Jump to content

My YZ 125 is EATING Spark Plugs!!

By BrandonCA,
in Yamaha 2 Stroke

 Share

Recommended Posts

Grand Master

BrandonCA

Posted
Posted

Hey Folks...

I will have some pics tonight, but I wanted to see if anyone had some initial ideas on the above happenings...

Short story...was riding two weeks ago and thought I just blew a plug...after some pushing and a short ride after my friends got back to camp to get the truck..we got back and pulled the plug. The electrode thing at the top was broken off at the base.:eek: Got the bike home and pulled the head...and the piston had all kinds of divots where the piece was bouncing around in there...and the head looked like swiss cheese. The piece wound up wedging itself thru the top of the piston edge all the way down to the ring. Amazingly, no damage to the cylinder.:worthy:

SO...took the head to a local guy our here, Al Holly, and he put it on a lathe and was able to save it. Got a new piston kit and put it all back together. Good compression, fired right up.

This weekend I go out to Cal City with some friends...ride it around camp a couple of times to work it in..never hitting higher than 3rd gear for about 10-15mins. Gear up and off we go...get about a mile out...and just as I put it into 5th gear..KAPUT. Push it back to camp AGAIN..pull the plug and presto..no tip on the plug. SEETHING!:banghead:

I called Al this morning, and his first guess was an air leak somewhere...I'll have to take it over to him this weekend..but I was just wondering if anyone had seen this type of plug eating before..and if so..what was the problem?

Again, sorry! I'll post some pics when I get home...thanks!

Link to comment
Share on other sites
Grand Master

BrandonCA

Posted
  • Author
Posted
well the piston could be smacking it and breaking it off due to bad squish? but im not sure, im sure some more mechanical inclined people will reply good luck with the problem:thumbsup: :worthy:

Yeah, that was my first thought...but to hold the piston up to the head...there wouldn't seem to be anyway it COULD hit it.

are you sure your using the right spark plug, and that its not too long so that the piston is contacting the electrode of the spark plug?

Pretty sure..I'm using a BSR 8...

Link to comment
Share on other sites
Grand Master

BrandonCA

Posted
  • Author
Posted

What's odd...is the first plug definitely BROKE off. Well, I say broke but since it was bouncing around destroying everything in one piece that seems accurate. What I forgot to mention...is the second plug..almost seemed to MELT off...no pit marks in the piston or head this time...just a DEEP groove in the center of the piston..with what looks like aluminum flake debrie all over...

Link to comment
Share on other sites
Experienced

AV8R

Posted
Posted

Detonation is eating the ground strap on your plug. Things get soo hot in the combustion chamber from detonation that the electrode is getting literally melted off. Possible that pre-ignition is also occuring.

When you got the head fixed this may have made the situation even worse by making your squish larger from removing material from the head.

Ideally I believe the squish for a 125 is 1.0mm.

Do a search on adjusting the squish band to find out how to check this and adjust the squish band.

Jetting may also be a little lean on the needle an/or main jet due to dropping temperatures contributing to the, or maybe even the primary cause of, detonation.

Link to comment
Share on other sites
Grand Master

BrandonCA

Posted
  • Author
Posted
Detonation is eating the ground strap on your plug. Things get soo hot in the combustion chamber from detonation that the electrode is getting literally melted off. Possible that pre-ignition is also occuring.

When you got the head fixed this may have made the situation even worse by making your squish larger from removing material from the head.

Ideally I believe the squish for a 125 is 1.0mm.

Do a search on adjusting the squish band to find out how to check this and adjust the squish band.

Jetting may also be a little lean on the needle an/or main jet due to dropping temperatures contributing to the, or maybe even the primary cause of, detonation.

Yeah...my neighbor and I thought the second piston looked alot like detonation. I'm just confused by all this...the bike has run fine for the last year and a half with no problems. Then all of the sudden..it's hungry for plugs? Nothing has been changed or modified.

yeah, I was just reading about the squish not to long ago...I will re-visit. AND post the pics when I get home. Thanks!

