Poor performance from my 2004 Suzuki gs500f

Question

Bike has 23,300 miles on it.Ridden daily. But has notiicably lost top end power. Use to top out at 105-110 mph, now struggles to reach 90-95 mph. Must downshift for slightest hills. "putt-putt" sounding backfire when at idle and throttle opened. New plugs in , set at .35, oil change, chain adjusted properly, Could an old airfilter cause this much of a decrease in perfomance? Frustrating, as I dearly love to ride and don't know how to trouble shoot this problem. Having a shop diagnose is not an option..(cash flow problem right now!) Any suggestions? Appreciate any help! Also, only mods made on bike have been removal of fairing, making it look like a GS500E and a taller rear tire.

Answer 1

putt-putt, backfire is the sound of a rich fuel condition

is it hard on gas?

are the plugs black?

take a compression and a cyl leak down test

Answer 2

Clean the air filter or replace it and start using premium gas. 93 octane.

If you've been using 87, your fuel system may be dirty. most of the guys I ride with use 100 octane once a month and use 93 else wise to keep things clean.

Answer 3

Do your air filter! And have you adjusted your valves? One of the biggest problems with bikes is valves and carbies. So check them both out!