YZ125 Blows up Spark Plugs

Have a 2003 yz125 recently rebuilt (yes it was all done right ) And I have the correct spark plug in there and after about 10 minutes of riding the spark plug will loosen off the threads, and its deffinatly tight enough. When I take the plug out it is totally busted, I can move the white part up and down. I have replaced the spark plug 3x so its not a faulty plug, any ideas? I thought it might have been overheating so I checked the water pump however that is in good working condition

Are you running high octane gas? No knocking?

Sounds like detonation to me.....Up your main or throw some race gas in and you'll probably solve your problem.

^+1 sounds like detonation to me

Retard the timing and feed it better fuel.

Ok, I'm not familar with that term detonation, I'm assuming running too lean? What I cant figure out is that it never did this before rebuild (but then again I needed a rebuild because the dumbass I bought it from put the piston in backwards) Its running regular gas right now which I believe is 87, I'll up the fuel in it, but something tells me that wont be enough, like im going to have to mess with the jets. Anybody know how to tell which jets are already in there? Does it say it on the jets? Also how much would you reccomend going up on it and should I change the pilot as well or just the main? Thanks a lot guys really appriciate it.

You have to run at least 93 octane in any modern motocross bike. 87 octane is a contributing factor, but its' probably not the only thing wrong.

You have to run at least 93 octane in any modern motocross bike. 87 octane is a contributing factor, but its' probably not the only thing wrong.

You don't have to run 93. Easier to tune with higher octane, sure.

But many states only have 91 now though and people can usually get along with that

by pulling a degree of timing or running richer. Personally, I prefer the former.

There was a guy ready to sell his YZ because high octane was difficult to get in his location,

he did the timing thread and we talked about tuning it, now he's happy with his YZ.

Just an example of how it's not an absolute necessity.

87 can be a contributing factor however. For example, if a guy goes to a common line

pump that ran 87 prior to him, He then pumps two gallons of 91.

The actual octane will be approx. 90 from the up to half gallon of 87 in the line.

Back to the OP, it is prob. detonation, check your jetting and timing.

As long as you're at it, I'd pull the head too and check the piston crown

and the chamber, especially around the perimeter for pitting/roughness.

Damaged threads can also allow loosening, but since your insulator is damaged too,

that points to detonation v. damaged threads.

Edited by carver

Ok, but that doesn't answer my question. What is detonation? How should I check my jetting and what different jets do I get? A higher number means richer?

In the FAQ section there is a thread on jetting called

is your 2T running rich, that is easier than posting it all again.

Here is a detonation/preignition post from the timing thread in the FAQ section.

Detonation/Preignition

You don't have to run 93. Easier to tune with higher octane, sure.

But many states only have 91 now though and people can usually get along with that

87 can be a contributing factor however. For example, if a guy goes to a common line

pump that ran 87 prior to him, He then pumps two gallons of 91.

The actual octane will be approx. 90 from the up to half gallon of 87 in the line.

I agree. Around here, 91 is the highest sold. I always pump at least the first two gallons into the truck to make sure I'm getting straight 91 before I fill up my gas can.

OP- You asked how to identify jets. When you remove your carb bowl and remove the jets, they will have a small number stamped on them.

The number corresponds to the size of the opening. The larger the jet number, the larger the larger the hole is, therefore making it richer.

Read the jetting and timing threads in the FAQ, this should clarify many answers you may have.

I would suggest figuring out what jets are in it and putting stock jets in. Then set the timing on the bike to stock and see how that does. You can fine to your jetting and timing later.

Now you just need to find what is causing your problem.

Ok, so get a load of this, Took off the head last night (just the head not the cylinder) And turns out I have an Athena head, with a stock cylinder. Could this be causing my problems? The head was scratched and pitted and so was the brand new piston :thumbsup:

Get you get some pictures, are you pulling the plug cause it stops running? Could the piston be tapping the plug as it heats up, does it have alot of compression feel at the kick starter, just curious if the head is milled down?

if the piston hits the plug......ALL kinds of things went wrong. it will never happen.

I'm not sayin a full on collision of the two, obviously it would never start and you would hear it and not wanna try another kick, just curious if you have the smallest gap that closes up with heat then starts to tap it while riding, may be less likely to hear it at that point, then it just stops running....any of this make sence?

Ok, so get a load of this, Took off the head last night (just the head not the cylinder) And turns out I have an Athena head, with a stock cylinder. Could this be causing my problems? The head was scratched and pitted and so was the brand new piston :thumbsup:

I think maybe. Now, a question. I searched and Athena does have 125 sized heads, but would the previous owner have bought one?????

They do offer different domes that insert in the head. God only knows what the clearance is like.

Could you double stack a spark plug washer to eliminate the possibility all together?

That pitting is detonation. You are going to have to run race gas. Backing the timing off may help.

That pitting is detonation. You are going to have to run race gas. Backing the timing off may help.

This is the right answer. You are runnig who-knows-what for compression, and dumping regular gas in it. Pitting of the head and piston crown, and spark plug damage are huge red flags, sure-fire signs of detonation.

In my opinion a stock head would be best. Right now you have no clue what compression ratio you have.

Ok thanks for all the info guys, Do you think if I bought a stock head instead of the athena and ran 93 octance, this would solve all my problems?

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