SO I've got a 2004 Yamaha 400 big bear, 2x4. It ran great till i had an issue with the carb being slightly gummed , needle valve didn't seat, over filled carb, over filling piston, and then the crankcase. Ran, sorta would run for about 2 minutes then cut out, couldn't figure out why, pulled oil plug and 2 stroke mix GUSHED out, towed home, took the 2 stroke mix out of the crank case, new oil and used the petcock whenever i was shutting it off. Wouldn't run right. in first or second it was hard to detect, but 3-5 it will cut out under and sort of load, like in 4th going from 20-30MPH it will go 202020, 21 21, 22, sputter 21, sputter 21, 23,24,25,sputter, 26,28.,29,30 (maxed out upshift to 5th) at pinned throttle in 5th im at 25 because the wind resistance creates enough load to sputter out and slow em back down. I installed  New 93 gas(and a  bottle of seafoam), new fuel filter, new spark plug, ect.

Well my problem persisted. but i had an ATV rally to attend and ride some backwoods stuff. rode all of 50 miles during the rally with just extra shifting to keep load low, and it was doing it all day (seemed worse later in day); at one point the battery cable vibrated loose and was disconnected and it was still running(i shut it off and it wouldn't restart at all, coultn figure out why no neutral light, so trouble shot and a battery screw had vibrated loose, but shouldn't that kill it?). So reconnected and issue persisted. Then it sputtered and died at bottom of a hill, like a kill switch was hit, dead. So i get off trail and i hear a whistling from rear end(ATV is off and stopped). pull seat and battery is smoking/steaming from middle, i disconnected it and let is stop. drove to store got new battery, restarted and  rode it to top of hill and onto trailer(issue persisted).  I thought voltage regulator might have killed the battery , and made spark weird at various loads so new voltage regulator was put in and no change. Issue persists.


Summing up issue, sputters under increased load regardless of gear, just more noticeable in 3,4,5th. New factory carb, fuel, fuel filter(additional inline one between petcock and carb), sparkplug, voltage regulator.. im stumped  

Any clues?

Gasoline in the crankcase and sputtering under load points to a float bowl issue in the carburetor.

It's possible the float in the new carb is defective or more likely, needs adjustment.

Good God, man!

One thing to do, make sure all your vent lines are clear, carb, case, all of them.
Second thing, park it in the side yard next to some blackberry vines and let earth reclaim it.

Vent lines are clear, checked them already.

the new carb fixed the crankcase issue (I saved the oil for bonfire starter.. things got exciting when i used it to start a bonfire of sticks later), could the brand new factory bowl be issue?

Check the compression.

You said two stroke? You're not mixing oil in your gas are you?

machine is a 4 stroke machine, BUT when your carb overflows and floods the crank case(normally oil) with gas. you get a 2 stroke mix... (well not really 5-30 isnt suppose to burn).


never checked compression, wouldnt that manifest in not running

I'm rather confused. That's a 4 stroke engine and you are getting 2 stroke mix in the crankcase? I'm assuming you mean the gas is getting into the crankcase. My first thought would be leaking needle/seat or float issue

I would replace the coil and cdi, probably in that order. If you have good compression, correct cam and ignition timing, and no fueling problems it doesn't leave a lot to check.

correct. I HAD an off brand carb, it had a bad valve or something and flooded the carb, then piston, then the engine. Gas filled crank case, and engine died.  I drained the resulting engine oil, and gasoline out of crank case, put new oil, new carb, new gas, new fuel filter, new voltage regulator. and the issue STILL happens.
The resulting oil and gas mix was used to start a bonfire and was VERY much flammable.

I had an engine that would run with 1/3 rated compression, idled fine, no power at open throttle.

it will have power then once under an arbitrary amount of load cut power, then  regian power for a few RPM, then sputter again. If i put it in 3rd and try and take off, it will sputter and slowly accelerate(duh 3rd, large load), but put in 2nd and no problems,  not sure how else to describe it. i can drive flat no problem(in say 2nd), but hit a steep hill and i get sputtering, a loss of power, and  as soon as the engine has to work.  even in 5th on flat i get sputtering because the load exceeds the threshhold where it happens.

Is the choke possibly on.   Or messed up as well

This is a very likely culprit.  Under load a weak spark can be "blown out".  Replace the ignition components.

.

coil arrives this week, CDI will be next.. YIKES the CDI will be more than i paid for the whole machine.

Yammie parts are not cheap.

Cdi for one of my old polaris wedge snogos is over $300 and hard to find a good one.

