Motorcycle Repair: 1981 XS400 missing/high idle and 1977 yz80 bogging/no idle, stop screw, air leaks

Question
Hi Falcon, I have two vintage Yamaha problems, I hope it is ok to ask them both in one post.  

The first is for a 1981 XS400.  It starts very easily on the first kick, but even after it has warmed up it idles very high (1500-2000 rpm I don't know exactly because the tach is broken).  I have disassembled and cleaned the carbs, checked for air leaks around the manifold boots, and adjusted the idle stop screw.  In what I think is a totally unrelated problem, if the engine is under load, such as being ridden, it runs smoothly to about 3000 rpm, then it starts to miss and sputter like one cylinder is not firing.  It will then no longer accelerate, no matter how far you turn the throttle.  

The 1977 YZ80 will only start by rolling it and popping it into gear.  It has less than 10 minutes on a topend rebuild.  The reason it has to be roll started is because the bike will not run unless the throttle is opened about 1/3.  Once it is running it runs very well, it just will not idle.  It also seems that it does not have much power in 1st, more power in 2nd, and lots of power in 3rd.  The engine actually sounds different in the different gears.  It "winds up" a lot higher in 3rd.  I know very little about 2 strokes, but could it be the reed valves?  Any help is appreciated.  Thanks.  

Answer
Hi McLain.

On the XS400 you may want to check the spark plugs.  It sounds as if the bike is running rich.  If the plugs are blackened, then it is running rich.

This could be due to a sticking choke mechanism.  Possibly the cable is frayed internally if the mechanism at the carbs is moving freely.

As far as the YZ80 I haven't the faintest clue what the problem is.  Most issues that would do as you describe for start-up and 1st gear, would affect all gears.

Sorry I can't help further with that one, but let me know what you find with the XS400.

Good luck and ride safe.
FALCON