolt. Who needs a petcock when you have a fuel pump? I also installed quick disconnects on the fuel lines to make tank removal easier. I ran them in the opposite directions as eachother, so when the tank is off, I can connect the tank lines to eachother, and the carb lines to eachother and not worry about getting junk in my fuel lines.

Next thing is the jetting. What are you currently running?