NC30 won't start just backfires

[SteveBids]

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

[jetblack]

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

[SteveBids]

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

[jetblack]

But are you definately getting fuel into the carbs?

[SteveBids]

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

[jetblack]

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

Partial blockage?

[johnnie5]

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

[hunter]

Sounds like CDI to me,

[Morespeedvicar]

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

[SteveBids]

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