wideload
XS650 Member
I've been though a lot of threads and the carb guide; I believe I have a fair grasp of the concepts, but not much real-world experience. I hope to get a bit of feedback and maybe some creative ideas about how to proceed after making a series of air/fuel changes.
This is a stock '76 that I have been riding a lot this spring and know that it has been running well. I replaced the original equipment mufflers with the Commando style. The new ones are clamped to the original head pipes (not sawed off). The stock main jets were 122.5 and the rest of carb was in its original settings as well. I hoped that the muffler swap would not require carb modification, but this was not the case.
My first test run, after muffler replacement only, produced moderate lean fueling symptoms at 6,200 to near red line at WOT.
I installed 125 Mikuni main jets (one step larger) and tested again. Testing this time indicated rich condition break up between 4,200 & 5,200 at WOT. The bike ran ok outside this RPM range, but the spark plugs showed fluffy black deposits on the body and insulator.
Next, I removed the muffler plug from the Commando baffle, hoping to lean it out a bit. No change at all.
I dropped the needle 1/2 step with a shim. this time rich break up occured between 4,500 & 5,500 RPM. Spark plug still indicating rich, but better than the previous trial.
At this point I tried removing the air filters, shooting for a leaner condition. That was the worst test running too lean at 5,000 and up.
Replaced the filters and dropped the needles one full step from the original position. Rich break up moved higher in the RPM range - 5,000 to 6,000.
Seems like I am getting closer on correct tune, but I am not sure if dropping the needle any further would be a good idea - seems like Yamaha engineers liked the middle range of the needle. Any Ideas?
This is a stock '76 that I have been riding a lot this spring and know that it has been running well. I replaced the original equipment mufflers with the Commando style. The new ones are clamped to the original head pipes (not sawed off). The stock main jets were 122.5 and the rest of carb was in its original settings as well. I hoped that the muffler swap would not require carb modification, but this was not the case.
My first test run, after muffler replacement only, produced moderate lean fueling symptoms at 6,200 to near red line at WOT.
I installed 125 Mikuni main jets (one step larger) and tested again. Testing this time indicated rich condition break up between 4,200 & 5,200 at WOT. The bike ran ok outside this RPM range, but the spark plugs showed fluffy black deposits on the body and insulator.
Next, I removed the muffler plug from the Commando baffle, hoping to lean it out a bit. No change at all.
I dropped the needle 1/2 step with a shim. this time rich break up occured between 4,500 & 5,500 RPM. Spark plug still indicating rich, but better than the previous trial.
At this point I tried removing the air filters, shooting for a leaner condition. That was the worst test running too lean at 5,000 and up.
Replaced the filters and dropped the needles one full step from the original position. Rich break up moved higher in the RPM range - 5,000 to 6,000.
Seems like I am getting closer on correct tune, but I am not sure if dropping the needle any further would be a good idea - seems like Yamaha engineers liked the middle range of the needle. Any Ideas?