SV1000SZ running poorly. Fault fining help please.

[Ratty]

Hi guys. Looking for a little guidance fault finding a problem on my brother-in-laws SV1000SZ. He just bought it and brought it back from down south in a van. His test ride went well and it’s run well for a couple of days. Bringing it down to show me last night it started to run rough and would not hold tick-over. The only thing he has done prior to this was wash the bike and a hose was used.
There is definitely lower output from the RH exhaust and its also missing on that side. I only had an hour to look at it last night as it was going dark, but these things I checked: -
Front plug drain hole is clear and no water present. Rear plug was very sooty but dry. I put one of my old ones in and it started well then went the same. I didn’t attempt to get at the front plug due to time.
Air filter is a K&N and could do with a clean, but is not filthy.
Engine very sounds good ( much better than mine ) with no obvious problems and the battery is very strong. £10 of new fuel was added after the problem started. It has loud cans on.
Bike has a good service history and 20000 on the clock. The bike had not been run for about 4 months up to the sale this week.
I can do most things mechanical but have never messed with FI, throttle bodies etc other than to attach vacuum gauges to mine.
I would be grateful if you more experienced guys could give me an idiot guide to sequence of fault finding this sort of problem. I’m suspecting a leak / blockage around the TB’s.

Thanks in advance. Alan

[NTECUK]

My RSV-R had petrol that had gone emulsified. ran very much like you describe.
To check for air leak .easy way is spraying wd 40 round the throttle bodies.