Almost ever since I got my bike, it has not been firing on all four on startup. I always apply full enrichment for start and bleed it off as I am able, and although it fires right away, one cylinder does not kick in for about 10-15 seconds. Then it starts to sparatically fire and will eventually kick in and be just fine. All plugs look the same. Can this be simply one carb set a bit too lean or can it be spark.
I know, the old quandry again.
That could be about anything.. As Paul said it just might be as simple as a spark plug problem, (like hair line crack in the porcelain, or slight cold fuel fouling).. Could also be a seeping cylinder head gasket allowing a slight amount of coolant into the cylinder & causing a misfire until it cleans out after a few strokes).. Could also be a plug wire acting up due to resistance or a slight leak..
You might want to look at the carb enrichener adjustment on all carbs (or if you isolate the faulty cylinder, the carb on that cylinder), maybe one of your carb enricheners isn’t pulling open far enough..
If the cylinder that is acting up is farthest from the fuel inlet to the carbs that carb might be low on fuel at start-up & takes a second or so to completely fill..
If the cylinder/carb that is causing the misfire is the carb with all the vacuum hoses to it maybe the hoses are filling with fuel during the post ride hot soak or leaking enough vacuum to cause it to go lean during start up..
If your bike has a pulse air system it’s possible you have one exhaust check valve hanging open a short while at start up & allowing exhaust gas to back up into the intake manifold.
Another possibility is one of your hydraulic valve lifters is bleeding down at night & causing a lazy valve opening until it runs a short time & pumps the lifter back up.
It could also be ignition or pulse generator related but that usually causes 2 cylinders to act up not one.. On the same note, it might be low coil supply voltage & the problem goes away as soon as the charging system kicks in after the engine RPM’s come up..
If it gives you rythematic popping into the intake as it misfires that usually says lean, so you could very well have a lean set pilot needle, or slight plugging of one of the pilot jet channels.. As a test, maybe try opening each pilot jet needle 1 extra turn (do one carb at a time) & see if that has an effect on your problem..
Other than that, maybe a slight intake manifold vacuum leak or worn intake valve guide (oils up & seals after start-up..
Does it make any difference if the bike is started on the side stand or center stand? How about if it’s started for a few seconds after it cools down, then shut down for the night (that should leave filled carbs as the engine heat won’t boil the fuel out)..
You might pull all 4 spark plugs, then let it sit all night, then crank it cold with no plugs in it & see what comes out each cylinder… While the plugs are out maybe run a cold cranking compression test to see if you have any low cylinders cold..