01 YZ426 two questions

I finally got to ride a couple of weeks ago. Got the bike unloaded hopped on, took off, hit a rock which proceeded to hit my crankcase cover and break it open, ride obviously over in 10 minutes. :) I noticed that the magneto also has a scratch on it. I replaced the cover but now notice that it sputters at 1/4 throttle and backfires. Is it possible I knocked the magneto out ot time, how do I check and fix? Secondly, I pulled the plug and noticed I had a CR9E NGK plug instead of a CR8E would this also be part of the problem? I installed the new plug before the ride and didn't really have time to notice any problems. What is the diff betweent he two plugs? They look the same. Thanks

I doubt you knocked it out of time. Probably just a jetting issue. Could indeed be as simple as having the wrong plug though...especially if the 9E is starting to foul. Use a NGK iridium CR8E. If the backfiring is still there, fiddle with the fuel screw to fine tune the low throttle response. Good luck!

The difference is the "heat range". This is basically the amount of exposed porcelain on the engine side of the spark plug. The less there is, the more readily the plug transfers heat away from the electrode. If the heat range is too high, the plug tip gets too hot and contributes to pre-ignition. If it's too cold, the plug doesn't stay hot enough to stay clear of conductive deposits of oil and fuel, and it fouls. Probably what's happening to you. I'd change back to the hotter CR8E and see if that cures it. If nothing else, it's the simplest.

I noticed that the magneto also has a scratch on it. I replaced the cover but now notice that it sputters at 1/4 throttle and backfires. Is it possible I knocked the magneto out ot time, how do I check and fix?

I guess it depends where you hit it. While you wouldn't have knocked it out of time, it could be possible to hit the trigger coil and change the gap. I don't know if Yamaha even has a spec for this, but if I had to guess, I think most machines run around .020 to .030 pole gap.

This is the coil on the outside of the flywheel......measure gap between the coil and the raised portion of the flywheel.

If this is where the damage occured, it might be worth a look. :)

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