Post by spindlehead on Sept 28, 2008 16:56:12 GMT -5
I had a very unhappy ride yesterday. Everything started out great. My gradual carb jetting/tuning has been coming along great and the bike has been running better than ever.
The bike was running like a top at the start of a group ride. About 30 miles into the ride (mixed street traffic and coastal highway), it started bucking, surging, sputtering and nearly dying at anything above about 1/8 throttle and around 3500 rpm. This only happened under load. In neutral I could wind her up to redline at all throttle positions. The feeling was very much like an over-rich needle setting. I also think I could hear a faint knocking or clattering sound from the top end during the surging. Hard to tell because my exhaust is loud. By shifting early and creative clutch-work from stops, I was able to sputter home.
Oh yeah, it's a '78 with a Mikuni VM34-168 carb, round K&N air cleaner and a shorty "free flow" exhaust.
These are the things I have verified:
- The main jet was tight
- There is no blockage in the main or needlejet
- Changing the needle height in either direction does not change the symptoms much
- The plug is showing rich but not fouled
- The carb is clamped tightly in the boot which has no cracks and appears to be well sealed to the head.
I am reasonably sure that this isn't a fuel system issue. A couple of fellows mentioned timing and valves in another thread. Could either of those explain the sudden onset of my symptoms?
The bike was running like a top at the start of a group ride. About 30 miles into the ride (mixed street traffic and coastal highway), it started bucking, surging, sputtering and nearly dying at anything above about 1/8 throttle and around 3500 rpm. This only happened under load. In neutral I could wind her up to redline at all throttle positions. The feeling was very much like an over-rich needle setting. I also think I could hear a faint knocking or clattering sound from the top end during the surging. Hard to tell because my exhaust is loud. By shifting early and creative clutch-work from stops, I was able to sputter home.
Oh yeah, it's a '78 with a Mikuni VM34-168 carb, round K&N air cleaner and a shorty "free flow" exhaust.
These are the things I have verified:
- The main jet was tight
- There is no blockage in the main or needlejet
- Changing the needle height in either direction does not change the symptoms much
- The plug is showing rich but not fouled
- The carb is clamped tightly in the boot which has no cracks and appears to be well sealed to the head.
I am reasonably sure that this isn't a fuel system issue. A couple of fellows mentioned timing and valves in another thread. Could either of those explain the sudden onset of my symptoms?
