Total Motorcycle Community Forums

Hi,

I have a 1984 Nighthawk CB550. I have been trying to diagnose this problem for a while, and even tried some suggestions from posting here, which included replacing the spark plug and wire to the 4th cylinder. I was hoping to get peoples opinion if it was a carb sync or fuel mix problem, as indicated by this post:

http://www.motorcycleforum.com/showthread.php?t=98336

My problem is the 4th cylinder mostly fires at idle (determined by the heat generated in the exhaust header), but then cuts out at city driving speeds. I can feel significant loss of power, and more appropriately the gain in power when i open the throttle and the 4th cyclinder kicks back in at around 5000 rpm. I have recently checked the resistances of the ignition coil supply the 4th cylinder and its as it should be according to Clymer. I have also checked and I do get a spark on the 4th (holding plug against transmission case) and the resistance in the lead is 5kΩ as it should be. So yeah, I am just wondering if anyone has any suggestions, if they think it could be compression related or sync related. I cleaned the carbs just last season, but do not have much experience (or tools) for evaluated carb synchronization or fuel mixture.

Thanks
Jeff

that sounds like a fuel or vacum issue. I would check the carbs and make sure they are all seated and the boots are good,if so get busy pulling them carbs cause you got a plugged somethin

Instead of paying big bucks for one of those units you can make your own manometer for around $12. viewtopic.php?f=18&t=34471

Yeah, I have looked into that as well. Thanks. I am just nervous about doing it I suppose. I am going to check for vacuum leaks first and foremost, because it is an old bike that had not been taking care of and the boots look pretty gross.

Do you think this problem would cause my bike to have difficulty starting from cold? I have completely killed a battery in one season turning it over a lot in 10 degree weather, and now occasionally need to throw boiling water on the carbs to start it without completely draining the battery. I am going to look into a better trickle charger for battery storage and need to take better care of my batteries. At this point I think I definitely need a new battery, but am wondering if the bike is damaging the battery or if I was just particularly rough on it.

I think the solution to your problem is HERE.

You mean I should sell my bike? If so, thats pretty funny.

Either way, I dont think I will sell it yet...I think I will do the following steps to troubleshoot, working back a bit:

-check the plugs (especially 4th) for good sparks, and swap the leads to see if I get the problem on cyclinder 1. I tried this before and the problem moves with the leads, which indicates its likely not the carbs. That was last year though and I haven't kept a log book, so I could be mistaken. I find that when I speculate a problem is due to something, I get fixated on that assumption and forget to keep an open mind. I'll work on that too.

- I will also check the deposits on the plugs to try and determine if my mixture is lean.

Switching the leads should rule out fuel or carb issue, right? If so then could it be an electrical problem? Any advice would be great, and I will let you know about the results from switching the leads on the 1 & 4 cylinders.

Thanks
Jeff

So, I checked the plugs for sparks, swapped the leads (coil connections), and swapped the wires. All resulted in the 4th cylinder behaving as mentioned in the original post. Looks like I will be pulling the carbs...

Today it was not firing at idle either, so I am going to do some research on setting the slow jet screw and try and make sure that is set properly. Man it is hard to not sound foolish on forums, but anyway, thanks for the advice and I will keep the post updated incase anyone has a similar predicament in the future.

Jeff

jbethune wrote:You mean I should sell my bike? If so, thats pretty funny.

I meant you should pick up a different bike, whether or not you sell that one. (-: It sounds like a nightmare, I had an '84 V65 that spent more time laid up than on the road. But I think there's a certain subset of folk who enjoy wrenching on their bikes more than riding them, and that's cool too.