XT500 Runs crappy... sometimes.

Okay, here's the rundown: 1978 Yamaha XT500, normally runs like a champ. Lately, at midrange RPM and mid to heavy accel, it bogs down, stutters, and misfires pretty bad. It has been doing this off and on for about three months worth of riding, but the problem got really bad today.

The carb was rebuilt about two years ago, and I have done some minor carb repair in the past year, and it has run fine since then.

I have never adjusted the points, because frankly I don't know how to do it. My thinking is it's either points or carb, but I was hoping someone here could help get me pointed in the right direction.

Any thoughts?

BTW, I'm a mechanic, but I work on new cars and trucks only and don't have a clue what's what on this bike. Translation: I'm good with my hands and confident with repairs, so I should be able to handle whatever you all throw at me!

Mine ran like that when I bought it, also a mechanic, but old enough to know points, had cars that had them! Mine had a very large gap, which meant not enough dwell to saturate coil, therefore weak spark, .014 is what is needed, then set timing, you can use a timing light, mine runs well now that the points are gapped, the guy who sold it had just put the points in, not well I would say. Getting a manual helps, you need to remove the points cover, and the cover on the left side in front of the sprocket, that's where the flywheel is and the marks for timing. Have fun, not too hard, set at max bump for setting points, and then set timing, in that order. Pics in a manual, if you have never done this, are real helpful.

If you need help on points adjustment, I'll give you help. Unless they are really fresh, go buy new ones and a condenser as well. Don't spend a lot of time on pitted old ones.

Point gap is what sets dwell. Dwel is the time whe the coil charges, when points open, electromagnetic field collapses, induces voltage into secondary, which fires plug. By the time the points need serious adjustment, they need replacing. Use a really good grade of non-conductive hi temp grease on point cam rubbing block. Gap set at .012-.016, usually set for about .014". Then, set timing so points just open at timing mark. Almost always marked with an "F" ; the "T" is TDC. You can use a timing light, it usually gives a better job. Rotating point plate same way as engine rotates gives you retarded, or later timing, other way advanced, or earlier timing. Flywheel should show both idle/static point "F" and full advanced. Full advance should be seen at above 4000 rpm or thereabouts.

On magneto systems with points inside magneto, adjusting the point gap is how you adjust timing: bigger gap=more advanced. I believe your has a separate point plate, so you set gap first, for your dwell, the rotate point plate to get timing correct.

If you keep after points, spending about 20 minutes a month checking and adjusting, they are great, but don't fare well if neglected. Change points every season, or when they get pitted bad enough to have a mound on one side, canyon on the other. Any stalagtite-stalagmite evidence on points shows need to change. I used to change each end of riding season, that way it would be fresh for spring.

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