450 fse 2004 help no start

[flam66]

Hi all, new to the forum and to the 450 only had it a short while, probably done about 150 miles with no problem, but while doing slow technical riding it stalled a couple of times and then refused to start, I thought I had flattened the battery so tried bump starting behind a car (a bit desperate I know) the engine didn't fire at all,
when I try to start it now, I get an occasional backfire from exhaust and the odd pop through air filter, seems like timing?
have a good spark, fuel pump priming, injector firing fuel, valve clearances to spec, cam timing correct, magneto still on keyway, I have read a lot about resetting Tps? my question is could there be any thing in the efi that could cause these symptoms?
In the meantime I am re-seating valves (slight pitting)as compression was at 90 psi, any help appreciated, thanks