SUZUKI VL800 2008 fuel injection

Hi , my Project VL800 Bobber 2008 K7 , is backfiring when revs increased , it will tickover ok for a couple of minutes then just cuts out.
I bought it as a project , it had been stood a while so fuel tank was cleaned and filter cleaned , then tested fuel pressure with gauge and it was as per manual .
I have changed the plugs  DPR7EA9 , also the fuel injectors have been serviced and tested and are ok.
The bike has aftermarket exhausts , but do not know what brand they are.
The bike will start ok with a fast idle then drop revs after about a minute.
It will then tick-over ok , but back fires on higher revs and if I hold the revs steady but not high revving it the exhaust note goes up and down . 
Then when I go back to tick over it will be ok then after a minute it will just cut off as if no fuel going thru.
Any suggestions would be helpful.
Regards

Check the carbs mite want cleaning out

What colour are the old plugs that have been used?
I would think the mystery exhausts would probably cause an issue.
Is it a standard air filter?

Hi thanks for replying , will check it out regarding blockages . Thanks

Hi regarding plugs will check it out and not sure if standard air filter .
Apologise for late reply on both comments, I will update with progress as still in same position regarding issues.
May have to get a mobile mechanic in .
Regards