HELP! I'm out of ideas... '98 YZ400

My brothers '98 YZ400 doesn't run worth a crap... a month ago we replaced the cam chain, adjusted the valves, and installed new rings. The first trip out, it did not like to idle, but while on the gas it was running great. So after that trip, we pulled the carb off and found that the choke was loose (about to fall out) so that was fixed and he went through the carb anyway because it hadn't been done since I bought the bike.

Since then, the bike hardly runs. It will only start using the hot start, and barely idles with the idle as high as it will go and fuel screw almost all the way out.

I can't find any reason why the bike shouldn't run... all the jets are the same as they were last season when the bike ran fine and there have been no mods that would require different jetting. We've been through the carb twice, everything is clean. I just rechecked all the valves and they are in spec. The hotstart is not sticking and I have not found any air leaks. Replaced the spark plug too... I don't know where else too look. :cheers::p:bonk:

PLEASE HELP ME. I'M DESPERATE.

Thanks in advance.

Russ

My brothers '98 YZ400 doesn't run worth a crap... a month ago we replaced the cam chain, adjusted the valves, and installed new rings. The first trip out, it did not like to idle, but while on the gas it was running great. So after that trip, we pulled the carb off and found that the choke was loose (about to fall out) so that was fixed and he went through the carb anyway because it hadn't been done since I bought the bike.

Since then, the bike hardly runs. It will only start using the hot start, and barely idles with the idle as high as it will go and fuel screw almost all the way out.

I can't find any reason why the bike shouldn't run... all the jets are the same as they were last season when the bike ran fine and there have been no mods that would require different jetting. We've been through the carb twice, everything is clean. I just rechecked all the valves and they are in spec. The hotstart is not sticking and I have not found any air leaks. Replaced the spark plug too... I don't know where else too look. :cheers::p:bonk:

PLEASE HELP ME. I'M DESPERATE.

Thanks in advance.

Russ

Did you take the slide out and apart?

If you did, the plate on the front of the slide is very easy to turn upside down. The plate is correct if when you look down the throat of the carb , engine side, you see a small hole in the plate. No hole? you've got the plate upside down. See a hole? Plate position is correct.

Yes, actually had to replace the slide plate because it was cracked. The hole is there.

Did you take the slide out and apart?

If you did, the plate on the front of the slide is very easy to turn upside down. The plate is correct if when you look down the throat of the carb , engine side, you see a small hole in the plate. No hole? you've got the plate upside down. See a hole? Plate position is correct.

I did that when I had mine apart.:cheers: It ran just fine upside down, except it just idled way too high. I finally figured it out and corrected my error.

xjruss, it must be carb-related since that's the only thing you messed with, and it ran good before that (accept for not idling.) Is the throttle position sensor installed correctly? Plugged-in? accelerator pump thingy/linkages/etc all working and installed correctly? Slide linkages/all related stuff all working?

Those carbs sure are complex.:-o

Silly question this, but got to ask; you didn't leave a rag in intake did you?

sounds like the slow jet is plugged. this can occur when you start a bike with a fresh oiled air filter installed. You should be able to see daylight thru the jets. Clean or replace all 5 jets.

Good Luck

May be a dumb question but did you mess with the fuel screw in the bottom of the carb?? That could be the curprit. It should be out say 1 1/2 turns out give or take.

It's also very easy to lose the spring/oring/washer that fits onto the fuels screw.

:cheers: I recently had a very similar problem with my 98'. The bike would run but not idle no matter what you did. After having the carb apart four times and trying everything already suggested to you, it ended up being a piece of rubber in the carb. I also believe it was found in the pilot curcuit which affects idle. I had to give it to someone else to fix. I missed it four times. So don't rule out the carb having debris somewhere. Hope that helps a little. Good Luck.

You said you replaced the slide plate that was cracked. Did you put the round rubber gasket from the original slide plate on the new slide plate? If the gasket is missing the bike will run the same as if you had the slide plate in upside down.

Thanks everyone for the suggestions... I haven't had any time to look at the bike lately but I'll be sure to give an update when I do.

I would double check cam/motor timing.......better safe than sorry.

I would double check cam/motor timing.......better safe than sorry.

I did this when I re-checked the valves...

I did this when I re-checked the valves...

Just a thought. Good luck

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