backfires is not crap getting threw it's fuel starvation .i personly would not take metering head apart unless you know what you are doing .just try the simple things first .i would start with changing the fuel filter if its been on a while then take out the fuel feed to the metering head and crank it over to see if you are getting a good flow rate .it should fill a glass jar in a few seconds . i think there is a small filter in the head also ? its been a while since i did mine .if a good flow rate put the fuel pipe back in and remove all injectors still connected to the lines and crank again .you should see spray fuel coming out of all of them .dont forget there will be quite alot of fuel vaper in the area so no naked flames
if you get one injector or more spraying week fuel then i would remove the injectors all together and see how much fuel comes out of the lines .if all the same then its a dodgy injector if not its the metering head .hope this helps