1999 Yamaha Bear Tracker 250 carb problem

[TRUE69]

I just purchased the ATV and it run really rough with the breather filter on and lid on the breather box. Once I take the lid off and remove the filter it runs better but it backfires through the carb.

What could I do to fix this.

The carb is fairly new.

And I put high grade non ethanol gas in it hoping it would help some.

Edited December 3, 2022 by TRUE69

[Gwbarm]

Sounds like your air filter is clogged causing it to flood badly. Carb could be partially clogged also. I would get a new air filter make sure all the air passages are clear no mouse nest and clean and adjust the carb. I just noticed you said the carb was new so may not need cleaning.

[Mech]

I'd try the filter without the lid on the box. If that cures the problem and the bike runs ok then it means there is a blockage somewhere in the intakes piping, somewhere before the air gets to the filter housing. If it runs rough when you put the filter back in without the lid it probably means the filter is dirty or over oiled. In that case I'd wash and oil the filter if it's a foam element, or replace it if it's a paper element. If you oil the filter use engine oil and give it plenty, then work the oil all through the element by squeezing it and working it in, and then squeeze it to get all the oil you can back out of it. If you wrap the oiled filter in a rag and squeeze it it works best. If you just use your bare hands to wring the oil out it keeps getting sucked back up off your hands, but if you use a rag the rag absorbs all the excess.

If a clean or new filter still makes it run rough I'd check the breather hoses on the carby are not blocked and that they are on the right places. If all that checks out ok, then I think the carby needs some adjusting.. possibly just the float level. You could check that theory by turning the fuel tap off and running it and see if it starts to run right for a while before it runs out of fuel, which should take about two minutes if you are just sitting revving it. If it runs right for a bit, turn the fuel back on and try it.. it might have cleared the spark plug and excess fuel and it might be ok.. If not, you will need to pull the carby and clean it inside.

 

[Bruce Sawyer]

Some of the ATV carbs have a bowl drain screw in the bottom or lower side of the carb bowl.

When My Yamaha Big Bear starts not running correctly I can removed the carb bowl drain screw with the petcock turned off and drain the bowl while the engine is COLD and then turn on the petcock and let some gas flush the bowl, turn off the petcock re-install the bowl drain and good to go.

Edited December 4, 2022 by Bruce Sawyer

[spock58]

My guess is the "fairly new" carb is a Chinese knock off. Sometimes they work great, other times not so much. Of course it may be the only one available for bikes/quads of this vintage. If it is one of those, possibly swapping jets to the original factory ones is the remedy. Good luck, it isn't an easy problem to solve!

[Tiha]

Popping through the carb is usually a lean condition. Could be a vacuum leak, when you put the air filter on it is sucking too much air from someplace else and not getting enough fuel. 

Make sure you float is set high enough. A float set too low can allow air to be sucked into through the bowl creating a lean condition. 

Chinses carbs are always a crap shoot. But sounds like you are close enough to make this one work. I had one that pilot jet port was not milled out. You just never know.