Backfiring 2001 CR80

I've actually seen the stator come loose and just turn. This put the timing off. You need to grab the manual and check the timing. Also you could pull the exhaust pipe and using a flashlight simply look at the exhaust ports to see if they are clogged.
The flywheel key would cause the timinig issue as well. I'd really focus on that becuase backfiring is very often caused by timing being off. If it was the CDI I'd expect that you would not get any spark. Just pull the left cover off and see if anything looks wrong. Is the stator or Flywheel loose? Can you move the stator without loosening any bolts?

3oz oil to 1 gallon gas is 32:1 ratio. Many use that. I use 40:1.

Your running a little light on the oil. You should be running 32:1 which is 4 oz of oil per gallon of gas. You are running at about 42:1. Here is a handy chart:

http://www.rctek.com/pdf/fuel_to_oil_ratio_chart.pdf

Also, does it just backfire when you kick it? Does it run good once it's lit up? If it just backfires once in a while when you kick it, that's not abnormal. Odds are it's just time to do some maintenance:

1. Filter should be cleaned at least every other week. Use a good filter oil, NOT motor oil (I know, that's what the user manual says, but honda hasn't changed the filter section of their manuals for 30 years).
2. The carb float bowl should be drained periodically. You should clean the carb once a month if you ride in either a very dry dusty climate, or wet muddy climate. You can minimize this with a 2.00 in line fuel filter.
3. Inspect the reeds periodically, if the petals are warped or frayed it will cause problems.
4. The Honda intake box sucks. Take it apart, and seal the carb boot to the box with a little yamabond (the stuff honda uses cracks in about 6 months), otherwise it will start leaking crap past the filter-not good.

If it's jetted correctly, spark plug will last forever-I change my daughter's in her '99 CR80 about once every 6 months just because.

Eric

Nice chart! I did my math wrong in my head. Even at 42:1 I wouldn't be too worried if your using a good synthetic oil. Some oils gaurantee they will lube at 50:1. Especially if you aren't riding it too hard.

Your right, normally 42:1 won't hurt it. The problem is if he has not rejetted accordingly it will be running very rich (less oil=more fuel per volume=rich condition). It sounds to me that it is primarily a trail bike, and in this case I wouldn't run it at 42, extended runs at a steady RPM are much harder on a little smoker than an MX track, I would stick to the 32:1. I also strongly recommend the Honda HP2 oil, it is really good. Klotz is good stuff too (and it smells awesome;-), but you really need to tear down the top end more often as it's a bit messy, especially on my son's RM80 where it tends to gum up the powervalve. The main plus for Klotz is I can by it by the gallon for 30.00, where the HP2 is 11.00 a quart.
Eric