Back fire

[scenic]

I just bought a 100hp Mariner oil injection. It sounds and pump water ok except every ten seconds or so it back fires once. I replaced the plugs and fuel filter, flushed out the carbs by undoing the screw where the float sits and pumping fuel through it, carb four had a bit of oil in it.
The motor did not run well before i replaced the above parts and did a clean.. The only thing is the back fire. Can someone please advise me on what to do to rectify this problem?

 

[racerone]

Backfire can be misinterpreted------------Try opening the low speed idle mixture needles one at a time , 1/8 turn at a time ( remember where it was )---------Just sounds like it is lean on the low speed mixture which will make the motor cough.

 

[scenic]

I give that a go, thanks for that.