Hi - hate to admit defeat but really use some help please

Quick background of problem

mallory park - last session running total loss battery starts giving up hard to keep revs up then stops dead.

Back to pits Change battery and top up fuel - won't start just occasional impressive backfire - try mates cdi and change plugs but exactly the same. Call it a day and go home

Back in workshop change both coils - no change
Whip off carbs check over and refit - no change
Dismantle fuel tap (mod done a while ago) - diaphragm dislodged a bit so corrected - no change
Lose the plot completely - engine out complete strip down (done a few of these so getting used it now !) can't find any problems, nothing broken, all lugs still on cams etc, new head gaskets reassemble - no sodding change
Bite the bullet brand new battery and brand new set of plugs - best spark I've ever seen ! - no change !!!!
Swap carbs for spare set - no change
Checked all wiring (race loom) and resistor + resistance of pulse coils all appear ok
Tried 2 different cdi's borrowed from mate - no change
Checked all earth points and even ran separate earth wires to coils etc direct from battery - still the same !

So to summarize more or less changed or checked everything I can think of but still won't run just turns over then sodding great backfire and flame out of the can.

Anyone seen this before ? Any ideas/help greatly appreciated, probably something really stupid !!

Really at a loss - starting to think it's never going to run again

Vac hose worked its way off?
If your getting a spark, then the only thing missing is fuel...

Thanks but hrc Fuel tap mod does away with vac hose - checked the blanking screws are all tight and sealing ok

But are you definately getting fuel into the carbs?

Yes - dont think I'd get the flames/backfire without fuel

Enough fuel to cause backfiring, maybe not enough to get it running?

Partial blockage?

remove the engine cover and check the timing trigger disk , grab hold of it and try to turn it , it shouldnt move , i think its on the RHS

i had this problem in the past

I didnt listen to my dad that said if you have backfiring through the carbs you have a timing issue

I dismissed it as timing is fixed in these motors

but help from here and i checked the trigger disk and it had seperated and moved

spark fuel and compression and it should run but not if the spark is at the wrong time

Sounds like CDI to me,

I'd try cleaning the plugs again and trying some easy start down the carbs or just squirt a few drops a fuel down each carb. If it doesnt try to run on that, youll need a strobe light to check the timings right. Oh yeah dont earth the wires to the coil, the cdi switches the ground and has a constant 12v going to them.
I had this same prob on me trail bike, the guy i bought them off actually bought another bike which had the same fault! Good for me tho, just took a while to figure out (nakered CDIs)!
Cheers
Ian

Thanks for all help - I'm going to check starter clutch tomorrow to see if timing has slipped somehow, I'll also take carbs off again and double check fuel getting in. Tried 3 cdi's now maybe try them all on my mates bike see if they work
I'll let you know how i get on