2003 Monster 620 bogs under load & burps near injector when engaging clutch

[onthefritz]

First Id like to thank those who have contributed to this site. Its been a huge help over the years. Unfortunate that my first post here is an issue. Hopefully someone else might have had similar issues and can point me in the right direction. Found a few threads alike but no solution.

Bike info: 2003 Monster 620, 10,700 miles
Arrow pipes on it, the bike ran fine with the pipes on it for 2-3 years prior.

The bike starts rough when cold. Idles low until it warms up. Once I put it in gear and give it throttle the bike will burp/cough near the injector/airbox area. Seems like it might have a rhythm to it, if I hold the throttle there. When I try to get acceleration/power while riding, the bike bogs. It backfires once (randomly), when I take it to higher RPMs or when shifting.

COMPLETE LIST of recent repairs/tests throughout this trouble shooting process.

Repairs/part replacements:
new spark plugs
oil change
new fuel filter
new air filter
New fuel pump
fuel pressure regulator
head exhaust gaskets
cleaned all fuel hoses/overflow hoses

Valve Adjustment (had a broken rocker spring and tight valves)
Fuel Injector flushed with solvent using auxiliary tank

Seemed to run better after these 2 repairs were done. Im only getting half the bikes power as if its only running on one cylinder. I can take first gear to a higher than normal speed.

List of tests:
Spark *good
pickup resistors *good
engine compression *good 175PSI/160PSI
belt timing check *good
battery voltage *good
clean spark *good
re-secured loose electrical wire connection *good
cleaned connections on electrical components *good
flushed fuel system *good
fuel pressure check *good 3 Bar when running, 2.5 Bar when not
second engine compression check HOT *good 185PSI/160PSI (have had valve adjustment since)
ignition coil *good
removed air box for injector spray function test *good
Swapped pick-up coil with good unit (no fix)
checked throttle body synch and adjusted (no fix)
checked spark under load - good

air vent/overflow lines have been checked and are clear
the whole fuel system was flushed and refueled.

[onthefritz]

FIXED!

Found and fixed the problem to the bike. Had the valves adjusted and found a broken rocker arm spring. It didnt fix the problem but the shop hooked it up to the dyno free of charge and ran some tests. They also hooked up an auxiliary tank to bypass my fuel system to make sure the problem wasnt inside. When they increased the fuel pressure they noticed it would run better. They ran a solvent through the injectors and it improved the issue some. I was instructed to remove them and take them to a carb/injector cleaning place. Used an ultra sound machine to test/clean the injectors and replaced the screens. They found rust buildup inside the injectors, due to water getting into the fuel.

Hope this helps others save time and money when experiencing the same symptoms.

Thanks for all the help!