Bought our son a brand new YZ 85 this past January. He put 20 hours on it. Ran fine until about 2 weeks ago. We had him practicing on our home track, he rode all day, and it was fine until he went to go back out for few more laps. Bike started bogging really bad, so we had him stop and take it to the garage. Changed oil and it smelt like burnt clutches. I immediately ordered them, but he had a race the following day and he still wanted to race. Took him to the track and bike wouldn't start. Took the carb apart and the jets were clogged. Got them cleaned and the bike started right away. He raced it and two days later, his clutches came in. Got those replaced. He went to training the following day and after a bit, it started to bog again. Cleaned the carb out again the following day. Went to start, now bike will start but only when you twist the throttle and smokes a lot. As soon as you let go, it dies.
Since then, we have...
Rebuilt the carb & changed jetting
Replaced the reeds
New spark Plug
New top end
Exhaust is good
Not sure where to go from here. Called the shop where we bought it, and they have no idea either. Said we can bring it in, but they are weeks out.
A friend of ours mentioned maybe the crank seals are bad. His sons brand new KTM 85 (few years ago) had similar situation at 20 hours.
I've worked on bikes most of my life, this has me stumped.
Thanks in advance!
When you say "changed jetting" I assume you mean put in new jets of the same size? There was no reason to change the sizes if the bike was previously running well.
Otherwise, there's a few items that could be at play here. Yes, crank seals are a possibility.
Remove ignition cover and inspect for obvious sign of left side seal leaking. Also inspect flywheel key and make sure it is intact and nothing else in there obviously came loose.
The right side seal, you can sort of inspect by putting the trans breather tube in a cup of gas with the bike running. It may either bubble the gas or suck the gas up if crank pressure/vacuum are getting past the right seal to the trans.
You may have cracked a ring. Only way to be 100% sure on the top end and the crank seals is compression test and leak down test.
Inspect exhaust and make sure pipe or muffler didnt somehow get blocked or full of fuel.
Might be an ignition problem. Makes sure the coil is still mounted tight and grounded well to the frame and no wires are obviously pinched or rubbed.
Running race fuel? Isnt there some shitty gas going around causing big problems?
Sounds like you have trash in your tank, or fuel line is coming apart and the trash is causing the needle to stick flooding the engine, that is why it is smoking. The crankcase is fuel of unburnt fuel.
Take the tank of and clean it out thoroughly, pull the petcock out and clean it and rebuild carb. Make sure your fueling can is clean as well.
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