Quantcast
Jump to content

  • Do you own an ATV? Join our Forum!

    As a member, you can post in our forums, upload your photos and videos, use and contribute to our downloads, create your own member page, add your ATV events, and even start your own ATV club to host your own club forum and gallery.  Registration is fast and you can even login with social network accounts to sync your profiles and content.

MarkinAR

2000 Yamaha 400 Big Bear, dies off idle

Recommended Posts

Working on my oldest son's buddy's 2000 Big Bear.  He was having some power loss at WOT so i thought it worth while to clean the carb and put at kit in it...or at least clean the OEM jets and use the new orings that come in the kit.  It starts and idles like a champ, but dies and wants to backfire with any throttle at all.  I assume lean because it will run decent with the choke on. Air screw adjusted fine, nice crisp idle.  This one has fuel/air screw, low speed jet, mid jet, and main jet.  I was assuming just trash in the low speed circuit so pulled it and cleaned again, still does the same. 

Two questions I guess:

1. Does it sound most likely trash in the off idle circuit?  This one has a few more passages and jets than I'm used to...and a darn diaphragm.

2. It now really wants to leak at the bowl drain screw.  It doesn't seem to tighten enough to completely stop the flow.  Seat looks good, screw was dirty but cleaned up fine with a touch of 2000 grit paper.  First for me.  Any idea here?

Share this post


Link to post
Share on other sites

Subject is wrong...it's a 400 Big Bear.  Must have fat fingered a key there.

sorry.

 

Share this post


Link to post
Share on other sites

Welcome to quad crazy.

If it idles fine the pilot is partially clogged. If it runs better on choke your not getting enough fuel. What happens when you hit it at wot? Look up the factory spec for jetting.

Your a/f screw should be at 1.5 to 2 turns.

Your float is off and has to be adjusted.

I’d go thru that carb again and make sure it’s clean. Blow air thru the carb. Check to see if your float is even working and seating properly.


Sent from my iPhone using Tapatalk

Share this post


Link to post
Share on other sites

Can't get to WOT, won't make it past just off idle unless the choke is on. Hitting the throttle from idle, it will sometimes cough back through the carb...which is another indication of fuel starvation.

the needle seems to be moving in and out of the seat ok, and float wasn't moved during rebuild...but could always be checked again.

As far as jets, it has the original jets that came in the bike.  I normally chunk the jets that come with the cheaper rebuild kits in a bin and forget them because I don't trust chinese jets.

Share this post


Link to post
Share on other sites

Can you see thru the jets. The pilot has 5 and the main has one.


Sent from my iPhone using Tapatalk

If not. Use a thin needle and clean them out



Sent from my iPhone using Tapatalk

Share this post


Link to post
Share on other sites


 I agree with @06kfx440 on the pilot jet may somehow still be partially blocked.  It happens, I would make sure all of your oem jests are clean  back in place, aftermarket cheap ones will not work properly.   Make sure all o rings and seals that you may have swapped match in size, oem versus what you put in.   

The way you are explaining the bowl overflow how I understand it, the screw is leaking ? Not the hose overflow?  If thats the case there is most likely a seat gasket or o ring not in place or not properly put back together.  If the overflow hose has fuel coming out of it then yes as @06kfx440 says  it’s a float adjustment. 

On the very rear of the carb you have one more jet.  It’s located on the intake side/back of the carburetor.  It’s called a pilot air jet. Take a look at that and unscrew it and clean it good.  

Make sure your diaphragm has no holes in it and it’s installed and seated properly at the top of the carburetor since it works on vacuum and don't  want any leaks. 

Back to the air fuel screw, I’m assuming you cleaned it well and put it back in properly and it’s hopefully the oem screw, but double check that the tip of that screw is very pointed and in good condition and the sequence of the screw going back in was right, screw, spring, metal washer and then the o ring. That’s important that it’s correctly done.  

Then turn the air/fuel screw in and out as the engine is running to see how much the engine reacts to the adjustments.  Screw it all the way in and see if the engine shuts down.  If not then the air/fuel screw  is having problems.  If it is responding good then set it at 1-3/4 turns out from a snug position and start the engine.  Slowly turn the screw in or out, my guess would be out and the engine should start to rev higher.  The higher the better since the air and fuel are mixing properly hence causing higher rpms.  (That’s a good thing) Keep adjusting until this high rev starts to break up a bit. Stop turning the screw any further and turn back a hair to that sweet spot where the rev was at its highest point.  That’s the sweet spot yiur looking for. Now adjust your idle down to about 800-900 rpm’s or a happy spot you feel comfortable with and see how she runs.

I agree with your initial diagnosis of losing power could be a dirty carb issue but unfortunately people think that a new jet or rebuild kit is in order when he truth is it’s only a matter of opening the carb up carefully disassembling it , cleaning and reassembling it.  But that’s water under the bridge at this point. 

This could also have been noting more then a fouled spark plug or a tired secondary ignition coil.   

Hopefully this helps you out.

