First of all thanks to everyone on this forum. I have learned so much and I really appreciate it. I have four different XS650s. I have read the AMAZING carburetor guides on here ten times or more.
I purchased a 1979 XS650 Special II with only 2,500 original miles from the original owner. It had sat for a long time, but it was running okay at best when I bought it. I have been slowly fixing all the small things and getting it going in the right direction. The carburetors were leaking so I rebuilt them with a rebuild kit. I also replaced the o-ring on the emulsion tube and the throttle shaft seals. While they were off of the bike I rigged up a spare set of 1973 carbs to the 1979 bike and it ran pretty good. When I put the rebuilt original carburetors back on the bike it had a really bad hesitation just off idle when you open the throttle. I played with the pilot screws until it started to get better and was close to running properly, but they pilot screws were almost 4.5 turns out. (1979 carbs don't have a spring on the pilot screws at all?) The hesitation got better when the choke was activated so it's starving for fuel. So... I took the carbs off and put in larger pilot screws, but it didn't change much at all. So, I kept on adjusting and changing. I also replaced the pilot screws with NOS Yamaha screws. I am currently at 140 Main Jets and 35 pilot jets (135 and 27.5 were original sizes.) I am still having the hesitation (low fuel) just off idle as you try to accelerate and I can't get it dialed in. With everything I am still forced to have the pilot screws 4 turns out to get the bike to run somewhat close to normal just off idle.
I have adjusted and checked the cam chain tensioner, the timing and the valve clearance. I also inspected the timing advance unit since that has given me troubles on other bikes but it seems to work great. Almost everything on this bike is original still. Same original ignition and charging system. The original owner did drill out the ends of the mufflers though. I have synced the carbs visually and they are very close. I also set the pilot screws using the dead cylinder method and when I do that it seems to be they are set at 3 turns out, but then it won't run properly when I drive it. If I back them out a full turn it gets better, but four turns out is pretty far and conflicts with the dead cylinder method.
The only things I have not done is sync the carbs with an actual fluid vacuum synchronizer and change the position of the jet needle clip. The bike runs really good once you get just past the flat spot at opening of the throttle. What else am I missing? Any ideas? Is it as simple as changing the jet needle position? I'm perplexed as to why it ran great with a spare, old set of carbs I had laying around. Thanks you all so much.
I purchased a 1979 XS650 Special II with only 2,500 original miles from the original owner. It had sat for a long time, but it was running okay at best when I bought it. I have been slowly fixing all the small things and getting it going in the right direction. The carburetors were leaking so I rebuilt them with a rebuild kit. I also replaced the o-ring on the emulsion tube and the throttle shaft seals. While they were off of the bike I rigged up a spare set of 1973 carbs to the 1979 bike and it ran pretty good. When I put the rebuilt original carburetors back on the bike it had a really bad hesitation just off idle when you open the throttle. I played with the pilot screws until it started to get better and was close to running properly, but they pilot screws were almost 4.5 turns out. (1979 carbs don't have a spring on the pilot screws at all?) The hesitation got better when the choke was activated so it's starving for fuel. So... I took the carbs off and put in larger pilot screws, but it didn't change much at all. So, I kept on adjusting and changing. I also replaced the pilot screws with NOS Yamaha screws. I am currently at 140 Main Jets and 35 pilot jets (135 and 27.5 were original sizes.) I am still having the hesitation (low fuel) just off idle as you try to accelerate and I can't get it dialed in. With everything I am still forced to have the pilot screws 4 turns out to get the bike to run somewhat close to normal just off idle.
I have adjusted and checked the cam chain tensioner, the timing and the valve clearance. I also inspected the timing advance unit since that has given me troubles on other bikes but it seems to work great. Almost everything on this bike is original still. Same original ignition and charging system. The original owner did drill out the ends of the mufflers though. I have synced the carbs visually and they are very close. I also set the pilot screws using the dead cylinder method and when I do that it seems to be they are set at 3 turns out, but then it won't run properly when I drive it. If I back them out a full turn it gets better, but four turns out is pretty far and conflicts with the dead cylinder method.
The only things I have not done is sync the carbs with an actual fluid vacuum synchronizer and change the position of the jet needle clip. The bike runs really good once you get just past the flat spot at opening of the throttle. What else am I missing? Any ideas? Is it as simple as changing the jet needle position? I'm perplexed as to why it ran great with a spare, old set of carbs I had laying around. Thanks you all so much.