Good place to get some parts.
https://rmstator.com/en_ww/partfinder/search/result?pf_id=1&dw1=ATV&dw2=2004&dw3=Yamaha&dw4=YFM+400+Big+Bear&category=Stator&cat=28

Grizzly central is another place you can ask.  There are a few guys there that seem to know every detail of every Yamaha ever made.

Try removing the inline fuel filter.
Many paper pleated filters will not flow enough fuel from gravity alone.

#1: are you using genuine Yamaha 'new parts', or cheap chinese shit off ebay?

#2: Diagnosis. Do it, instead of shotgunning parts.

#3: New battery. Go to a bike shop, not your local Slowreilly's or Autoboner. Ditto get a new NGK spark plug and install.

#4: Start it, get it warmed up, run it Wide open throttle to the top second or third gear and kill ignition and get in Neutral as fast as you can and coast to a stop. Pull spark plug. Black is either too much fuel or shit ignition. White is good ignition, but starved for fuel. Golden brown like McD's french fries is the goal.
It will not hurt to ride it around a bit and repeat this test. Record your results!

#5. Ride it around while it continues to screw around, if it continues to do so. Do the ignition kill get to Neutral coast to a stop routine. Pull plug, check results.

Likely things to check ONE AT A TIME based on your post:
FRESH gas, NOT from a can full of old gas. Use a new can, get ethanol free fuel.
Air filter. Get a new FACTORY one. Not some Uni shit. Whether it 'looks good' or not. Did you check to see if the airbox was full of fuel when your carb was overflowing? That's gonna make it run rich as hell. There's a drain cap in the airbox if you need it.
Like I said, the RIGHT NGK spark plug. .
Try unscrewing the spark plug cap (should unscrew easily, don't force it if it won't go), and tape the wire directly to sparkie Pluggie and see if it changes. Sometimes trimming 1/8" (but no more) off the end of the wire helps.
Have you adjusted valve lash? I'm betting nobody has.

If spark plug is white, pull the fuel valve and inspect for plugged screens. BE GENTLE cleaning the screens. This is a good time to flush the tank.
Ditch the shitty aftermarket fuel filter and use only OEM or TYGON fuel line only to replace it. No polyurethane bullshit.

@ me with results and we'll walk through checking the charging system, which is also the ignition system.

Ok from the top.
Fresh gas ethanol free, fuel filter is free flow.
Genuine Yamaha prts so far.(carb(minkui (spelling)), coil(spark plug wire, boot, and coil all one part), voltage regulator, air filter,  New battery.

Spark plugs test were black, super sooty , I swapped in a new plug (auto lite , localMC store closed so no ngk, original was NGK),  and the fresh plug was carbines up in 2 minutes.
Airbox is clean and unobstructed, even with air filter removed ,issue still happens.
I dunno how To check valve lash,
@jos51700


Attached File

Left plug is after 1-2 min wide open then kill switch, right is “old” plug that has was in when it broke down on trail(battery explosion described above)

What does the Yamaha factory service manual say to do?
Not some POS Hayes or Chilton.

When you get to a point where you have spent that much time and money fucking around with it take it to a dealer and let them troubleshoot the issue.
Or get rid of the POS and buy something new.

@fsjdw2

OK, Good photos and detailed response. Before I forget, make sure to look through the tube feeding air to the airbox, and make sure a shop towel, mud clod, hornets nest, towel or something didn't get stuck in there.

Also, make sure the choke plunger is free to move in its' bore. This is probably culprit numero uno.
In the pic below, this is the big black knob (snicker), but on your quad it's connected to a cable connected to the lever at the handlebar. A kink in the cable or an improper function of the plunger means you're cruising with the choke on 24-7.


You can see the wee return spring. Doesn't take too much to hang this up. You can use a wrench to unscrew the hex from from the carb body, and this will pull out (snicker) with it and then diddle the lever to see if it reacts accordingly. When the little plunger is bottomed out (more snickers), then the choke circuit is closed off, and this ceases the extra fuel flow for startup. If this wasn't screwed in properly on the new carb (boogered threads, bound cable, or other issue), it can make the quad run rich.







If that isn't it:

Process: Identify if black is ignition or fuel related.

MAKE SURE THERE ARE NO FUEL LEAKS. SHUT OFF THE FUEL VALVE BEFORE THIS PART. I have literally burned a bike to the ground because I didn't do this first.
With a fresh new plug (I did enough bikes I just kept a new plug in my box), pull the plug cap, stab in a new plug, and while holding the plug body against the engine (so it can ground), crank the motor (the original plug should still in the motor). Your spark should be bright blue, easily visible in daylight (maybe not noonday sun), and you might even be able to hear it. Notice that the actual plug testers have a HUGE gap, but sometimes it's a bit huge for jap bike ignition systems:


Theory: just like the tank on your toilet (which is much easier to watch for illustration), the carb has a float that rises with liquid level, and float controls a valve (the needle in the needle and seat system) that shuts off liquid at the right height.
In your carb, if the liquid is too high, it will run too rich.

