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YZ125 cutting out problem

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Hi everyone, I have a 2002 YZ125 that i recently rebuilt with a new bottom end after the crank gave out, and it runs great, starts extremely easily and everything seems to be fine except for when i run wide open. Almost the second i hit full throttle it will die, just completely die like your pressing the kill switch until you let the engine stop and then it will start again. And yes, i have cleaned the carb and checked the float height and all that, and it all seems to be in spec according to the service manual.. This problem has me completely confused and maybe you guys that know something about this could help out..

I also have a video to show you whats happening

http://www.youtube.com/watch?v=bn38YlrTNPQ

Any help is appreciated!

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Did you change anything else when you did the rebuild - new pipe, reeds, something else? Maybe you just need to up the main jet a couple sizes?

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I didn't change anything else, i did try raising and lowering the needle but both of those made it run worse. I could be wrong but it doesn't seem like a lean main jet would make it completely cut out.. Seems like it would just make it miss or run badly up top?

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I didn't change anything else, i did try raising and lowering the needle but both of those made it run worse. I could be wrong but it doesn't seem like a lean main jet would make it completely cut out.. Seems like it would just make it miss or run badly up top?

Yes, that is sort of correct. I couldn't see a bike with a lean MJ doing that, unless it was WAY lean.

What's your jetting compared to stock? It sounds like it starts blubbering (rich) at the start.

How many hours since the rebuild? I would pull the head and have a look.

Make sure the piston isn't in backwards, etc.

If all that checks out, I'm going with Arnego, electrical.

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I just went out to try it again, this time it only cut out once in 8 full throttle runs, idk what changed.. It was also missing a lot in the higher RPM's so i took the plug out to see if it looks rich or lean and it looks alright, almost black but not sooty or anything. What do you think?

DSCF9485.jpg

DSCF9484.jpg

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Yeah, rich but not enough to make it cut out at WOT. Especially since you are experiencing the problem intermittently, I'd say it's electrical.

By the way, is that a resitor plug you're running? It should be a BR9EG or equivalent.

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Just checked my jetting, it has a 370 main, how does that sound? Im at 5500 feet elevation and it has a FMF gold series pipe and pro circuit shorty muffler.

I ordered a BR9EG plug from RMATV, so unless they sent me the wrong one its a BR9EG...

Also, the needle is in the lowest position if that helps..

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Just checked my jetting, it has a 370 main, how does that sound? Im at 5500 feet elevation and it has a FMF gold series pipe and pro circuit shorty muffler.

I ordered a BR9EG plug from RMATV, so unless they sent me the wrong one its a BR9EG...

Also, the needle is in the lowest position if that helps..

Have a service manul? Check that, it should have a chart for altitude and jetting.

370 sounds lean to me, but I haven't jetted a keihin 125 carb, only Mikuni, so don't take my advice on that.

Needle in the lowest clip, as in from the top? That would be way to rich for that altitude.

BR9EG is fine, but that's a BR9IEX, I believe, the iridium plug.

The R is for resistor, and it's for cancelling radio frequency interference.

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Have a service manul? Check that, it should have a chart for altitude and jetting.

370 sounds lean to me, but I haven't jetted a keihin 125 carb, only Mikuni, so don't take my advice on that.

Needle in the lowest clip, as in from the top? That would be way to rich for that altitude.

BR9EG is fine, but that's a BR9IEX, I believe, the iridium plug.

The R is for resistor, and it's for cancelling radio frequency interference.

The needle is in the lowest position, as in the clip being as high up as it can go. So its as lean as it can go..

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I had a problem with one of my street bikes that it would drop one of the cylinders over a certain RPM. Drop below that RPM and it would be fine. Ended up being the coil for that cylinder. Don't know if that's your issue but maybe?

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BR9EG is fine, but that's a BR9IEX, I believe, the iridium plug.

The R is for resistor, and it's for cancelling radio frequency interference.

You are actually correct, it is a BR9IEX. Should i change it out for a BR9EG or is that plug fine for the bike?

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You are actually correct, it is a BR9IEX. Should i change it out for a BR9EG or is that plug fine for the bike?

Why so surprised??:busted:

That plug is fine. The difference between the br9eg and br9iex is the latter is an iridium plug. Supposedly the iridium plug has a more efficient spark.

Personally, the only difference I can tell between the two is the price.

I run a br8eg, or a br8iex when I steal my buddies in my yz250. The difference in the 9 and 8 is the heat range.

Personally I would use the plug your manual calls out for until you get the bike running correctly.

If your jetting is good, I would start checking electricals. Start with the timing and the coil.

Let us know what you find out.

Edited by GMO397

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