 

 

  • Like 1

Share this post


Link to post
Share on other sites

The bowl drain leaking has me stumped, never had one that wouldn't tighten up and close off.  it leaks out of the drain spout like the drain screw is loose...but it's not  may have to figure out if there is a scratch or something in the seat that isn't sealing off.  That drain screw doesn't have any kind of oring or gasket, it's just metal on metal.

Air screw was missing the washer and oring...previous owner left them out...so had to correct that.  Idles like a champ.

Of the 2 jets that are mid and main, they are both the same size/style jet.  I'm assuming the largest would be the main like usual?  If I remember right they are 115 and 75 or something close to that.  Low speed is the usual 35 or close to it.  Normal Mikuni carb.

Share this post


Link to post
Share on other sites

Try valve adjustment. Be surprised at the difference that'll make. Also check heat range of the plug your using and compare that plug to the ambient temperature your running in. Hot plugs and cold plugs do make a bike run differently

Share this post


Link to post
Share on other sites

Valves and spark plug is always a good place to start but this one ran decent before the rebuild. Money is on clogged circuit, bad diaphragm etc. Will tear down a 3rd time and try again shortly. Without the owner present so I can go through it methodically. 

Share this post


Link to post
Share on other sites

Have you checked that drain screw carb diagram for a washer of any sort ? I find it strange that nothing goes there. You can always add a drop of the high heat silicone and send it in. Be sure to wipe away any left over silicone. 

  • Like 1

Share this post


Link to post
Share on other sites

I did check the diagram, was surprised to find no oring. Checked another mikuni I have from a 220 and it didnt either. May have to go with a dab of silicone. 

  • Like 1

Share this post


Link to post
Share on other sites

This carb still has me beat, can't get any fuel moving off of idle.  Took it apart and cleaned a couple more times, jets are clear, passage below the pilot jet is clear, no unhooked hoses, throttle diaphragm works properly, float is set at spec 13mm... Think I'm sending it back to the owner and telling him to pull everything apart again and drop it in an ultrasonic cleaner and try again.  I'm almost 100% positive a passage somewhere inside is restricted enough that it can't draw sufficient fuel to run.  Everything is there and adjusted properly per diagram and service manual.  Hoping letting it set for a few days full of fuel will dissolve whatever it is that i can't see or dislodge with torch cleaning rods and carb cleaner and low volume compressed air.  Really want to hit it with 120psi of compressed air but it would be my luck i blow out a plug.

Share this post


Link to post
Share on other sites

After soaking the carb in lacquer thinner for 30 minutes I'm no better. Thinking of bypassing the tank to rule out slow fuel delivery or water in the tank issues next. 

This one has been a real "bear"

Share this post


Link to post
Share on other sites

This should not be that difficult and you can blow the carburetor out with as much pressure as you want as long as the float and needle is out.    You might have to clean the passages out with a piece of wire as well but it should clear up between the cleaning and the air pressure.   

Slap it  all back together and set the air fuel mixture screw as we’ve discussed before a s see what you got.  Throw in a new spark plug as well.  

Share this post


Link to post
Share on other sites

Have done all of the above, thus my frustration with this POS. I even moved the float to a mm or so higher than spec thinking the jets just needed buried a bit more in fuel. 

Fingers are crossed there's a fuel restriction in the petcock or water. It flows fuel freely visibly but it could be restricted by 50% coming out of the petcock and I couldn't tell by looking bc I dont have a frame of reference. 

Share this post


Link to post
Share on other sites
On 8/18/2019 at 12:49 AM, MarkinAR said:

Subject is wrong...it's a 400 Big Bear.  Must have fat fingered a key there.

sorry.

Added ATV info to topic title and moved to Yamaha ATV Forum 😀

Share this post


Link to post
Share on other sites
8 hours ago, MarkinAR said:

Have done all of the above, thus my frustration with this POS. I even moved the float to a mm or so higher than spec thinking the jets just needed buried a bit more in fuel. 

Fingers are crossed there's a fuel restriction in the petcock or water. It flows fuel freely visibly but it could be restricted by 50% coming out of the petcock and I couldn't tell by looking bc I dont have a frame of reference. 

This may be a suggestion you may have or have not heard anyone suggest you do. I have been a mechanic working on bikes and small Equipment for 50 yrs and I have had at time put a lit torch to the body of carburetors. Remove all plastics and o ring you can find before doing this. I use a lit soldering torch the same used for soldering plumbing. It will burn out any impurities that may be in the small holes that compressed air cant blow out. After heating the body of the carb to a hot enuf flame to burn dirt out then use compressed air again. Try it and see where itll take you. Put all OEM settings back to where they r suppose to be.

Edited by skidooer_3
Forgot some info

Share this post


Link to post
Share on other sites

Gonna take the 8lb sledge to this mikuni before its over. 130psi, flipped the main nozzle bc I did have it backwards, soaked in thinner, sprayed with carb cleaner, torch tip rods, passage ways clear. I'm stumped. Even bypassed the tank with fresh gas from a bottle to rule out a line issue or water and nothing works. Just has to be a blockage but cant find it. I've rebuilt 100 and never been stumped before. Sometimes they are cranky but this one refuses to run past 10% throttle. 

