TOPIC:

Hi, hopefully someone more knowledgeable than I am can help with this one !

Z650B1, running on 3 cylinders but not number 2. Dyna coils & ignition (the ignition unit very recently replaced), S&B air filters (fit to outside of back of each carb, not those awful push-ons that can block airways !)

Compression test is within spec and virtually identical on all 4 cylinders.

I've changed all 4 spark plugs for new ones, and done a bit of swapping around on the off-chance that one is bad. Number 2's new spark plug looks clean and unused compared to the others (which I guess tells me it's either not firing, or fuel isn't getting through).

I've rebuilt the carbs several times over the last year or so; we seem to have a real issue with the quality of fuel here in the UK (I'm using E5 with a fuel stabiliser, to stop it going off after about 2 weeks).  It usually seems to be cylinder number 2 that gives me problems !

So my first question is; am I right in thinking that if I had an electrical problem with the ignition or a timing issue, it would affect numbers 2 & 3 equally ? Is it even possible to not fire on number 2 but still fire on 3 ? 

Assuming that is correct, is there anything else anyone can think it could be other than either a vacuum leak in the inlet rubber or a carb problem ? What else could I be missing ?

Thanks !

i bought a KZ1000A model a number of years ago.  The bike ran on three cylinders and I fiddled with all those things you mentioned.  Ultimately, I pulled the head and discovered that I had a galled (stuck) exhaust valve on the cold cylinder.  The likely culprit is usually simpler than what I went through, though.

Something I would suggest is to move the spark plug leads on #2 and #3 and see if the issue follows the lead that had been on #2. If it does then the issue is not with the carb, but with the coils/leads.

The other thing I would ask is just which carb set you have. Some were pilot screw only, some were air screw only and some had both types of screws. 

Agree swop plug leads 2 & 3 over if it follows the lead then it's not carbs.
If it does follow the leads then check connections both ends of the ht lead. Cap to lead, lead to coil. 
Both sides of a coil use the same windings but the circuit does split to provide power to both leads. I have only once found a break after the split which meant one lead was live whilst the other was dead. So yes it is possible but its unlikely. 
My guess would poorly connected plug cap or knackered plug cap or knackered plug. 

Thanks for the replies !

Good idea about swapping the leads over - I'll give that a try when it's daylight tomorrow. They're the dyna leads with the integral cap rather than separate screw-on NGK type caps, which hopefully makes things easier.

The plugs are brand new, but #2 didn't work with both old and new plugs (even the old ones were very new), so hopefully the odds of getting two bad plugs that just happened to end up in the same cylinder are pretty low !