Hey all,
I bought my first motorcycle, a 1979 cb750k, last July. When I bought it, I really had no idea what to look for and couldn't spot any potential issues. When I bought it, the previous owner rode it over to my place and it didn't seem to have any issues with starting (warm).
In the coming days, I came to realize that it was very difficult to start while cold, idled rough, and occasionally had weird acceleration/surging issues. Also, before it was warmed up, it had a tendency to die while idling. I didn't ride it around much after that because the drivers in St. Louis are genuinely terrible and I hadn't yet taken a basic rider's course, so I wasn't comfortable.
Fast forward to this year. I cleaned the carbs and discovered a cracked accelerator pump as well as a float valve that was stuck from being gunked up. I replaced the accelerator pump. The bike starts up much easier than it did last year, so I think I've mostly remedied that issue. However, it still idles pretty rough and dies sometimes before it's warmed up.
I've got a host of questions because I don't know what's normal for these old bikes and what's not, so I'm just going to list a few of them here:
1. When I was cleaning the carbs, I noticed that the head of the pilot screw for carburetor #1 is raised much higher than the other pilot screws. I attempted to "lightly seat" all of the pilot screws and back them out to the appropriate specification outlined in the manual (I think 1 1/2 turns or something). I didn't really have much of a feel for what lightly seating the screws feels like, though, and I didn't want to damage them. After doing this, screw #1 was again raised up much further than the other screws. My question is this: is this normal, and for some reason carb #1 is designed differently, or is there debris/corrosion that made me think it was lightly seated when it really wasn't?
2. While trying to diagnose problems, I ended up killing my battery from starting/idling/starting it too much. I walked away, waited about 20 minutes, and was able to start it back up. When I did that, I revved it up to ~3k rpm and held it there for a minute or so. I noticed what I think was backfiring during this and ended up hitting the kill switch because the pipes were very, very hot and I think I smelled burning and saw some smoke coming off of something, I think it was an exhaust pipe. Can backfiring be caused by a weak battery, and would you expect it to get hot enough to flash-vaporize water after about a minute of 3k rpm at a standstill?
3. How long should I be holding the choke while starting it up? For the first few minutes, I have to fiddle with it constantly to keep it within an acceptable rpm range. Too much choke and it revs high, then declines and idles rough/dies. Too little, and it idles rough/dies.
4. The engine sounds mechanically... off, to me, but I don't really know what I'm listening for. I've got a video on my phone that wasn't meant to be showcasing the engine noise but after listening it might give you an idea of what it sounds like at idle. Please let me know if you think something sounds wrong (knocking?).
I've uploaded a couple videos that aren't really great quality, but they may be of some use.
The first, I was filming the exhaust. I suspected that maybe one cylinder wasn't firing because the consistent puffs of smoke out of the left exhaust, but only intermittent puffs out of the right exhaust.
The second, refer to point #4.
#1:
#2:
If better videos would help, let me know and I'll upload as soon as I can.
Sorry for the long post, I know it's a lot to read.
Thank you all in advance, advice would really help me out. I am essentially a complete noob and have no idea what to look for, where to start, or even what's wrong in the first place.
I bought my first motorcycle, a 1979 cb750k, last July. When I bought it, I really had no idea what to look for and couldn't spot any potential issues. When I bought it, the previous owner rode it over to my place and it didn't seem to have any issues with starting (warm).
In the coming days, I came to realize that it was very difficult to start while cold, idled rough, and occasionally had weird acceleration/surging issues. Also, before it was warmed up, it had a tendency to die while idling. I didn't ride it around much after that because the drivers in St. Louis are genuinely terrible and I hadn't yet taken a basic rider's course, so I wasn't comfortable.
Fast forward to this year. I cleaned the carbs and discovered a cracked accelerator pump as well as a float valve that was stuck from being gunked up. I replaced the accelerator pump. The bike starts up much easier than it did last year, so I think I've mostly remedied that issue. However, it still idles pretty rough and dies sometimes before it's warmed up.
I've got a host of questions because I don't know what's normal for these old bikes and what's not, so I'm just going to list a few of them here:
1. When I was cleaning the carbs, I noticed that the head of the pilot screw for carburetor #1 is raised much higher than the other pilot screws. I attempted to "lightly seat" all of the pilot screws and back them out to the appropriate specification outlined in the manual (I think 1 1/2 turns or something). I didn't really have much of a feel for what lightly seating the screws feels like, though, and I didn't want to damage them. After doing this, screw #1 was again raised up much further than the other screws. My question is this: is this normal, and for some reason carb #1 is designed differently, or is there debris/corrosion that made me think it was lightly seated when it really wasn't?
2. While trying to diagnose problems, I ended up killing my battery from starting/idling/starting it too much. I walked away, waited about 20 minutes, and was able to start it back up. When I did that, I revved it up to ~3k rpm and held it there for a minute or so. I noticed what I think was backfiring during this and ended up hitting the kill switch because the pipes were very, very hot and I think I smelled burning and saw some smoke coming off of something, I think it was an exhaust pipe. Can backfiring be caused by a weak battery, and would you expect it to get hot enough to flash-vaporize water after about a minute of 3k rpm at a standstill?
3. How long should I be holding the choke while starting it up? For the first few minutes, I have to fiddle with it constantly to keep it within an acceptable rpm range. Too much choke and it revs high, then declines and idles rough/dies. Too little, and it idles rough/dies.
4. The engine sounds mechanically... off, to me, but I don't really know what I'm listening for. I've got a video on my phone that wasn't meant to be showcasing the engine noise but after listening it might give you an idea of what it sounds like at idle. Please let me know if you think something sounds wrong (knocking?).
I've uploaded a couple videos that aren't really great quality, but they may be of some use.
The first, I was filming the exhaust. I suspected that maybe one cylinder wasn't firing because the consistent puffs of smoke out of the left exhaust, but only intermittent puffs out of the right exhaust.
The second, refer to point #4.
#1:
#2:
If better videos would help, let me know and I'll upload as soon as I can.
Sorry for the long post, I know it's a lot to read.
Thank you all in advance, advice would really help me out. I am essentially a complete noob and have no idea what to look for, where to start, or even what's wrong in the first place.