Problem running engine after removing rear Carb

Hi, Just got a VS800 intruder with Chinese diaphragm carbs - Mikuni BS36 (I think). I had a lot of backfiring and so removed the front carb, cleaned jets, checked float valve and found that the diaphragm looked like a sieve. Bought a new diaphragm and fitted it. 

Connected up and the bike ran well but now some backfiring from the rear cylinder now. Took diaphragm out and found it was OK - no holes but there was a crack in the carb body which snapped off!!! I couldn't afford a new one so found an aluminium welder specialist and had it welded up. Of course I had to remove it to have it welded. 

Put it back together, with a smidging of gasket goo just to be sure  and now find I can't get the engine to run at all. I realise that the Haynes manual suggests that it is not recommended to undo the couplings from the first carb to the second but had no choice. I have seemed to have put all the couplings back as they came off with the same settings but still no run.

The problem getting it to run has only occurred after removing the rear carb. I seem to be getting a spark but nothings firing up.

What have I done or not done - I'd appreciate the help from anyone who is more familiar with this set up.

Thanks for reading this chapter!!!

if you removed the cables running from the first to second carb you will need to sync them.  but sounds like a jet issue.. if your getting spark then I'm inclined to say examine the fuel delivery.

Given the work you've done,. it sounds more like a carb synchronisation issue ..... or the general set up of the cables. If you look through the workshop manual, there is a section describing where the cables have to be set at identical lengths. I'd suggest double checking everything there.
Since we are looking at start up from cold, I wonder if it may be connected with the fuel enrichment (choke). You need to check if you are getting a spark .... but also let the motor run for a short while, then check the rear plug to see if it is showing signs of getting fuel  .... the plug colour & if it is getting 'wet' with fuel could give further clues.

Section 4-29 in the Factory Service Manual.. it's a PITA.  I use the Emgo carb sync gauges.