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Discussion Starter #1
Hello, registered just now to ask for help as this is the last resort
2004 Yamaha Big bear 400 2wd
new top end not even fully broke in yet
New carburetor < complete assembly
new coil and plug

it'll start fine cold and run thru the gears fine no issue with or without starting on choke
once it gets warm or roughly 8-10 of idling it will idle but any touch of the throttle and completely dies
and now it's also showing signs of small amounts of fuel in the intake tube on airbox side.
I've checked the jets the needle the floats, plugs and diaphram and all seem fine and as is it's factory new
i can not seem to figure it out
for the first 8-10 of a quick ride it'll run like it's being chased but once it gets warm just dies
any idea's or help on the matter would be greatly appreciated
Oh and also i checked the valve clearance and is within spec as well as the timing
 

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Glad you mentioned the valves, that would've come up inevitably. This carb, is it an eBay speacial or a Yamaha carb. eBay carbs spell trouble. Some other people who have much more experience than me with that wheeler are the users @reogem and @dezz. They are both 4wheeler gurus.
 

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Discussion Starter #4
I can take a look and see if I can find a part# on it but I'm currently at work I'll have to check in the morning.
 

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Hmm. Yeah, hopefully these other guys can chime in and help you out. It must be super frustrating!
 

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Have you checked and cleaned all your grounds? Pull all your grounds, clean them to shiny metal on all contact points, wire wheel the bolt and screw as well if you can to clean threads, then a dab or dielectric grease, or fluidfilm etc and re-assemble. Also check that the coil bolts contact points are clean....believe it grounds through the body. You'd be surprised how many issues stem from bad grounds. ;)

Sounds to me like it could also be a control/ignition relay is getting hot and the electronics are then crapping out.....had this happened many times to me on old cars over the years.
 

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You haven't been running ethanol gas in it have you? That's a big no no. I recently had to dump my tank out treat it and refill it with pure gas. My 2005 Bruin was running like junk. Someone had put ethanol gas in my tank.
 

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You haven't been running ethanol gas in it have you? That's a big no no. I recently had to dump my tank out treat it and refill it with pure gas. My 2005 Bruin was running like junk. Someone had put ethanol gas in my tank.
True statement, however, most people can't get ethanol free regular gas. Most people believe that ethanol free premium is the deal. Wrong. Yamaha engines are low compression engines and run better on regular octane content. My suggestion is that you use a fuel stabilizer such as Lucas. This will help with the effects of ethanol. I did not know this myself until recently. It is a combination of what I have gathered over reading threads.
 

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Discussion Starter #9
On the grounds I've cleaned up the normal battery ground and I remember only 1 off the engien, the fuel is what ever comes out of the fuel pumps at local gas station regular 89 octane with up to 10% enthanol but I can't get anything less in my area, I originally thought it may be the coil but I've tried the old and new and it still acts up, only thing I can come up with is a bad carb or sensor of sorts, I'm almost at a loss, I thought it may have been jets but wouldn't explain why it would be fine cold and not warm unless slide needle or something, sensor wise I tried the "parking brake" mod and didn't change it. And yes this is very frustrating since I've gotten it about 4 months ago I've basically rebuild the entire thing trying to figure out what's wrong
 

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Discussion Starter #10
Oh and you said about ignition relay would you be referring to the stator? Or the coil its self I have not checked the stator yet as I would have to crack the outer case
 

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Welcome to GC Burn. When it starts acting up you could hook up a spark tester. You can get these cheap at any parts store and the gap is adjustable. Set the gap for whatever the gap is on a new plug is .Just unplug the spark plug and plug the tester into the wire. The spark should be solid and bright blue. If it's pink or red the coil isn't doing it's job. That's a way to test the coil and eliminate it but it really sounds like carb related.
 

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True statement, however, most people can't get ethanol free regular gas. Most people believe that ethanol free premium is the deal. Wrong. Yamaha engines are low compression engines and run better on regular octane content. My suggestion is that you use a fuel stabilizer such as Lucas. This will help with the effects of ethanol. I did not know this myself until recently. It is a combination of what I have gathered over reading threads.
I only run ethanol free gas. Don't have a need for stabilizer.
 

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Running ethanol gas is perfectly fine. There is nothing wrong with that and your engines will run perfectly fine on it too.

It's the storage of ethanol that creates issues. If your bike sits between rides for a period longer than a couple weeks, then you'll have problems. It absorbs water and breaks down. This is the same if the gas sits in a gas can for periods of time.

Now to put this thread back on topic on the Big Bear.

How have you checked the jets? Did you swap the jets from the old carb into the new one? Or did the new carb come with new jets?
Did you inspect the O-ring on the choke plunger (or just replace it)? If this o-ring is in bad shape, the bike will act like the choke is still on, and will.run rough once warm.

Try removing the air filter or the airbox cover and see if running improves.
 

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Discussion Starter #14
I've checked the jets in the sense that I've made sure their clean and clear, I can't exactly see the number stamped so I can't be sure of the sizes,now you say about the choke o ring ; that's seems plausible since it will usually start without any chicks applied and using the choke seems to have minimal effect
 

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Discussion Starter #15
Oh and on that ive attempted to use the old jets directly swapping them with the new one's but doesnt seem to make a difference, it's baffling that for 5-10mins this BB will run like shes off the showroom floor
 

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Check that O-ring. Dezz is probably right. Think about it. A choke let's the quad run fine until it gets warm. Then it starts getting to much fuel and it starts to cut out and dies.
 

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Discussion Starter #17
Yea I'm going to go and take a look at the oring on the choke, wonder if I could just pull the choke out completely if it were blocked off to give me a positive on it being the problem with some modifications of course else I'll just examine the oring
 

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Discussion Starter #18
I'm looking at it now,brass needle and brass fitting seem ok, only oring I see on it is where the plastic piece screws into carb
 

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Plastic? On a 660 it is all brass. I guess replace that one and see if it helps. You are only out $.50. Lol.
 

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Discussion Starter #20
So i unscrewed the choke cable looked at it seems good, but i was unable to start the BB as it would not fire so looks like i've got more to look into at this moment than originally thought,unless it's all related to the carburetor which i believe it is , I may end up ordering a new one from local shop depending on $$$
 
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