This has me stumped trouble shooting it!

I have a suzuki boulevard c50, 2005 with 21000 km on it, its been converted into a custom Trike. I haven't weighed it, but I think with the mod and I'm a big rider (280lb), so it is underpowered for what they built.

my problem is, last week it would pull ok in town, it has a lot of trouble maintaining highway speeds going up any hills, but it could mostly maintain.

I got it serviced, new tire (general maintenance) last week rode home fine, went to work, then on the way home it started to pop and back fire a bit and run a bit rough. The next day it was back firing on decel like a gun shot at times, and popping and sputtering on acceleration.

So far I've changed the spark plugs, air filter, fuel filter, flushed the gas tank, changed the fuel pump (hail marry play) fuel pressure regulator is fine (40pis) compression to both cylinders is 220pis, air box is clean, disabled the pair valve too.

It's running really smooth now, not a pop! pulls pretty well up to 3rd gear in town....but it wont maintain spreed on the highway! Flat out it consistently scrubbs speed going up any hill, and even flat out in 5th it'll top out at 110km/hr even down hill.

Only thing left I can think I can think of is fuel injectors?

Any ideas on what eles to trouble shoot would be greatly appreciated!?