Link to comment
Share on other sites
Mentor

olddude

Posted
Posted
Yeah...my neighbor and I thought the second piston looked alot like detonation. I'm just confused by all this...the bike has run fine for the last year and a half with no problems. Then all of the sudden..it's hungry for plugs? Nothing has been changed or modified.

yeah, I was just reading about the squish not to long ago...I will re-visit. AND post the pics when I get home. Thanks!

How old is your bike?

I went through a problem like this awhile back and my problem ended up being a bad crank. Or, actually the crank bearing to be more exact. I went through a couple top ends before I finally figured out what was going on. After the second one I said that I was going to go all the way back through it all the way down to the crank, even though I had just done the crank a short while back.

Mine did the same thing as yours, it would run like a bat out of hell then all of a sudden it would cut off and when you would check it the compression was gone. Pull the plug and the ground strap would be gone and everything would be all beat to hell. Man I tried everything, different plugs, more octane race fuel, reeds, pipe, everything. The last time I pulled it down I almost didn't change the crank because it looked and felt good. There was no indication that the bearing was bad but I pulled it down anyway. Once I got the crank out it still looked ok and didn't feel to have any play but as I was rotating the rod and looking at the bottom cap where the oiling hole is I could see something that didn't look just right. I could see that part of the cage was missing or it looked to be. I broke the crank down and when I got it all apart I could see that the bearing cage had broken in a couple places and there was a needle bearing that was gone. How this thing felt tight I don't know but this explaned the other mysterious looking marks on the top of the piston that could not have been made by the ground strap.

What was happening was the piece of bearing cage would break loose and find it's way to the top of the piston, this would break off the ground strap and all the pieces would tear the heck out of the piston and head, cylinder....I was blaming the spark plug when, yeah, the ground strap did break off but it was really not it's fault because of the extra pieces of junk floating around in the head area.

I'm not saying this is your problem but you have to start from scratch and check and eliminate everything that could be causing a detonation problem. If you haven't changed anything and it all of a sudden started acting up then something has changed, you just need to find out what it is. Pressure test the crank case, check the reeds, clean and check carb to be sure something has not broken or has fallen out. When checking the reeds check the gasket or o-ring good because that is a easy place for a leak to happen.

Link to comment
Share on other sites
Grand Master

BrandonCA

Posted
  • Author
Posted

Ok...here is the first piston..

YZ125Piston015.jpg

YZ125Piston016.jpg

YZ125Piston019.jpg

YZ125Piston023.jpg

You can see clearly the divots from plug...I didn't get pictures of the head...but it looked just like this.

Second piston...

YZ125Piston005.jpg

YZ125Piston009.jpg

YZ125Piston009.jpg

Sorry about the pics..but the lighting was terrible in my garage...

Here's the head the second time around...I can't tell for sure if the small ridge was there before...it's kinda hard to see in these pics...

YZ125Piston012.jpg

YZ125Piston013.jpg

YZ125Piston014.jpg

Oldman...I'm taking it to a local here in the valley on Saturday to get it pressure tested...and I guess we'll go from there..he did ask if the crank had any play in it if you pull up on it..best I can tell it doesn't...but we'll see...

Link to comment
Share on other sites
Community Regular

snoop76

Posted
Posted

YOU ARE RUNNING WAYYYYY HOT OR LEAN!!!!! that is pre ignition NOT anything to do with your spark plug, unless its a br4 or something along those lines!!!! CHECK FOR AIR LEAK!!!!!!!!!!

I didnt read the whole thread but theres my response to the pics!!!!!!

EDIT: Per-ignition or detonation will eat pugs too.

Link to comment
Share on other sites
Grand Master

BrandonCA

Posted
  • Author
Posted
Snoop and olddude I think you both are on the right track. Split the cases and do it right. Generally a bike that old has bad crank seals or bad crank. Don't waste your money on another top end until you fix that first.

That would certainly be in order...but it's got a new bottom end that's less than 2 years old.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

ThumperTalk

Register a new account Sign In Now
 Share
Go to topic listing
×
×
  • Create New...
Terms of Use
Copyright
Privacy Policy