Diversion refused to start, trying to blow me up

[aquilaalba]

Hi all, new owner of a XJ600S Diversion here. I bought mine (bit battered) XJ600S about two weeks ago with the intention to taking my tests on this bike. Sods law - it was starting on the button every time all the time up until when I've passed my theory and went onto booking the practical tests. Now it permanently refuses to start. Engine turns freely, but on the first 5 sec spin it will do nothing, on the second 5 sec spin it will end up in flames (literally, loud bang and fire from an aftermarket exhaust). I did check (in a way) the fuel flow and seems ok as carbs are filling up every time I undo the bolt to drain one, on a prolonged attempt to start it actually did all of a sudden, yet it was running (to me anyway) on two cylinders tops. At present it's on charge to rule out low battery, other thing I noticed yesterday is not so high fuel level (this would be stupid...) so will add more fuel and try it on reserve setting, yet I've put a canister's worth of fuel in it and only done few short runs around the block so don't think there problems' there. Any ideas? It's quite consistent as it blows fire of an aftermarket exhaust (4 to 1) every second attempt... When I did managed to start it (but it run for maybe twenty seconds and cut out) it was on PRI setting of the fuel tap. Any help appreciated. Also will need to swap front brake pads and check front shocks as the travel on them seems bit lengthy - it dives quite low when braking.