I am new to this forum and I apolgize if my motorcycle is considered off topic. It's a 1979 XS750 SF. I have made one other thread and had such helpful comments that i think this is the place for questions, vs other forums.
I'll keep the back story very short...
Picked this bike up a little over a month ago. It ran pretty smoothly. At least it wasn't so unstable feeling that it ever was noticed. I let it sit for like a week at one point, then decided to start riding it and tinkering with it. During that down time, it rained pretty heavily and i had been at work and didn't expect that weather. The next day or so I rode the bike and started noticing irregular engine behavior, and also it was stalling during idle almost every time i let off the throttle, even after warming up. So consulted a local friend. He advised that perhaps i got some moisture in tank due to heavy rain and old seals.
So, commence the restoration work.
Emptied and cleaned tank. Thoroughly, and with proper substances and methods. Emptied the float bowls and let them dry out. Removed the K&N air filters and cleaned them with their proprietary cleaning solutions/methods. Refilled with Ethanal free fuel. Put fuel filter pods between pet cock output and carb input, to monitor the fuel passing to carbs for a while. Drove it like that for a bunch of miles, not the finest spec of anything was in pods. Removed pods. Also installed new spark plugs, with carefully measured gaps and with carefully torqued install. Installed new tank seal of course.
The startup was EXCELLENT. Starts up so easy that no choke is needed at all, and it's so fast that it's already running before i can even take my thumb off the button.
But the rocky performance did not at all go away like i had hoped. At this point I parked the bike in driveway and made a short video demonstrating the irregular performance during idle. But it's tough to hear the pattern. Basically if you just imagine the idle sounding like the valves are making morse code instead of a repeated circular pattern, by taking out a beat here and a beat there and a couple beats here and another beat several beats later, so on and so forth, it just sounds unstable and inconsistent. But not in that spitty/harley exhaust way where the engine is stable but the exhaust pops in a weird pattern. My engine is not stable. Sometimes it jerks up a bunch of RPM's just for like a 1/2 second. And sometimes the opposite.. it dips down in RPM's just for a 1/2 second.
Inspecting the carb boots, i noticed there were hairline cracks all over the place, and that they would expand if pulled a bit. Found out that this can lead to bad balance between of air/fuel mixture. Ordered boots, installed them today.
Rode again. Exactly the same as before putting in fresh carb boots.
However, it does seem like it's not overheating like it used to. I had noticed that the engine would get pretty warm pretty darn quickly. But now it's not doing that, i don't think. I let it idle for like 10 minutes, then rode it for just 5. Previously that would have caused some heat. Now nothing gets too hot. And today here in New York is a very hot and humid day. So i think the carb boots have helped in that regard, as was to be one of the expected improvements from them being new and not leaking in air and throwing off the mixture.
Today during the ride i also THINK i noticed some weird light metallic grinding sound coming from the engine. Pretty scary sound, so i shut it down. Only hear this when actually riding though. Hoping i was just hearing the speedo shaking randomly or something.
I've been in touch with the previous owner through all of this. The carbs were rebuilt in September of last year (2018), and were sync'd then too. Compression was tested to be good.
Is my next logical step to start down the path of valve alignment and carb re-syncing? Would carbs go this off line in just a year? I have heard that out of sync carbs can cause the engine to shake at idle and low speeds. My engine truly shakes. But not always. It follows the pattern of erratic cycles. Shakes a lot sometimes.
I'm happy with what is seemingly really good air flow and fuel flow. I think if i can resolve the engine issue, the overall performance will be quite smooth and easy now that all of this basic down and dirty work was done.
Any ideas about the symptoms though?
I'll keep the back story very short...
Picked this bike up a little over a month ago. It ran pretty smoothly. At least it wasn't so unstable feeling that it ever was noticed. I let it sit for like a week at one point, then decided to start riding it and tinkering with it. During that down time, it rained pretty heavily and i had been at work and didn't expect that weather. The next day or so I rode the bike and started noticing irregular engine behavior, and also it was stalling during idle almost every time i let off the throttle, even after warming up. So consulted a local friend. He advised that perhaps i got some moisture in tank due to heavy rain and old seals.
So, commence the restoration work.
Emptied and cleaned tank. Thoroughly, and with proper substances and methods. Emptied the float bowls and let them dry out. Removed the K&N air filters and cleaned them with their proprietary cleaning solutions/methods. Refilled with Ethanal free fuel. Put fuel filter pods between pet cock output and carb input, to monitor the fuel passing to carbs for a while. Drove it like that for a bunch of miles, not the finest spec of anything was in pods. Removed pods. Also installed new spark plugs, with carefully measured gaps and with carefully torqued install. Installed new tank seal of course.
The startup was EXCELLENT. Starts up so easy that no choke is needed at all, and it's so fast that it's already running before i can even take my thumb off the button.
But the rocky performance did not at all go away like i had hoped. At this point I parked the bike in driveway and made a short video demonstrating the irregular performance during idle. But it's tough to hear the pattern. Basically if you just imagine the idle sounding like the valves are making morse code instead of a repeated circular pattern, by taking out a beat here and a beat there and a couple beats here and another beat several beats later, so on and so forth, it just sounds unstable and inconsistent. But not in that spitty/harley exhaust way where the engine is stable but the exhaust pops in a weird pattern. My engine is not stable. Sometimes it jerks up a bunch of RPM's just for like a 1/2 second. And sometimes the opposite.. it dips down in RPM's just for a 1/2 second.
Inspecting the carb boots, i noticed there were hairline cracks all over the place, and that they would expand if pulled a bit. Found out that this can lead to bad balance between of air/fuel mixture. Ordered boots, installed them today.
Rode again. Exactly the same as before putting in fresh carb boots.
However, it does seem like it's not overheating like it used to. I had noticed that the engine would get pretty warm pretty darn quickly. But now it's not doing that, i don't think. I let it idle for like 10 minutes, then rode it for just 5. Previously that would have caused some heat. Now nothing gets too hot. And today here in New York is a very hot and humid day. So i think the carb boots have helped in that regard, as was to be one of the expected improvements from them being new and not leaking in air and throwing off the mixture.
Today during the ride i also THINK i noticed some weird light metallic grinding sound coming from the engine. Pretty scary sound, so i shut it down. Only hear this when actually riding though. Hoping i was just hearing the speedo shaking randomly or something.
I've been in touch with the previous owner through all of this. The carbs were rebuilt in September of last year (2018), and were sync'd then too. Compression was tested to be good.
Is my next logical step to start down the path of valve alignment and carb re-syncing? Would carbs go this off line in just a year? I have heard that out of sync carbs can cause the engine to shake at idle and low speeds. My engine truly shakes. But not always. It follows the pattern of erratic cycles. Shakes a lot sometimes.
I'm happy with what is seemingly really good air flow and fuel flow. I think if i can resolve the engine issue, the overall performance will be quite smooth and easy now that all of this basic down and dirty work was done.
Any ideas about the symptoms though?
