I've just got my '84 FZ400 up and running for the winter commuting season, and I'm having a few running problems. The bike starts OK, but any choke kills the motor instantly and means I have to leave it for a few minutes. Not a big problem but it does mean I have to keep the motor running and warm it up using the throttle. The real annoyance is a *huuuuuge* flat spot at about 5000-7000 rpm. In first gear the bike can just about pull through it with a lot of farting and spitting, but in the higher gears the only way through it is to gain momentum using lower revs then drop a couple of gears and nail it. Above 7000 rpm the bike takes off like a good-un and feels strong. Extra annoying because this rev range coincides with 70mph in top, so the cruising choices are 65 mph or 85+. I've stripped the carbs, which are pretty clean. I've made sure the main and pilot jets are clear, cleaned out the air screws, checked the diaphragms are intact and moving smoothly, and also the little jet in the base of the diaphragm chamber in each carb also looks clear, although I can't remove them for some reason. The floats all function and are set as I'd expect (not been able to find any official service specs for the bike), and the fuel lines are clear (also i tried the fuel tap in "prime" position) The only thing I haven't disassembled is the choke circuit. The motor itself has recently been replaced with a secondhand one, and it behaved the same before the swap except for the choke behaviour. Also the flat spot seems to get worse the hotter the motor gets, am I correct in thinking this implies a overrich mixture condition? Anyone got any ideas before I go off in search of a new set of carbs? Cheers and thanks, Corin Grieves.
