QuestionI have an 1982 Yamaha XJ 750 Maxim with 13K miles. It is in great shape and runs very well (when well warmed up) on the road at rpm's above 4K or so. I start it with full choke, and let it run for about 5 minutes, gradually reducing the choke. After warmed it idles at 1100. But unless I keep choke at 1/2 or so, throttle response from idle sounds as though the engine is either running too lean or too rich. Even after having riden for many miles, throttle response under ~4K rpm is met with the same (too lean or too rich) kinda sound/engine feel. Throttle response at engine speeds above 4K or so and mph speeds of >50 or so, are very lively. Is this typical of the bike?
AnswerKen, you are asking MrHonda about a Yamaha question, but I will do my best here.
Many bikes built in the 1980s were jetted really lean to comply with EPA regulations. Generally, you could put a shim under the needles and drill out the limiter plugs on the idle mixture screws and richen them up about 1/4 to 1/2 turn and you were good to go.
Today, we are dealing with gasoline that is 10% alcohol, which has a different viscosity and energy package than 100% gasoline of days past. This fuel mixture leans out the carburetion calibrations even further, right in the range where you are having issues.
Assuming that there are no issues with air leaks/varnished up carb passages/jets, then the bike would probably respond to a "jet kit" or by doing what I mentioned previously... raise the needles and richen up the idle mixture screws.
Carbs out of synch will also give some part throttle stumbling and lean issues.
Look at the spark plugs to see how they are burning. Probably they are going to be pretty white and lean-looking.
Bill Silver
