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Hey guys I'm looking for some assistance here. My grandfather passed away recently and has left me a 2000 yamaha big bear 400 4x4. It has not run in at least 3 years. The battery is completely shot and I was not able to pull start it. I removed the fuel line from the carburetor and it was plugged solid with a build a green gum/varnish type material. After screwing with it for a while I finally got the carburetor removed and I took it apart and cleaned it. I also completely drained the fuel tank and replaced it with new fuel. After trying it for a while it did finally start after I boosted it with my truck. The problem is it is running so badly it won't stay running at an idle. If I hold the throttle open about 1/8-1/4 inch it runs great. If I try to go anymore it coughs,sputters and backfires terribly. On the same account if I try and let it idle it seems to idle so low it just dies. it will rev up with the choke closed but as I said it just backfires. I know that is caused by a lack of air to mix with the extra fuel. What I don't understand is why when I open the choke it wont rev up at all. It just dies if I try to give it any throttle. I remember riding this when I was younger and It was a great machine. It always started right up and idled perfectly. Obviously sitting for so long has taken its toll. I am wondering if the battery being so bad it is some how causing so sort of issue with trying to run it due to an electrical issue. Any ideas what I should try? I would prefer not to take it to have it checked out if I can but I guess as a last resort that is an option. Any help would be great. Thank you.
 

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Hey..md544600..Welcome to GC from Wisconsin.
I would clean the entire fuel system.
Pull the tank off empty out any fuel, looing side for sediment, flush it out.
Probably easiest to replace the fuel lines and if it has a filter replace it also.
Pull the carb off, clean it or have it cleaned and install a new needle and seat.
Run some fresh gas. Let us know what you find.
 

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Discussion Starter #3
Hey..md544600..Welcome to GC from Wisconsin.
I would clean the entire fuel system.
Pull the tank off empty out any fuel, looing side for sediment, flush it out.
Probably easiest to replace the fuel lines and if it has a filter replace it also.
Pull the carb off, clean it or have it cleaned and install a new needle and seat.
Run some fresh gas. Let us know what you find.

I will pull the fuel tank off and completely flush it. I will also add in inline filter when I do it. I will pull the carburetor off and go through it again and see if that makes any difference. I am also planning to install a new spark plug. I guess my biggest question was regarding the electrical system on this unit. If the battery is bad will that cause any running issues with the engine.
 

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Welcome from Colorado.
It could.
Get a new battery, you'll need it.
 

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Good idea to get a new battery although that machine should run good with a weak battery.
 

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Discussion Starter #6
Hey..md544600..Welcome to GC from Wisconsin.
I would clean the entire fuel system.
Pull the tank off empty out any fuel, looing side for sediment, flush it out.
Probably easiest to replace the fuel lines and if it has a filter replace it also.
Pull the carb off, clean it or have it cleaned and install a new needle and seat.
Run some fresh gas. Let us know what you find.
So I tried all of this with the exception of the needle and seat. I pulled the tank, completely cleaned it and I added an inline filter to add an extra layer of protection just in case. I also replaced the plug and cleaned the carb again. I have a video i'll see if I can add. After doing all the work it is exactly the same. It will start if I have the choke about half way, any more or less it will not start. Once it does start after a few seconds of running it will idle quite high with the choke open half way. If I move the choke to the fully open position it just dies. If I close the choke completely I can rev it up but it backfires and sputters badly out the tail pipe. With the choke open half way it does seem to back fire out of the air box. I pulled the air filter and felt inside the air tube and there is fuel in there so I assume its getting to much fuel. I also noticed this time that when I put everything back together that when I open the petcock for the fuel, after a few seconds fuel started leaking from the tube attached the bottom the carburetor. After getting it to start it stops leaking. That to me would be an indication the needle is sticking open and never stops the fuel from filling the float bowl. Am I correct in think this? I sincerely believe it is a carburetor related problem.
 

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You are going in the right direction. You definitely have a carb problem. Replace the needle and seat as they are not stopping the flow of fuel. Additionally, did you bent the metal float tab when you had the carb apart? This will also give you fuel flow issues if float height has changed.
 
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I should have mentioned this before. While you had the tank off you might as well adjust the valves.
 

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Discussion Starter #9
I should have mentioned this before. While you had the tank off you might as well adjust the valves.

Why do people keep saying the valves need to adjusted on these? Is this a common problem on these units? I should mention my grandfather rarely ever used this machine. It's a year 2000 and it only has 1800 km's on it. I found some carb's on amazon from around $60.00 for these. Are they ok are or should they be avoided?
 

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As you have found these engines are sensitive to minor input.
At 1/2 choke it acts different than with no choke or full choke.
Proper valve lash is critical for best performance. There is a range and if the machine was adjusted on the low side of the range when new, any amount of change due to fear may not allow the valves to close fully during the fuel burn.
My experience with 550's, 660's and 700's is all I've checked needed a slight adjustment around 1000 miles.
Most all the miles on my grizz are above 7000 ft. where the grizz starts out 21% down on power, so every little bit helps.

As for your carb. it seems cutting back on the air isn't a sign of too much gas, but to much air.
 

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Discussion Starter #11
You are going in the right direction. You definitely have a carb problem. Replace the needle and seat as they are not stopping the flow of fuel. Additionally, did you bent the metal float tab when you had the carb apart? This will also give you fuel flow issues if float height has changed.
I have picked up a carb kit and I will install that when I get home and see if I get any change. I do not believe anything is bent. I did also confirm the larger jet is in the main position. I see some people have switched them but I am sure they are in the correct spots. Anything else I should try while I'm in there?
 

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Discussion Starter #12
Idle speed adjustment??????

So I installed the new carb kit. I did find a problem with one of the pieces I didn't notice before. It seems to run now but It wont idle. If I open the choke fully it will idle to low and die. I believe the screw on the front just below to intake boot is the screw thats adjusts the idle correct? What I need to know is two things. 1. do I have to remove the carb to adjust it? Will turning it in out increase the idle speed?
 

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So I installed the new carb kit. I did find a problem with one of the pieces I didn't notice before. It seems to run now but It wont idle. If I open the choke fully it will idle to low and die. I believe the screw on the front just below to intake boot is the screw thats adjusts the idle correct? What I need to know is two things. 1. do I have to remove the carb to adjust it? Will turning it in out increase the idle speed?
That screw your talking about is the idle air screw. Screw it in all the way (but do NOT tighten it). Then back the screw out to set it. Every ATV will be different, so you will need to find what works best for your machine......but 3 turns out is a good starting point for your machine.
 

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Discussion Starter #15
It seems like its not getting enough fuel. If the choke is closed all the way it will run, but at very high idle. As you open the choke it will start to sputter and back fire through the air box. If I try and open the choke all the way it will idle down very low and die. If I try and give it any throttle, with the choke in any position it will just back fire through the exhaust and die. I am getting very annoyed I can't figure out what's going on.
 
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