Running rich as hell, the other (and far more common) option that produces a black spark plugs condition, can be a few things.
Valve lash is a little more involved. If you have feeler gauges, you can be taught to do it, but it's really more of a shop-level maintenance item. If the valve lash is non-existent, the valve gets held open slightly and you're blowing combustion pressure back down the intake. Carburetors work in both directions, so the fuel gets mixed with air once on intake, again on blow-back, and AGAIN on re-intake, so the bike runs pig-rich. Typically, it's tough to do this because it's a fine line where the bike will still run with poor valve seal. This quad still has full exhaust on it, right?

The other option is remove the carb, and do some looking. You do NOT need to remove the throttle cable.

There are typically four screws holding the bowl on the bottom of the carb. Be careful when you pull the bowl OR turn the carb over, it's going to spill fuel. There's a pivot pin for the float and the presence of the bowl is usually what retains it, so hold the carb level while you pull the bowl straight down after you remove the screws. It'll probably stick, and be careful not to tear the gasket. The pictures below have a screw holding the pivot pin.

Once that's off, identify that pin and keep an eye on it, you won't be finding a replacement for it at Lowe's. Invert the carb, and look at the frame for the float, where the float pivots on that pin. The frame is usually flat sheet metal, and should be parallel to the bowl gasket surface when the carb is upside down (and not tilted). It's easy for these frames to get bent, and if the floats are bent up, they'll close off the fuel supply late, and the fuel level in the carb will be too high, and cause a rich running condition. Photo shows different carb, similar system. Frame is the shiny silver part.




Some places talk about blowing in the fuel inlet and all that crap, skip all that. you're just wanting the needle (under the tang) to seat when the floats are in the right spot, and that's when the float frame/arm is parallel to the gasket surface. if you hold it upside down and slowly lift the floats, you'll see that the needle will seat and stop moving, and if you let go so gravity is on the floats, that's close enough to measure. Pic below shows the float a little high, but remember, it's upside down. There's usually a spring-loaded pin in the needle. If so, you want to hold the float so it's not compressing that pin, and see if it's level when it JUST starts touching that little pin in the float. Some needles omit this and just hook directly to the float.



While you're there, look to make sure all the jets are still in place like above, and write the numbers down on them. If it's a used carb, it's possible some t-rard used a wire or drill bit to 'clean' the jets out, and ruined them. The numbers are listed in the service manual to make sure that Kawasaki-san didn't grab the wrong jet out of the bin during assembly, but it's ALWAYS possible some idiot drilled the jet. The jets should be snug, but if you try to make them tight, they'll just break off as they're brass. Also not found at Lowe's. If they're snug, they won't rattle out,

SOME carbs have washers under the main jet, some don't, and some carbs have intermediate jets and some don't and so on. Your exploded view knows for sure.

Pull the pin that retains the float, and the needle will CAREFULLY come out with the float. If the needle resists coming out, there's your huckleberry, it should be free to move. Look for ANY trash, specs of dirt, rust, ANY PARTICLES OF ANYTHING in the needle area, on the needle etc. THIS NEEDS TO BE CLEAN LIKE YOUR WIFE ON YOUR WEDDING NIGHT. These solids will keep the needle from plugging the hole, which is what shuts off the fuel flow into the carb. A little spritz with carb cleaner here will not hurt anything, but make sure it blows it all out, instead of just moving it around to come back and haunt later.

Let us know what you find here. Pics are great. Pics inside the venturi (the horizontal tube through the carb that the engine breathes through) won't hurt either.

Looks like you have done everything right. For the next time you can use online owners manual for your bike here - https://www.atvownersmanual.com/yamaha/grizzly-660/2004/service-manual/. I guess it will make your life a little easier and will help you to solve the issues faster. No need to download anything or pay, just open it on the laptop and it is ready for your use.

ill be able to test this stuff out next week. ive been down with COVID(not that bad just ZERO energy), and im catchingup on other stuff.

Does it have one of those vacuum fuel pumps like the rhinos had?

ETA: nvm, thinking SxS instead of 4wheeler.

Needle valve be fucked.

Check the vent hose for the gas cap. Very silly but I’ve seen all sorts of weird problems from this being kinked or inadvertently closed up when guys stick it back down into the handlebars.