400f carb problems

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#1
hi to all, i've not been on here in a long time, but i've been having problems with my wr and thought maybe someone could help me. The bike is a 1998 WR400F, it's been acting up for a while now. the bike runs great for a while and then it starts to afterburn (pop when i let off the throttle) and soon after it does that the plug fouls. recently it's gotten worse, the plug fouls much quicker. i was looking at the carb to adjust it a little and i noticed some gas, so i started the engine and cracked the throttle a coulple times and gas would come up out of my accelorator pump around what looked to be a brass bushing. so now that the story is known, i'm thinking that i have a damaged accelorator pump diaphram. would the accelorator pump diaphram cause the afterburn and plug fouling? does anyone else have any sujestions? ~WR400Fanatic
 

Danman

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#2
Popping when you let off the throttle is usually a sign of a lean running carb as far as I know. My guess is that you did damage something in the accelorator pump. I would take it apart and have a look. You will need to have that replaced anyway to get the carb working right again and if it takes care of the popping deal then your all good, but I dought that it will.

Has the bike sat for a long time without running? If so your jets may be plugged up with bad gas causing a lean condition.
 
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#3
the bike has sat for about 4-6 months, so bad gas is a good posibiblity. the guy who had the bike before me may have fooled with the air/fuel settings and/or jets, so i think that i'm going to get new stock jets and put everything back to original specs when i put the new accelerator pump in so then i will know what i am working with. thanks for your input, advice is always welcome
 

Danman

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#4
You might also want to soak the carb to clean out any passages in it that are gummed up with stuff. Going back to stock settings is a good idea and them slowly make changes from there. I think they were pretty corked up from the factory though, but it should get you in working order.
 

a454elk

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#5
I have the 99 WR and had the same problem with gas that sat in it too long, it gunked up the jets, mainly the idle jet. When putting it back together, gas was squirting out of the accelerator pump due to an "o" ring that fell out, my fault. Whe you take it apart, there is s small o ring that fits into it's own little sopt, you'll see it. Make sure it is there and isn't cracked. Mine popped as well on decel which I think was caused, like Danman said, from being lean, it was sucking in air through there. Go back to stock if everything else is stock, I hadn't had the need to change anything on the carb. I am now having problems with it idling right though, even though it's all cleaned out, I think it's my float now. It spits gas out the overflow when I turn on the fuel. This all occurred because I left race gas in it. The bike isn't jetted for it and by me leaving it sitting in the carb, allowed it to gunk up quickly. I got the gas for free so I thought I'd use it, big time bummer.;)
 
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#6
thanks a454elk, it all seems to make sense to me, what i can't figure out though is why my plugs are fouling though...i think perhaps someone has fooled with the pilot jet settings and it's fouling out when i'm just sittin there ideling. i'm sorry to hear your luck with your wr, they are great bikes, but man i don't like the carbs on them...pricey peices they are. anyhow, i ordered a accelorator pump diaphram for mine and it should be in wed. as for the o-ring, i'll check it out and get a new one if need be. as for the race gas you said you used, i read somewhere (i think it was off-road.com) that you're not suposed to use race gas in the WR, that it can damage it's internals...but i'd say you already know that. i have a fresh tank of 93 octane waitin for wed. i've been dyin to ride. well, thanks for the help, and i'll keep ya posed~peace
 

a454elk

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#7
The bike is great but I learned the hard way about the race gas deal. Pulled it apart today and checked the float level and set the pilot screw back to stock setting. Still have a tough time keeping it idling, all the jets are clear so I"m stumped right now. Are your plugs real black or are they just dead? Kinda curious cause for them to foul it takes quite a bit....
 
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#8
my WR's plugs were just a little sutty. i'm not sure why really....the crazy thing is, i mixed up a great batch of gas (93 octane plus octane booster with a cleaning fomula kinda thing). after i got that gas in it, the poping stopped and the plug didn't foul out and i rode it probobly about 2-3 miles so it seemed OK. but the acc. pump is still messin up, but my parts come in today. however, when i tip the bike over gas will not come out the tubes on the bottom, and i think it should. i think my float maybe sticking. i'm gonna give the carb a good working over on friday i think. i don't really have much help on your carb. if i'm new to these bikes, but if i come accross something i'll let ya know.
 

a454elk

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#9
Took my carb apart again and this time I fully disassembled the accelerator pump and found dirt on the under side and the diaphram was worn through, a hole was on the rubber side. I guess that'll explain the problems, the jets are completely clear but I think that the jets are getting clogged due to the dirt getting past the diaphram and into the circuits. It also backfires because it sucks air into the pump on deceleration. That should explain the backfire on yours but I think your jets may be screwed up or the air fuel mixture is set too rich. Maybe your pilot jet was changed to a real rich one. My local dealer ordered the diaphram for me at 18 bucks.:scream:! Oh well, we'll have it by the end of the week, I hope. Update to follow....
 
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#10
i put about 5miles on the 400f this weekend, not much, but it's only been in the teens, so it's not bad for me. i haven't fouled a plug yet on the bike, seems to be running really good. i still haven't got my parts yet, they are suposed to be in monday or so. anyhow i think my float is sticking, i can't get it to dump fuel out when i tip the bike over. what do you think? i'm gonna pull the carb apart this weekend hopefully and i'd like to try to clean out my jets and float and replace the accl. pump. do you have a manual for you bike? my manual says something in the back (under tuning) about adjusting a pilot air jet, it says you can't/shouldn't adjust one with out the other. welp, g'luck w/ yours, i'll let ya know how it goes next weekend.
 

a454elk

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#11
If the gas doesn't come out when turing it on it's side then it sounds like it's sticking. Take the float needle out and clean the channel and needle real good. One thing, if you just had the carb apart and then you replaced it and turned the bike on it's side, it won't come out right away, it usually takes a bit of throttle turning to make it happen. I have the manual for mine and the pilot air jet is the adjustable one through the bottom of the carb when it's together. That one states it should be 1 5/8 turns out. It controls the flow of gas instead of air like a 2 stroke. Not sure on your carb if there is more than one adjustable jet. MIne has only one. good luck
 
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#12
welp, i finally got my carb rebuilt for my birthday. i also changed my oil, new plug, new throttle cables and a few new stickers to make it pretty. some of the jets were nasty, and the acc. pump was crap. so things are workin alot better now. we'll see how it goes in the long run though. but i put about 3-5 miles on it tonight (it's about 25 deg. F where i live), it popped a little everynow and then, but overall i think it is better. thanks for the help.
 

a454elk

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#13
Mine is being cleaned out as we speak. The accelerator pump diaphram is new and I think there was still gunk in the idle circuit which caused the idling problem. Should be done soon, good news on yours!
 
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#14
a454elk said:
Mine is being cleaned out as we speak.QUOTE]

Elk.

It may take a little longer, I'm waiting on some stickers to arrive. :ohmy: :laugh: :laugh: :laugh: :moon:

In my experience, most of the time the problems with these bikes can be traced to the accelerator pump or pilot air system. Be sure the little rubber boot that the accelerator pump rod goes through is in good conditon and is all the way seated in place. If the boot is not in place, dirt and water can get down into the diaphram.

Just my $ .02
 

a454elk

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#15
Sticker it up fool, your license is being revoked as we speak.:moon: I seemd to have lost the rear tire I had set aside for you, too bad I guess! BWUAHAHAHAHAHA!