Problems with YZ400

[Trimation]

Wondering if someone can please help me out?

I have a 1998 YZ400 that was working perfectly just a couple of weeks ago. I rode it a couple of days ago and noticed that it is cutting out in the upper parts of the gears before I should be hitting the rev limiter. I can't even go very fast on the road because it is cutting out so badly when I'm trying to hit top speeds. I thought that maybe I was getting a bad spark so I changed the plug and that seemed to make it better but it is still happening. What else could it be? What should I do?

Thanks,

Jedediah

[PumpkinHumper]

1. dirt in the carb.

2. clutch slipping

3. jumped timing (timing chain wore out)

Try one of those out.

[biznet1]

It could be fuel delivery, but it may also be due to a worn gear and/or bent shift fork. The upper gears can get rounded on the edges causing it to jump in and out of that gear. The symptoms can be deceiving because it makes it seem to cut out. If you are in second gear can you make it "cut out"?

[Trimation]

No, it only seems to do it in third, fourth, and fifth.

[PumpkinHumper]

No, it only seems to do it in third, fourth, and fifth.

That sounds like the clutch slipping to me. A slipping clutch will often only show up in the higher gears.

[KillerMedic]

If it is not the 3 things that yzman has suggested, it could be your jetting. If it is getting cooler where you are, like it has here over the past 2 weeks, and you were borderline lean in the summer heat, you may be way lean now. This could cause the power "cutting out" at higher rpm's (where your main jet flows fuel) and higher load on the engine (upper rpm in higher gears.) Are you getting significant "popping" from the exhaust when you are at higher rpm and cut the throttle?

[Pincushion]

Only does it in certain gears and only high RPM's. Thats a tell-tale sign of worn shift forks/dogs, but this problem doesn't happen overnight while the bike is in the garage.

Could be bad fuel too. Drain the tank & carb and start with some fresh 91 octane. Start simple (and cheap) and then slowly work towards other more-difficult options.

-fresh fuel

-fresh air filter

-check the spark (swap the CDI with a known working one if possible)

-clean the carb

-check timing/valves. Has the timing chain ever been replaced? It may be getting sloppy.

If none of these gain any improvment then its time to move onto more invloved troubleshooting.

[Trimation]

Thanks for all the replies. You've given me some things to try out. If nothing else trying them all will allow me to learn the bike better. I'll let you know what I find out.

Thanks again,

Trimation

[KillerMedic]

Good luck with it and definitely let us know, I am curious as to the cause.