Honda Twins banner

1 - 3 of 3 Posts

·
Registered
Joined
·
4 Posts
Discussion Starter #1
Hello,

I have been working on my buddies 1985 CB450SC Nighthawk over the past couple weeks. The bike is able to idle, but whenever any throttle is given it starts to die. it seams to die inconsistently, one sec it would rev alright for a sec then the next it would die immediately. Do you know what would cause this?

I have confirmed that the left cylinder is not firing/not firing correctly. I have:
  • Confirmed left header is not heating up the same as the right (left not getting more than 120 degrees after a few min of idle)
  • Checked coil input wires, Using a circuit tester confirmed power is being sent to the black/purple wire (I assume right cylinder?) and it did not light up on the green wire (I assume left cylinder?).
  • Using a timing light confirmed a consistent spark/timing for right cylinder, while a weak/sporadic light for the left cylinder.
  • removed spark plug boots, trimmed wires, and reattached.
  • I am not seeing any damage to wiring in the harness and have no idea how an issue like this could only impact one cylinder.
  • Replaced and Gaped spark plugs at .7-.8mm. Left plug still looks new while right looks like its been being used.
  • Spark seams kind of weak generally, could this be stater related? /timing pick up related? But I would have assumed spark would be stronger with a fully charged battery.

Originally I had assumed it was a carb problem so I pulled off the carbs to find a bunch of stuff wrong (incorrect jets, seated pilot jet screw, clogged accelerator pump nozzle, and more). I then cleaned the carbs (all passages clear), put new gasket kits in, put correct jets etc.
Now the bike idles better, but still dies at throttle.
After Carb work I also confirmed that:
  • Slides are moving and choke is not incorrectly engaged(confirmed using endoscope)
  • Petcock is not restricted by using gravity fed fuel.
  • Both Carbs are receiving fuel.

Any ideas on what would cause this to happen?
Have you seen this before?
Is there a ground or something I should check?
I would have expected failure of the ignition system to impact both cylinders. I have been looking at a manual I found a link to on this site but have not been able to wrap my head around how the ignition is working and what could cause this.

I plan on starting to do the Ignition Diagnosis detailed in this forum in the next day or two.

Thank you!!
 

·
Premium Member
Joined
·
24,208 Posts
Fuel system and ignition system symptoms are very close to one another making it hard to pick which is at fault.
Start with the ignition because it's a set of fast tests and if it's right you have a solid foundation to work from. Ignition Diagnosis for CB/CM 400/450 and CB450SC manual...
Once that's right if the problem still persists then it's carb rebuild time and the only way to get them right takes time and patience.
 

·
Registered
Joined
·
13,085 Posts
Hello,

I have been working on my buddies 1985 CB450SC Nighthawk over the past couple weeks. The bike is able to idle, but whenever any throttle is given it starts to die. it seams to die inconsistently, one sec it would rev alright for a sec then the next it would die immediately. Do you know what would cause this?

I have confirmed that the left cylinder is not firing/not firing correctly. I have:
  • Confirmed left header is not heating up the same as the right (left not getting more than 120 degrees after a few min of idle)
  • Checked coil input wires, Using a circuit tester confirmed power is being sent to the black/purple wire (I assume right cylinder?) and it did not light up on the green wire (I assume left cylinder?).
  • Using a timing light confirmed a consistent spark/timing for right cylinder, while a weak/sporadic light for the left cylinder.
  • removed spark plug boots, trimmed wires, and reattached.
  • I am not seeing any damage to wiring in the harness and have no idea how an issue like this could only impact one cylinder.
  • Replaced and Gaped spark plugs at .7-.8mm. Left plug still looks new while right looks like its been being used.
  • Spark seams kind of weak generally, could this be stater related? /timing pick up related? But I would have assumed spark would be stronger with a fully charged battery.
Originally I had assumed it was a carb problem so I pulled off the carbs to find a bunch of stuff wrong (incorrect jets, seated pilot jet screw, clogged accelerator pump nozzle, and more). I then cleaned the carbs (all passages clear), put new gasket kits in, put correct jets etc.
Now the bike idles better, but still dies at throttle.
After Carb work I also confirmed that:
  • Slides are moving and choke is not incorrectly engaged(confirmed using endoscope)
  • Petcock is not restricted by using gravity fed fuel.
  • Both Carbs are receiving fuel.
Any ideas on what would cause this to happen?
Have you seen this before?
Is there a ground or something I should check?
I would have expected failure of the ignition system to impact both cylinders. I have been looking at a manual I found a link to on this site but have not been able to wrap my head around how the ignition is working and what could cause this.

I plan on starting to do the Ignition Diagnosis detailed in this forum in the next day or two.

Thank you!!
Please check your Conversations
 
1 - 3 of 3 Posts
Top