'99 WR400F Idle Problem

Hey guys, It's nice to be back to TT.

After a 2 year hiatus from owning a bike, I picked up a cherry 1999 WR400F (I previously had a 98 WR400F). The bike is a 2 owner bike. The first owner was an older guy (60s) who bought the bike in 1999. He rode it very little then sold it in 2007 to the person I purchased it from. The 2nd owner bought it for his son who rode it 4 times before deciding the bike “wasn't for him”.

So, here I have a completely stock, very clean, cherry (the original tire is still on it, no frame paint wear, mint plastics, etc) 1999 WR400F with ~500 miles on the clock. When I test rode it, it was hard to start but after kick 6 it fired right up. I let the bike warm up and took it for a ride. I noticed that it idled very low and even stalled once accompanied by with a small pop. It ran good so I took it home with me.

Last weekend I went to take it out for the first time. Started in 6 kicks and once warmed I took off. I rode it for about 10 miles then stopped to take a break. After stopping I couldn't start it for the life of me. After kicking it another 20 times I checked the plug - which was soaking wet. I changed it and it fired up first kick.

Every time I start it cold since changing the plug, it starts (with the help of the choke) in 2-3 kicks.

Now, on to the problem. The bike runs good but doesn’t idle. It idles nicely with the choke slightly on but as soon as I turn the choke off it stumbles and eventually stalls. It leaves a bit to be desired as far as throttle response compared to my other WR. It just doesn’t seem as snappy. The old WR had a YZ pipe on it. I’m not sure what else done to it (I bought it as an $600 beater so I really didn’t care at that point).

I’ve searched the forms and read threads suggesting everything from adjusting the air/fuel screw to rebuilding the accelerator pump. Can anyone help me narrow this down? I haven’t had the time to try anything so I’m looking for a place to start. I’m relatively new to these 4 stroke bikes so any help is appreciated.

Thanks,

Dave

Since the bike is new to you, I would go through and do a maintenance check like valves, filter clean, clean the carb (especially since it seemed to sit a lot), of course oil too. Then see where you are. it most likely isn't just one thing, but a combination of things.

Since the bike is new to you, I would go through and do a maintenance check like valves, filter clean, clean the carb (especially since it seemed to sit a lot), of course oil too. Then see where you are. it most likely isn't just one thing, but a combination of things.

Oil has been changed, filter has been cleaned. I was waiting to check the valves for when I change it to YZ timing. Nothing seems to be out of the ordinary though.

I haven't even touched the carb. The next time I will be able to work on the bike is Saturday. First item on the agenda will be a good carb cleaning. I'm hoping that will solve it. If not, what should I look to next? Fuel mixture screw?

On a side note, will I have to re-jet for removing the airbox lid and slapping a YZ slip-on on it?

Thanks.

Have you tried to just raise the idle speed a little by turning the black knob?

Have you tried to just raise the idle speed a little by turning the black knob?

Nope, I haven't tried anything. I know very little about the carbs on these bikes and before I started poking around I wanted to either: 1.) get a manual or 2.) consult with someone on here. I ordered a manual but it won't be here until Monday.

Like I said, I'm going to give the carb a good cleaning on Saturday. I'll also try the knob. What direction should I turn this idle knob?

Also, can someone help me clarify the Yamaha YZ400 throttle stop screw part number? I've searched the boards and nobody posted a specific part number. The general advice seems to be "get a YZ screw" but no there's no mention of part #'s. I searched google and as dozen sites offering micro-fiches and I cannot find a part number for the life of me. All I find are 426/450 part numbers. Are these interchangeable?

Thanks for your help!

I'm kinda new to these bikes too. I've only had mine for a month. Not sure which way to turn the screw. If you turn it and the idle speeds up, that's the right way. :-) For the throttle stop screw on the 400 I think the consensus is to file it down to about 8mm tall and put it back in.

I'm kinda new to these bikes too. I've only had mine for a month. Not sure which way to turn the screw. If you turn it and the idle speeds up, that's the right way. :-) For the throttle stop screw on the 400 I think the consensus is to file it down to about 8mm tall and put it back in.

Good news. The first thing that I adjusted was the pilot screw. My plan was to reset it and set it to the stock setting (1 3/8 turns out from snug). So I counted how many turns it took me to snug it...3 1/2 turns! So I snugged it, and turned it 1 1/2 turns and backed it in a tiny bit (1 3/8) and it started up first kick and with a little idle adjustment idled like a champ.

Thanks for your help!

I'm kinda new to these bikes too. I've only had mine for a month. Not sure which way to turn the screw. If you turn it and the idle speeds up, that's the right way. :-) For the throttle stop screw on the 400 I think the consensus is to file it down to about 8mm tall and put it back in.

I think you actually file 8.5mm off it, it should be 12mm when you put it back in... I googled the topic and found this:

http://dirtbike.off-road.com/dirtbike/article/articleDetail.jsp?id=266303

hope that helps.

well, there ya go. Sorry if I miss led you.

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