Question about Yamaha YZ 125 Motorcycles

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1999 Yamaha YZ 125 will not start

I have replaced the spark plug, checked the pressure from the piston, rebuilt the carbeurator, put in new gas, and changed the air filter. everything seems to be right but it still will not start. it idles, but will not do any more then that and I have to have it full throttle. one person says it might be the sti box (electrical box) but i dont know where i can get one of those.

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If it starts and idles, but will not rev up, it is your carb. Recheck your rebuild and make sure float height and all is correct.

Posted on May 30, 2009

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Posted on Jan 02, 2017

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It idles fine. When I press the throttle, it bogs down and stalls. I replaced the spark plug when this initially occurred.


The problem is that your carbeurator is gummed up. Get a can of carbeurator cleaner spray at any auto parts store. remove the air filter and put two squirts of cleaner into the air port hole. start it up. with the assistance of a helper, run it and squirt one squirt of cleaner at a time into the air hole until it no longer bogs down.

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I have a 1999 wrangler and it tops out at 55 miles per hour but it doesnt mis and runs great around town


Top speed on a wrangler is typically about 15 miles faster than what you are getting. Check the air pressure, air filter, spark plugs and fuel filter. If all of these are good your carbeurator may be partially blocked. Begin with a fuel additive that will clean your carbeurator. I have had success with a product called seafoam which you add to your gas tank at a rate of one ounce per gallon of fuel. Use it for a couple of fuel tanks full of gas.

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yes it is a gummy carbeurator. Put in fresh gas if you havn't already and put in a new spark plug. Then get a can of carbeurator cleaner and follow the directions to get your carb up and running again. While you are at it I would also install a new air filter.

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Yes. Your carbeurator is gummed up. Get some new gasoiline and add some carbeurator cleaner to it, usually one ounce per gallon. Dump the fuel u have in there and put in the new fuel.

Also, get some spray carb. cleaner, remove the air filter and put a couple of squirts into the air intake chamber. Then start it.

Finally if it still won't start, use a squirt of starter fluid to get things going and get the carbeurator cleaning into the system.

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Since you have good spark and good gas the only things left to check are the air filter and the carbeurator. If the air filter is dirty, replace it. While you are at the auto parts store also pick up a can of a gasoline additive called seafoam which will keep your carb. clean. Finally get a can of spray carb. cleaner and put a couple short squirts directly into the air intake which is right there when you remove the air filter.

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Your carbeurator is gummed up, a very common problem after a winter of sitting idle. Go to any auto parts store and buy a bottle of carbeurator cleaner which is added to your gasoline, usually one ounce per gallon. Also buy an aerosol can of carbeurator cleaner which you spray directly into the air intake by temporarily removing the air filter.

If you havn't replaced the spark plug and the air filter in a while do those things also, and use freshly purchased gas, not gas that has been sitting around for months.

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Its starts but will cut off


The problem you are having is due to dirt and gunk in the carbeurator. Spray a carbeurator cleaner into the air chamber [remove the air filter] and run it a couple of times then let it sit for two hours while the carbeurator cleaner does its job of dissolving any debris. Then start it up. IF this does it, great, if not repeat the cleaning technique.

If all of this doesn't do it, then the carbeurator will have to be removed and either replaced or rebuilt.

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