'84 KZ550 Ltd no low end power

Bike in question is a 1984 KZ550 Ltd, also known as an F2 model here in the Great White North. Approx. 32,000Km, or about 20,000 miles.

The problem:no low end power, as in it almost dies if I try to accelerate when I'm below 3500-4000rpm. Once I hit 3500rpm, it takes off like a rocket.

What I've done:I installed a stock exhaust system on it. It had aftermarket mufflers prior to this, but they were all rusted to pieces. I anticipated rejetting but upon checking the jets, mains are stock, and so, as near as I can tell, are the pilot jets. I was told by my local cycle guru that pilot jets were almost impossible to get for these carbs, so they are likely stock. I cleaned the carbs, blew with air, cleaned again, and balanced them once I got it going. Plugs look normal.

I'm planning on checking the needle position, and also removing the exhaust to check for a blockage of some kind. I know the exhaust sat for a long time before I got it, maybe some critter got in there and made a nest...

Any other ideas?

Thanks in advance
Brian

maybe the mechanical advancer is sticking?

I thought about that, but according to the manual (Haynes) the advance on the F2 models is controlled electronically, and not mechanically. Mind you, I never looked at it, just took the book at it's word so I guess it's worth a look-see.

Brian