Bike won't run under load

I have a 78 KZ650 that was running pretty well until recently. It would take a little while to warm up, but once it was warm, it would run pretty good - the only issue was a little backfiring under deceleration. Yesterday I took it out, and it was running good while in neutral, very responsive to the throttle... But when I took off, it would need alot of throttle to get going, and as soon as I let the clutch completely out the engine would really struggle. I could ride it in first gear up to about 15 mph, but it would sputter in 2nd, and die in 3rd. Does anyone have an idea of where I should start looking?

Please give a little more background. Is the engine, airbox, exhaust, points, etc. stock? When was the last time it was tuned up? Has it been used on a regular basis? Does the inside of the fuel tank show any rust? Does it have a fuel filter? Is maintenance (oil changes, etc.) performed on a regular basis? Thanks, Ed

I just changed the oil. It gets ridden alot. I just got the bike last year, and I haven't had it tuned up. The air filters are the 4 pods, not the stock air box, but everything else is stock. I don't see any rust inside the fuel tank, but I haven't changed the fuel filter. That's a good idea - the last time I had the bike out and it was running well, I almost ran it out of gas, so it's possible that I sucked some sediment that was in the bottom of the tank into the fuel filter. Any other ideas?

Checked compression? Checked valve clearances? Air filter not clogged? Good spark? Changed sparkplugs with proper gap?

Checked mechanical ignition advancer for proper operation?(it's behind a CD sized cover on right side of engine and should return to position at rest when twisted by hand with engine off).

Is it firing on all cylinders ?
Fire it up. Give it a minute to get warm and then spit or spray water on the exhaust header where it exits the motor. If the cylinder is firing the water sould sizzle right off.