yamaha big bear won't start

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Old 04-22-09, 10:01 AM
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Question yamaha big bear won't start

I have a 2000 yamaha big bear 4x4 400.It was runnig great shut it off now it won't start.It has fire and is getting lots of fuel.I changed the plug ,put in new fuses,changes the oil, cleaned the carberator and it still won't start.It wants to back fire when you try to start it.
 
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Old 04-22-09, 07:38 PM
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Is it a machine that sat for like a couple of months or longer after it was shut off or were you trying to start it again the same day or shortly after?
 
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Old 04-22-09, 08:08 PM
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yamaha big bear won't start

I had been driving the machine back and forth to the bush all day.I shut it off went to start and and it won't start.
 
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Old 04-23-09, 06:34 AM
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What did the old plug look like?

If you're getting hot spark you can assume you have a good signal generator/CDI on it. That leaves the carburetor which is probably flooding.

Try purging the engine of gas by shutting off the fuel from tank and if you can drain the carb bowl. Then pull the plug and spin it over with full throttle. Try to start it with half throttle and no choke.

If it starts and runs but loads up a little you need to put a carb kit or at least a float and valve in it.
 
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Old 04-23-09, 08:09 AM
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yamaha big bear won't start

The old plug was black like it was carbonded up when I pulled it.
 
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Old 04-23-09, 08:20 AM
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If you have a hot spark (bright blue, because a poor spark can cause a fouled blackened plug), you likely have a carburetor leaking past the float valve.

It's a simple fix and I would replace the float and float valve as well.
 
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Old 04-23-09, 10:22 AM
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yamaha big bear won't start

I tried everything you said and it still don't want to start.It just wants to back fire.Do you have any idea what is causing this.There is plenty of blue spark.Some times it seems to be getting gas and sometimes not.Do you think it is still in the carburator.
 
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Old 04-23-09, 11:27 AM
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The plug coloration would be typical of either weak ignition or overly rich fuel mixture. If you have the strong blue spark you'd have to think in terms of the fuel system.

A lot of things of can happen when you shut off an engine if you have components that are close to failing, such as the coil or CDI. The part will heat up and go from close to failing to failed. However, if you have a strong blue spark while you have the no start situation, the ignition components are still ok and the fuel system becomes suspect.

If your plug is sooty it's now indicative of an overly rich fuel mixture = usually a high fuel level in the carburetor = leaky float valve. It can be some other components of the carburetor that aren't sealing well and may require a complete kit to rebuild it, but the float valve is a good place to start.

Also backfiring is common with trying to start a flooded engine. A backfiring engine which is running can indicate an overly lean mixture.

If you have the flooded condition, the first thing you have to do is "dry" out the inside of the engine = shut off the fuel supply and let all the air you can into the combustion chamber. Then button it up and try to start it.
 
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Old 04-23-09, 05:55 PM
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yamaha big bear won't start

I appreciate all your help. I'm going to take the carburator apart tomorrow and check it all over again.
 
  #10  
Old 04-23-09, 07:13 PM
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No problem. Hope you get it going. It seems like a flooded engine can be a tough one to get going. There are fuel residue issues that complicate things.

As a rule if you can get something to fire in the engine, like a back fire it's just a matter of time to get it going. In your case, though you would have to take care of what flooded it to begin with.

As you work through things with your carb, check other things as well = fuel filter, fuel flow, etc.
 
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