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2000 Yamaha 400 Big Bear, dies off idle


MarkinAR

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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?

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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.


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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.

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 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.

 

 

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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.

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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.

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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.  

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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. 

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  • Admin changed the title to 2000 Yamaha 400 Big Bear, dies off idle
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
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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. 

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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 ?

 

 

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Don’t let it get to you. It could be an internal failure on the carb that you can’t clear.  It’s almost impossible to see everything. Is it a CV carburetor? If so have you checked the diaphragm for any leaks? It could be a vacuum blockage inside the carb and not a fuel delivery. The vacuum blockage can be restricting the main needle from coming up ? That vacuum chamber I’m sure you know  runs up through the top of the carburetor. 

It’s worth a look  

 

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It is a CV and the main jet raises with air flow. You can blow in the inlet port and it raises fine. Wonder if I could give it throttle and manually raise the slide at the same time...that may rule out the diaphragm. I have a feeling if it ever runs up throttle once correctly it'll pass whatever the issue is. 

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37 minutes ago, MarkinAR said:

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. 

I have boiled carburetors in hot water for 1/2 hr and sometimes that worked for me. If its internal and it cant be reach with high air pressure then you have to soften it up some and hope it either dissolves or melts apart. Think me crazy if you like but sometimes disperate things require disparate actions. Lol

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After another good soak in lacquer thinner and a ton of high pressure compressed air, it now idles like a dream and will scream at WOT.  However, idle to 15% throttle still falls on it's face.  I can feather the choke a touch to get it just above idle and once it's open, it will scream.  But 0-15% throttle is still an issue.  At least it's an improvement.  The plan for right now is to let it sit with gas in the carb and run it again later tonight to see if it'll work normal.

It reads like a vacuum leak since vacuum leak is almost negligible at WOT, but there is none on the intake boot and none on the diaphragm so I'm at a loss where that vacuum leak would be.  Maybe a thin layer of silicone where the throttle diaphragm cap meets the carb body?  Diaphragm itself is fine, no tears or holes, moves up and down fine with air.

Unless the diaphragm itself leaks a minuscule amount between itself and it's seat...that could be an option that i couldn't diagnose from the outside of the carb and may cause the off idle loss of power.

Edited by MarkinAR
add'l info
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So I've got it to the point now that it stumbles off idle much less but if you double pump the throttle it'll pick up and run. No vacuum leaks anywhere and I put a thin line of RTV where the throttle diaphragm sits just in case. I would say a bit more fuel by bumping the float up but it screams at WOT so I'm hesitant. Ideas?

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You know, as I'm sitting here at work not thinking about an ATV, I just had a thought that there may not be an o-ring on the back of the choke nut where it tightens into the carb body.  I know it didn't leak enough vacuum to suck carb spray in, but I'm wondering if it's a 1/2mm too deep in the choke throat... May have to double check that tonight.  I can lightly feather the choke just enough to put tension on the choke cable and it perks up and runs great.

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