2004 Yamaha YZ 125 Logo

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Posted on Jan 09, 2010
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I have a Yamaha yz 125 and it keeps fouling plugs and bogging out could it be the top end?

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  • Yamaha Master 2,050 Answers
  • Posted on Jan 09, 2010
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A compression test will tell you your condition. (Throttle wide open while doing that) If that's good, your jetting is too rich, could be the issue.

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Related Questions:

1helpful
2answers

2000 yz125 keeps on fouling out plugs

Fouled plugs are typically caused by jetting that is too rich. This could also be from a dirty air filter which can make the bike run rich. Clean up your filter and see if that is it. If the filter is not too bad, you might want to drop a size on your pilot or main.

Another possibility is poor compression - this can make a 2-stroke run richer than normal as the fuel can not be burned properly. Rings don't last long in these 125s. I used to re-ring each week when I was racing 125s.

You might want to run a B8ES plug, too - this is one step hotter and will help the plug from fouling as often.
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Yz 125 bog

Sounds too lean to me or lacking fuel.

Partially blocked jet in carb. May have to remove / clean.

Check also that carb butterfly / piston is travelling to full open position.
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New top end- now does not idle-bogs out when given gas

Sounds like an issue with the choke not opening.
2helpful
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2003 Yamaha 125 yz plug fouls pretty quickly. running 32/1 fuel oil ratio, and using NKG plug 2411/B8ES. Do we need to run a hotter plug?

NGK BR9EG is the stock plug for your bike. You are one heat range hotter already. I suggest you go to a BR8EG to keep you in the racing plug style but one range hotter than stock. I question what 2 stroke engine oil you are using. Stick with Yamalube 2 stroke oil and you should be okay. If you are using standard motor oil like you have in the gearbox then the fouling is definitely the oil.
1helpful
1answer

Yz 125cc 2003

Black spark plug means too rich , you will have to replace the spark plug since it has foul up and reset the air mixture screw on the carburator 2 and 1/2 turns outwards.
Check the air filter too.
Hope this helps.
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