Share this post


Link to post
Share on other sites

Have you checked the top of the air fuel screw to make sure it’s not damaged ?

Try something  stupid and run the bike, have  a can of starting fluid handy and ready  to go.  Hit the throttle until the 10% and starts to die off and spray some starting fluid in and see if it revs higher. If so then it’s clearly a fuel delivery issue in the carb.  If not then there could be an issue with the flywheel or pick up coil once the engine rpms come up?? 

Long shot but maybe your looking in the wrong spot ?

 

 

Share this post


Link to post
Share on other sites

It runs decent with choke on and it ran before carb rebuild so it has to be fuel delivery. Just never had one I couldn't get clean and run at least decent. 

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Similar Tagged Content

    • By jolio
      Just got a '98 Yamaha big bear 4x4 in working condition (so far). Looking forward for some good information on this site about how to keep it alive.
    • By Muskyjim
      I recently purchased a 99 big bear 350 2x4 that was sitting for a while got it running and went to take it for a ride after about 10 minutes into the ride it stalled and wouldn't let me shift down into neutral tried rocking it back and forth but will not go into neutral...can anyone point me in the right direction on where to start?
    • By Christy Miller
      Need a wire diagram for 
      99 big bear 350 4x4 can anyone help I have no spark my roommate drove it ended up losing the battery and it grounded out a.wire in the middle of harness was burned to BEAR any suggestions ty
    • By KotaGman
      My bear big fires up with the pull start fairly easy. My electric start worked fine but now doesn’t even try. I bought a new battery and the same. The odd thing is, when I start it up at times, the neutral or reverse light will light up at times... depending on if it’s in neutral or reverse of course... wondering if anyone knows where I should replace? 
      IMG_1575.MOV
    • By gareth leifheit
      Ive got a Big Bear 350 4x4 that has been sitting for a couple years. It was put away running but now the pull start wont move and the starter wont turn it either. Any ideas what I can lube up or bang around to loosen it up? Thanks
  • Similar Topics

    • By jwline
      Friday evening went riding, rode for a good hour or so everything was fine. I went to take a short ride to the restroom (less than 100 yds away) and all of the sudden everything flickered and then just died out. Accompanying the problem I noticed the killswitch completely locked up. Once daylight hit took the killswitch apart to find it had shorted out and melted the plastic inside. Has anyone else had anything similar to this and if so what did you do to fix the situation? I have a new killswitch on the way to replace the obvious problem but was wondering could there be a bigger issue at hand. We tested with a voltage meter to make sure everything was still getting power from the battery and as far as we could tell it was.
      Any advice/info would be GREATLY appreciated.
      Thanks.
    • By JJ78
      I have a yamaha big bear 400, tested all fuses and relays. When I turn on key I get nothing, no neutral light or headlights. Could it be the key switch bad?
       
    • By billyz57
      New to Quadcrazy, Has any Yamaha owners had similar issue?
      1987 Yamaha YFM 350 Starts on Pull, but just spins on starter, and won't start.
    • By Blenderite
      Hello guys! New to the forum, and fairly new to ATVs. I've driven them, but never owned one. Now I do own one. Just picked it up today. It's a 1999 Yamaha Big Bear 350. It's been through some stuff. The guy I got it from told me the previous owner removed the front diff because he didn't want to replace the half shaft. So it's 2wd, even though it's supposed to be 4x4. Lovely. I can deal with that for now.
       
      More importantly, I need to get it running. The carb needs to be rebuilt. The guy I got it from said he sprayed it all out and it still didn't work. I'm concerned he might have damaged it or not put all the parts back. I was looking at a replacement carb, and the OEM parts is $600. I'd rather not spend $600 if there are any other options. I have seen carbs on Amazon and eBay that are in the $40 - $100 range that say they fit. Anyone have experience with these? I'm going to get a rebuild kit for the current carb just to make sure that won't fix it.
       
      Thats the first thing with it. Secondly, it was completely rewired. I need to find a original wiring diagram so I can put it back how it was, with a key and all. Currently it's wired with just a rocker switch. Any sources for a service manual that would include wiring diagrams?
       
      Thirdly, is the Big Bear 350 a good atv? I picked it up for $700. It does start and run when you spray starting fluid into the air intake, so I am not concerned about the engine being bad. I plan to use it for the farm, running around and carrying things with it like minerals and feed and such. Not really for mudding and things like that. How good is the 99 350?
       
      I am certain I will have more questions as I examine it more, but that's all for now. Thanks in advance for the assistance!
       
      -Blenderite
    • By jolio
      Just got a '98 Yamaha big bear 4x4 in working condition (so far). Looking forward for some good information on this site about how to keep it alive.
  • Recently Browsing   0 members

    No registered users viewing this page.


×
×
  • Create New...