I thought the Kawasaki Police Bike KZ P19 was running a little rough - nothing significant just a little less smooth than what I'd like. I replaced the plugs with the recommended NGK BR8ES item number 5422 (10 dollars including tax at Napa Auto Parts). Since I had the gas tank off, and I'd never down it on this motorcycle, I synchronized the carburetors using my Morgan Carbtune.
The plugs were okay - one was a little sooted up from running rich but they were all good enough to provide a hot spark. The carburetor synchronization was way off though. One was matched to two, and three was matched to four - but the left two and right two were off from each other significantly. To get them in synch I had to decrease the vacuum (gasoline flow) on three and four to match one and two, and then set the idle speed back up to 1000 rpm.
The spec for the vacuum is less than 20 mm Hg, ideally exactly the same - but that's not quite possible usually since the vacuum indicators are moving a bit as the engine idles. When I started three and four carbs were pulling about 80 mm Hg more than one and two - so off quite a bit.
I just used the motorcycle's gas tank while doing the carb synch, rather than fooling around with a temporary gas tank. I have a long piece of fuel line and cut a piece of metal pipe that would fit inside the fuel line so I could splice my long fuel line to the fuel line connected to the carburetors. I sat the gas tank on a step ladder cross bar with a couple of short 2x4's underneath to stabilize it and ran the engine with the petcock on prime.
It's running smoother now and there's less engine vibration.
One thing I've noticed a few times with used motorcycle ads on Craigslist is a statement something like - running rough or not running, needs carbs synchronized. You won't correct a motorcycle that won't run or is running really rough by synching the carbs. A carb synch is more of a fine tuning operation. When the carbs are out of synch the amount of fuel getting to each cylinder is not the same - that will hurt fuel economy and cause the cylinder(s) with more fuel to try and run faster which causes some vibration...but it isn't going to cause a bike to not run at all or run really rough.
There are exceptions I can think of that could cause a bike to run really rough and have the carbs not in synch - one would be that someone started twisting on the synch screws without a manometer which I suppose is a possibility, but I'd be worried what else might have been done to that bike if I was buying it.
Carbs that aren't in synch (pulling the same amount of vacuum) can be the symptoms of something else wrong - intake valves being out of adjustment, or the rubber carb boots having leaks, so you have a vacuum leak upstream of a carb. In these cases you could have a bike that's running poorly - but you need to adjust the valves or replace/repair the carb boots not just synch the carbs as the Craigslister's sometimes suggest.
