Hi All,
First time poster. I have a Yamaha 1986 XT600 that cuts out at freeway speeds. Heres the specifics.

The Bike: 1986 Yamaha XT600 - 4000 original miles, completely stock. Sat for a couple of years with fuel in it.

The problem: It cuts out and sputters and misses when cruising at 60+ MPH. Cruising in town below freeway speed is fine except for a barely noticeable hiccup between gears upon throttle decrease when you pull in the clutch to shift. Idles and starts fine. Strange thing is that it accelerates STRONG all the way to 100MPH as long as it is at FULL throttle. In fact, even while you are at 60+ MPH attempting to cruise and it is cutting out and sputtering, if you give it FULL THROTTLE, it accelarates FINE with no miss right up to top speed. In fact all full throttle acceleration, idling, etc is fine EXCEPT the cutting out at 60+ MPH cruise.

So Far: I have replaced the fuel and cleaned the carburetor including the jets and slides. I have had a Yamaha mechanic clean the carb as well as drill out a plug for a "non-adjustable" jet but the same symptoms occur. They have been re-jetting but they tell me it has little to no affect on the freeway speed cruise problem.

I had thought maybe ignition but discounted that idea completely because it runs up to high RPMs fine and accelerates fine as long as it is at full throttle. It is only the 60+MPH cruise where it stumbles. A little more throttle at 60MPH has little to no effect but nail the throttle and off we go with no miss!

HELP