Backfire through metering head

I have an intermittent problem with my 93 fuel injected clipper in that occasionally on startup the revs will die off and the car backfires through the air filter/ metering head. I am getting to the point where I am starting to pull my hair out as I am unable to trace the problem.
I have changed the leads,cap,plugs,injectors, fuel filter, the fuel pump relay and the lift pump in the tank, I don’t really know where to go from here so any advice would be appreciated.
Thanks