Problem Remains...

[justifiedman]

My totally stock 1975 XS650 continues to have the same issue, a bad miss under heavy acceleration. I have less than 1500 miles on a professionally rebuilt engine. It ran perfectly until recently when the abrupt miss began. I have a new fully charged battery, new plugs. I have adjusted the cam tensioner, adjusted the valves, set the points (after lightly filing and cleaning them) set the timing, adjusted the carbs (in that order) and it did not change things.
I don't know if this is significant, but after setting the timing statically, I used my timing light and found that the timing mark was not steady, everything is blurred whether at 1200 RPM or 2000-3000 RPM. Also, when setting the carbs beginning with the 3/4 setting on the idle mixture screws (then using the method in the Factory Manual of pulling the plug wire off of one side and adjusting the other side) the right side responded as it should by speeding up the RPM while turning the idle mixture screw, but the left side was significantly less responsive upon switching over to doing that side.
Having looked over the possible causes in the "Trouble Shooting" section of this most excellent website, based upon my description, are there a few items that, historically, tend to be the most likely culprits to start with? Thanks

 

[5twins]

A non-responsive mix screw often indicates the float level is off in that carb.

The bouncing timing marks are usually caused by all the play in the advance system. The advance rod can have excess in-out play in the cam and the advance unit itself can be wearing and getting all sloppy (well, sloppier than normal, lol). There's not much you can do with the advance unit except to keep it lubed well. Keep the advance rod greased up well where it turns in the cam too. You can shim the excess in-out play out of the rod though. That sometimes helps with the split image.

 

[fredintoon]

Hi jm,
what 5twins said. In addition, when did the carbs last get cleaned? I mean thoroughly, totally apart, ultrasonic cleaned?
Micro-crud in the skinny little jets, passages and needles that control the carbs' low throttle settings can often cause that problem.

 

[xonix_digital]

I'm having the same issue and I am looking to replace my ignition coil. Just posted up asking for feedback on specific models. My coil is from 1980 and is surely ready to retire. Thoughts?

 

[fredintoon]

Hi xonix
XS650s weren't built up to a standard but down to a price so your stock coil was the cheapest unit that'd work acceptably when the bike was new.
It ain't new no more, eh?
So yes, a new hot coil will improve the bike's ignition.
It won't fix anything else, though.