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Hi all, I am new to this site, but not new to vintage bikes. I worked at a Kawa and Honda shops back in the 70s, last year I picked up a 1980 KZ1000 Ltd B4. I have gone through the bike with a fine toothed comb, here is my problem:
Exhaust is V&H 4/1, air filters are cheap air pods. Valve adjustment complete, all tolerences are a tad to the loose side. Carbs have been gone through and cleaned and rejetted. New Dyna ignition, including ignition and pick up coils, igniter is no longer used. Throttle is smooth, opening and closing with no binding. Throttle cables adjusted with proper slack. Ignition timing verified with timing light, battery charged. Plugs are reading good - a tad rich if anything (even though the pipes are bluing some???). 99% certain there is no vacuum leak, although the symptoms I have indicate such. So...when "blipping" the throttle to increase engine speed, the bike is slow (3 - 10 seconds) to drop to normal idle speed. The timing advance is free with no binding, but I an starting to suspect perhaps the springs are worn out. Any ideas most appreciated. I have restored several vintage era in-line 4's - but this one has me stumped! I am sure I am not the first person to encounter this - so the advice and experience of this forum greatly appreciated!

Yes, the worn out advance springs will cause your symptoms. I have had em that looked and checked out perfect but when you gassed the bike it would not come down to idle for about 20 seconds. What I do now on all used advance springs is cut off one coil no matter what they look like or how they check out.

weak advancer springs or carb synch or vacuum leak

you can tell if it's the advancer springs by dragging your finger on the dyna rotor after revving the engine slightly . if it returns to normal idle , there is the problem......

This may be coming from left field but I had that issue when I was moving to a stage II kit and it only stopped when I backed the mixture screw out more (3 1/2 turns)

Thanks to all for the inputs. I will certainly be modifying the advance springs, and the finger trick on the ignition cam is also a great idea. Backing out the air screw to 3.5 turns is something I would never have thought of, but will put that on my list of things to try in case I need it.

manifolds that old dry out and leak. had that problem on my 79 kz1000, slow to return to idle. i replaced them and problem solved! im not saying this is your problem, but something to look into.

flying dutchman

I would not recommend it but I remember a article on the Vance and Hines pro stock bike (pretty sure it was the 1000 Suzuki) and they filled the slides with lead to get the effect of returning to idle extra fast when blipping the throttle.

Always thought someday I might do this but sounds like if you didn't really do it right it would be easy to crack and get a chuck sucked into the motor.