So I've recently decided to rebuild my 903 after nearly two years, I've had four new pistons and the head recon I haven't touchscreen the crank and cam timing as there was no need but obviously the dizzy had to come out, so is there a certain way to get the dizzy to match up with tdc on cy1? As it's getting fuel and spark but its back firing through the carb is the dizzy causing this? Any help really appreciated

Just me but recheck all the HT leads are in the correct place.
No 1 Cylinder at TDC
Dizzy rotor contact bit pointing at No 1 cylinder.

Yeah that's what people keep telling me to do haha I probably have it firing on the wrong cylinder. Will do this when I finish work

Bit worried you've said you haven't touched the crank timing - if you've had the head off and pistons out you will certainly need to make sure the crank and cam shafts are timed up when you re assemble it all.

Thought it wouldn't matter due to it being a pushrod engine and the timing was fine on them befkre

Ah, apologies, hadn't realised the camshaft wasn't in the head on these - never run a 903. No if the timing chain wasn't disturbed you should be OK on that.

It's starting up now just need to alter the dizzy slightly but don't suppose you know how to top the gearbox oil up?