Effect of unsynched carbs on rpm drop to idle time

In everyone's experience, how much of a role do unsynched carbs play on the time it takes for a revved engine to drop to idle (under load or neutral)? I have done a bench synch, and have addressed all other issues like air leaks with success and have had the idle and idle drop time decrease. There is still some hesitation with the engine to drop down from rev, and it seems to be more than "normal" from what I have seen on Youtube / others' videos. There is also some hesitation at takeoff from stop, and slightly less power than expected at low rpm despite mixture seeming to be good. Thanks for any thoughts!

If a slight sync problem is the only issue, it won't have much affect on the hanging idle.

As you said, it's often a vacuum leak. But it can also be a slow-to-react advancer mechanism. Often, people don't realize their bike is only running on 2 or 3 cylinders, and that can cause the hanging idle issue as well, so you should do a water spray test on the header pipes.

I plan to do the vacuum synch soon, but just haven't gotten around to buying a long fuel hose to run the bike off of while doing the synch.

I have addressed a major vacuum leak via changing all four carb boot holders which made a huge difference in power, idle smoothness, and overall power. Further carb spray tests do not show any other air leaks present that I can identify. The bike was originally only running on 2 cylinders upon purchase, but a new igniter along with new plugs has fixed that issue. After another 1K miles the new spark plugs are correct color along with mixture screws to spec, leading me to think no major mixture issues.

FWIW, I actually did a spit test about 2 days ago in 15 F. weather. Cylinder 3 took a few minutes to begin firing but once warmed up, all four cylinders were firing and passed the spit on header test. I've never had issues with that cylinder before so I chalked it up to a weak battery and cold temps, but I'll check again this weekend when it's warmer (fingers crossed).

I will have to look into the advancer mechanism this weekend as well since I haven't taken a look in a while. My mechanic supposedly had adjusted the timing back in August during an oil change, but should have verified myself.

A bike will stumble and could hang if the carbs are out of sync. Cables could cause the hanging too. Bench syncing is the final step before adjusting the pilots. Bench syncing will get you close but on the gauges one or more carbs can be way off. At WOT it won't matter much.
Steve

I'm hoping the issue is in fact the carb synch out of whack. The bike pulls fine and has no stutter beyond 2/2.5k rpm, so that would be in check with what Steve is stating. Cables were recently adjusted and lubed, although I'll check again to make sure there isn't some sluggishness there too.

Edit: Came across another bike that seems to have the same "laziness" idle return that my bike has give or take some. See the revving at around the 0:22 mark. Does that sound about normal?....perhaps I'm just expecting too much out of the old girl.

Poor sync can result in a hanging idle. That's one of the reasons why Kawasaki calls for doing a sync every 5000 miles (3100 miles). It's always good to do maintenance, because when any running problems come up you will then know what the problem is not.

Just this evening I ran into bad hanging idle on my nephews Yamaha 650 twin. He had just cleaned carbs and put in new jets etc. He did a pretty good job but had not synced it yet. I hooked up the gauges and #2 must have been way out because I ended up running the sync screw in a couple of turns. Anyway once they were in sync no more hanging idle. Also had a GS750 do the same thing but not as bad. Sync fixed it as well. A good bench sync is a big help to getting them in the ball park but vacuum sync is the only way to get them right.

The fact that #3 took time to start firing is probably a clue. It could be lean for some reason or there could be a cylinder condition issue. 15 deg F is probably not a great time to be tuning a bike anyway (not to mention not fun). At normal ambient temps the sync will probably be different and idle mixture settings will be far from ideal.

How long since the last valve adjustment? The valves tighten with mileage so #3 may be on the ragged edge

Valves were checked w/ oil change in past August...probably 1-2k miles ago and looked good. I'll throw in a new set of spark plugs, check gap, and cut off a bit of the old wire on #3 as maybe it's a spark issue until warmed up. Also will get the tubing and do a carb sync and idle mixture adjustment. I've been chasing an old wives tale that E10+ gas will burn lean, so maybe a tank of E0 will change the odds.

Pretty sure E10 only runs lean with FI and closed loop program (O2 sensors).

I may not have mentioned it but I really think you need to pull the carbs and see why #3 is not doing it's job at idle until warm. My guess is enrichener jet/circuit (the jet in the bottom of the float bowl on Mikuni BS carbs) or partially/fully clogged idle jet/ circuit or both.

A carb sync should reveal that either #3 needs adjustment or not doing anything at idle.

Went out and checked the bike and plugs this morning, as well as threw new plugs in. It was way too cold (15 deg.) to do anything else or even start, but plugs seem mostly fine....possibly a hair lean but should be able to fix with the idle screws. I'll edit my post with a pic of them soon for reference.

The #3 wire seems to be getting very brittle and the cap was somewhat loose from the wire, so I'm guessing the plug had no spark and was making a connection once the wire rubber expanded from the engine heat perhaps. Looks like new wires and caps are in order for spring along with a new battery.

Last, checked valves and while valves were all to spec, a few were towards the tighter side of spec, so I'm thinking valve adjustments along with the sync will help greatly towards the lagging drop to idle. Does this make sound sense to everyone else?