Question about FMF POWERLINE 4-STROKE MUFFLERS - YAMAHA FITS MODEL: YFM400 Big Bear 4

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Carb issue on Yamaha big bear 400 high end hesitation what could be a cause

Replaced carb all runs fine until full throttle.

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Sounds like you are running lean at the top end. Unless you are a moto-psychic, proper tuning gear is important to prevent holing a piston or overheating. But a few things can be tried before dropping by your local Yama shop:



1. New parts may combine with a less than spotless work area to cause problems. Start with clean hands and work space!! If you are running the engine for more than a 30-60 seconds at a time, get out a good fan and keep things cool. It\'s cheap insurance. Now, since you posted in November and are likely north of the equator, I\'m assuming you have fresh, winter blend gas in the tank....



2. See if the engine revs differently with petcock set for run or reserve. If so, it\'s the petcock or tank filter that need attention.



3. Check the tank vent tube for blockage and sufficient flow. A vacuum will occur in the tank if it\'s wrong and counter fuel flow.



4. Check that spark timing or quality haven\'t been accidentally whacked. That will affect top end. Check all connections and wires.



5. Drop the float bowl and see if fuel flows well (note you may need to apply vacuum to the "automatic" fuel cock, if you have one, to get fuel to flow). Check fuel tank filter, fuel petcock, fuel line and carb needle and seat for blockages. You can use a paper coffee filter under the fuel drop area to see specifically if anything is there (and catch loose parts).



6. That done, get the carb manual or PDF to know what you are getting into. Carefully clean the fuel passages in the carb body. Maybe there\'s a bit of manufacturing grit or casting flash in the system. Carb spray may not do it. I\'ve found steel guitar strings and welding tip cleaner rods are the trick here. These are small holes. Be careful and don\'t force anything!!!



7. Lastly, that new carb may mean you need to increase carb jet size or adjust the metering needle to get enough fuel at high revs. This is especially important if you are above 5000 feet and the carb is jetted for less. The shop is best for this BUT, if you must DIY, get the carb manual or PDF to know what you are getting into. Jets: go rich one size at a time. Metering rod: move up/down one place at a time.



Hope that gets you down the road!!

Posted on Dec 19, 2014

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So the problem is usually your rubber fuel line is getting old and letting little chunks pass, so a good fuel filter is always a good thing, To remedy the problem' big bear 400 is same as many Yamaha ATV with a Mikuni carb . so turn off the gas at petcock or drain tank by removing the fuel line from the carburetor, at the carb. it will drain a little. take off battery and box remove the throttle cable from the carb from the right side under the plate with 3 screws egg shape. the cable goes into a brass connector, and into a round hole , its spring loaded , roll it around, don't lose anything. on l side of card choke screws in there, remove the whole thing by unscrew at carb. loosen band around front of carb and remove or roll over . The bottom side up, shows 1 screw slotted and !!!! don't mess with the slotted screw !! 4 screws around the 'Bowl" or the float bowl. remove these evenly and lift the entire bowl off , don't drop the screws! under the bowl facing you is the float and 3 brass things with slot heads these are the jets, these jet clogged and you can clean them with carb cleaner and Qtips , take them off if you must but don't get them mixed up , the big hole jet goes on the big one in the middle spray them clean but don't stick anything in them!!!the one in the middle with the larger hole is the high end jet and assists in high revs.

reverse all this to reassemble and make sure all the hoses(2) from the carb to intake and to airbox front are on and seal well.
if this doesn't work,? clean your air filter

Posted on Sep 27, 2016

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1) Check for water in the gas.
2) Turn off the petcock and drain the carb. turn it back on.
3) Try it.
4) Start it with the choke on and put your hand over the air filter hole. Will cause some back flow.
5) Try again.

Worked for me. Might be dirtier on yours, or have mud in it or something.

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