Hi! I have a 1982 CB650 Nighthawk I bought about a year ago and have done a lot of work to since. I replaced the head gasket, cylinder gasket, piston rings, and all the o-rings involved there. I also rebuilt the carbs (everything but the accelerator pump diaphragm which seems OK) I adjusted the valves (.002 intake/.004 exhaust), the spark advancer (and oiled the springs?) and have the mixture screws out 1 and 1/2 turns each. I synced the carbs and cleaned them twice (focusing on the idle curcuits) but it didn't seem to alleviate the problem. I also bought and installed brand new carb boots and have checked for vacuum leaks everywhere.
Here is what happens when I start the bike: I close the choke and start the bike, It always starts quickly and after about 10 seconds or so I open the choke because it is no longer needed (its not terribly cold here). I can then rev the engine at this point and the RPMs always quickly return down to around 1500. During this time, I can feel that the exhaust coming out of cylinder 3 & 4 is perfect, consistent, and powerful, like those cylinders are firing perfectly. 1 & 2 however feel weak, like they aren't firing nearly as powerfully, with an occasional full-blast puff of air. When I open the throttle, all of the exhaust from all 4 pipes feels right. I thought that maybe the idle curcuit of 1 & 2 might be clogged, but I verified twice that it is clear (removed all the jets and cleaned with cleaner/air)
All cables are working perfectly and the choke/throttle butterfly's open and close fully without issues.
I also swapped around the spark plug cables and the same issues occurred, which I assume means that the cables are not the issue?
After the bike runs for a while and warms up, I hear the rpms (hovering around 1500) start to fall and rise a bit like it might die. When I open the throttle, unlike when cold, the rpms are very slow to return to idle speed. Previously they would also suddenly start to rise uncontrollably, but I haven't experienced that since I cleaned the carbs for a second time. Soon the bike will die, and when I restart it, it will only run if I hold the throttle open. A few times when this would happen, I would open the main idle screw until it was propping open the throttle (way more then normal) to make the bike RPMs mimic idle (1500) and after a short time it was like something suddenly fixed itself and the RPMs would shoot WAY up to 4500 and I could then adjust the idle screw back to its normal adjustment and the bike would run normally again for a short time before the same thing happening again.
I'm currently a bit stumped. Are cylinders 1 & 2 not firing at idle and become flooded? Does this eventually cause the bike to die when it warms up? Is there something wrong with the timing via the spark advancer? (I know that the timing chain is in the right place) Someone suggested that the floats might be sticking (explaining why the dying at idle issue would suddenly fix itself)
Any ideas? Thanks! I am going to pull the spark plugs and look at them and I will report back with their condition.
