Yz-125 fouling plugs!!

**Yz125-2012** · 21st November 2017, 07:21

I’ve had my 2012 yz125 for about 6 months but I bought it in winter so I haven’t ridden it much and I’m just starting to ride it again but it’s fouling plugs every hour or so, I’ve checked the electrical system it’s fine, I’ve cleaned the carb and air filter.
The weird thing is it’s completely stock. Any ideas?

**FJRider** · 21st November 2017, 09:46

Pre-mix fuel or self oiling .. ???

**Banditbandit** · 21st November 2017, 11:15

Rings? Check the compression ..

**scott411** · 21st November 2017, 11:18

Originally Posted by FJRider

Pre-mix fuel or self oiling .. ???

its a YZ125, it will be premix,

few questions,
what two stroke oil are you running?
what oil mixtureare you running?

what sort of riding are you doing?

**FJRider** · 21st November 2017, 14:47

Originally Posted by scott411

its a YZ125, it will be premix,

few questions,
what two stroke oil are you running?
what oil mixtureare you running?

what sort of riding are you doing?

My guess is ... bike has been sitting for a few months and the oil in the tank has settled in the bottom of the tank and fuel line to the carb. topping up/replacing the fuel in the tank will not have cleared the gunk oil in the fuel line. Each time it runs ... some fuel flows and small lumps of oil sediment flow through and foul the plug.

Next time clean the lines properly before you park it up.

**flashg** · 21st November 2017, 17:07

YZ 125 awesome bike.
One possibility, it could be sucking gearbox oil through the crank seal (that can happen) on the clutch side. If the mains bearings start to get a little bit of play, the crank seals may not be able to do its job completly as they also lose their flexibility over time.
Also another possibility, ( this is my pick) if its been sitting like you say for months, oil might have seeped from gearbox into the crankcase. A pool of oil will cause the problems as you have described.( check gearbox oil level) although a small amount missing won't be noticeable. Eventually that oil will get flushed away and it'll start to run normal.
I've owned and ridden 2t's for nearly 50 years and never heard of anyone cleaning fuel lines before parking a 2t bike up for a few months. The only stuff ever going through my fuel lines will be my premix. Hell, even my 2t lawnmower sits for about three months over winter, (oil has never settled to any extent) and still starts second pull from cold (its 25 years old)
Worn rings and low compression makes 2t's hard to start, doesn't make them foul plugs or burn extra oil. (Where would that oil come from ?)
I'm hoping the OP isn't running extra oil in the premix as that will lean off the engine, causing a whole lot of new problems.
If it was my bike, I'd be taking the head, barrel and piston off, and have a good look down in the crankcase area to see if there is oil sitting there, and take necessary measures to correct it. Don't ignore it.
Best of luck.

**Katman** · 21st November 2017, 18:49

Originally Posted by FJRider

My guess is ... bike has been sitting for a few months and the oil in the tank has settled in the bottom of the tank and fuel line to the carb. topping up/replacing the fuel in the tank will not have cleared the gunk oil in the fuel line. Each time it runs ... some fuel flows and small lumps of oil sediment flow through and foul the plug.

Next time clean the lines properly before you park it up.

**Graystone** · 21st November 2017, 18:56

Simple things first (if you haven't covered this off already), dump any fuel that was in there through winter and ensure new stuff has correct mix. Make sure you're riding it in the power-band fairly often too.

**husaberg** · 21st November 2017, 19:51

Check the airfilter, something may have moved in to the airbox during its layup and the float in the carb.both the level and the integrity of the sealing of the seat. Fouling plugs is also a sign of richness on a two smoke.

**flashg** · 21st November 2017, 20:11

Originally Posted by husaberg

Check the airfilter, something may have moved in to the airbox during its layup and the float in the carb.both the level and the integrity of the sealing of the seat. Fouling plugs is also a sign of richness on a two smoke.

I'm hearing ya husa, although he didn't mention hard starting when hot or serious lack of power, which I'd expect if running very rich. I'm still thinking gearbox oil has seeped into crank area. The smell of burning gearbox oil would be a telling sign to the trained nose. Can't beat the smell of a good 2t oil.

**flashg** · 21st November 2017, 20:27

Originally Posted by Graystone

Simple things first (if you haven't covered this off already), dump any fuel that was in there through winter and ensure new stuff has correct mix. Make sure you're riding it in the power-band fairly often too.

If the bike starts (the fuel isn't stale)and he hasn't added extra oil (he'll know) no need to throw out the fuel. On modern 2t's you don't need to run them in the power band often, if at all. They don't foul plugs that easily. When I'm riding my 2t I don't worry about whether I'm in the power band or not, and it's never been an issue. Currently 135 hours on this bike (300cc 2t) and never pulled the plug.
Oh shit, that reminds me, I'll do that after work tomorrow, big ride on Sunday.

**Graystone** · 21st November 2017, 20:30

Originally Posted by flashg

If the bike starts (the fuel isn't stale)and he hasn't added extra oil (he'll know) no need to throw out the fuel. On modern 2t's you don't need to run them in the power band often, if at all. They don't foul plugs that easily. When I'm riding my 2t I don't worry about whether I'm in the power band or not, and it's never been an issue. Currently 135 hours on this bike (300cc 2t) and never pulled the plug.
Oh shit, that reminds me, I'll do that after work tomorrow, big ride on Sunday.

Disagree, I've had fuel about 3-6 months old which would run, but it really struggled to make power and was fouling plugs, dumping the old fuel fixed it completely. The 125s are also a bit more peaky than the 300s, though 2012 is a bit newer than my last 125.

**husaberg** · 21st November 2017, 20:34

Originally Posted by flashg

I'm hearing ya husa, although he didn't mention hard starting when hot or serious lack of power, which I'd expect if running very rich. I'm still thinking gearbox oil has seeped into crank area. The smell of burning gearbox oil would be a telling sign to the trained nose. Can't beat the smell of a good 2t oil.

rich would still start easy in a lot of situations other than hot , But he didn't mention burbling though, so it is a longshot.
the only reason i mentioned it was my CR5 was a utter pig when rich, i thought it was the primry side seal letting in oil as it was that gooey.
turns out it was just the needle jet worn oval (keihins are not replaceable) so i just compensated with a fatter needle.
it didn't actually have a lack of power, just had a 125's powerband, until it cleared the needle region.
Smart money would be on a crook stator or coil. because truely fouling plugs is pretty rare without electrical issues.

**sidecar bob** · 21st November 2017, 20:40

Perhaps you could check that the correct plug is fitted, or that there may be an option for a hotter plug depending on operating conditions.

Or you could re jet it, replace the gearbox seal, flush the fuel system, do the rings, replace the air filter, change the electrics.
Fuck knows, I'm only a mechanic.

**flashg** · 21st November 2017, 20:50

Originally Posted by sidecar bob

Perhaps you could check that the correct plug is fitted, or that there may be an option for a hotter plug depending on operating conditions.

Or you could re jet it, replace the gearbox seal, flush the fuel system, do the rings, replace the air filter, change the electrics.
Fuck knows, I'm only a mechanic.

You're so right bob, leave the cheapest option till last haha
Because this is KB.