Advice needed, I had a very enjoyable day out at the Guzzi meet at Gaydon a couple of weeks ago ( if you haven’t been I would highly recommend it) 79 miles there and 79 miles back, fantastic ride up the fosse way from Bath and back, the Guzzi performed faultlessly (this bike seems to be made for these types of road) until I got 5 miles from home when it started loosing power, spluttering back and backfiring, only just made it home.
I haven’t had the chance to look at it until yesterday, the bike started but had to reve it to keep it going, still misfiring, checked the exhausts both hot, checked the plugs, both had a good spark, plenty of petrol in the tank but what I did notice was when I turned the ignition on the fuel pump was taking longer than normal to pressurise the system ?
I am thinking from the totally un qualified that it is may be fuel related, it happened quite suddenly, any ideas ?, going to check for blockages in the fuel lines, fuel filter blocked ?, injectors blocked ?, or is it something in the electronics ?
any advice appreciated.
I’m certainly no mechanic however backfiring suggests unburned fuel getting through the engine to the hot exhaust which suggests an ignition issue of some sort, given the spluttering you mention and needing to rev it, I wonder if it could be ignition timing not quite spot on. 
Is pump and filter inside the tank? I’m betting on filter has split or hose coming off inside the tank. (Was it the Breva or Griso that was notorious for hose coming off? Or both!)
Also check your rubbers and exhaust clamps for any air leaks.
I found cracked exhaust clamps, it only affected it at 40 - 50 mph.
Applied sealant and new clamps and ran lovely after that
I agree with the fuel pump/ filter possibility certain models they used to split this would reduce pressure and affect would be immediate. Also a Cali that was running fine I left for 4 months it then misfired and ran really poorly. Assumed the fuel was off stripped injection system etc etc only to find changing the plugs cured it straight away. Resistance on one plug was huge, so standing for four months the plug failed. Utterly bizarre. I always change plugs as a starting point. The real pain job is disconnecting the fuel line, quick release it is not well not for me anyway😳