Left bank won't fire at low speed. It is a 2004 790 bonneville triumph.

carbs need adjusting for vacuum alignment replace the spark plug on that side and make sure the wire doesn't begin to short out when hot?

Could be one of the carbs playing up. Maybe a blocked or restricted jet.

Bonneville / T100 :: EFI Performance Tune 3 - Triumph Twin ...

www.triumphtwinpower.com/bonnevillet100-performance-tune-3.php?...
Rating: 5 - ‎44 votes I removed the O2 and AIS on my 2009 SE Bonnie. ... First thing I noticed was the smooth idle, next was the deeper exhaust note from my TEC 2:2 .... The power increase all over the band was very good , no popping or back fire at low rev's and

Was the bike working okay after the overhaul? Mis firing due to bad setup or very bad fuel.If it has not run right since the work was carried out ask the repairer to take a look.
If it only started to go awry after using wheap fuel, (it should still be of uniform quality with other fuel,)
I would drain the tank and refill with premium. Try not to get any air into the system if you do this, makes life easier

GOOD LUCK

under the left side cover you'll find three screws holding it in.

A good plan, but if the bike is already missing now and again, I would be tempted to strip and clean the carbs.
It would be worth cleaning the tank out as well.