Help please, burning piston after piston?

[RobbyB]

A small piece missing from the top of the piston, or skirt? When the fuel comes in, it goes to the cases and flushes up top through the transfers. Did it leave a mark in the head?

The 'Chip' was on the intake side of the top of the piston. There were about 6-7 marks on the head, you could barely feel them. Again, I figure the material that did this (if any) was soft due to there being no damage to the port. I'm also wondering if the piston simply caught the port??

 

[whenfoxforks-ruled]

What I am getting at, is maybe the piece missing atop your piston was from detonation. The marks in the head are another matter. If the ports are not chamfered correctly, a ring can snag, not the top of the piston. The top of the piston is a smaller diameter than your bore. The skirt is tight and rides the bore though.

 

[Uchytil]

I've had a few (and still do) of the 97-Present YZ125s and they are pretty stout littile tiddlers. The engine is soooo simple too. If you question the build why don't you check out Eric Gorr for an overhaul? Heck, if I was near you I'd taer that sucker apart.

 

[NO HAND]

You might want to check if the head was 'shaved'. It's fashion for kids to find a local machinist to shave the head and ruin a perfectly fine engine. Been there, done that... :( . Often it doesn't show until you break out the caliper.

 

[RobbyB]

What I am getting at, is maybe the piece missing atop your piston was from detonation. The marks in the head are another matter. If the ports are not chamfered correctly, a ring can snag, not the top of the piston. The top of the piston is a smaller diameter than your bore. The skirt is tight and rides the bore though.

Thanks. Your probably right, the chance of the top of the piston catching is almost impossible, especially with a new jug and piston. The mark on the top edge of the piston looks as though it came up and caught something that was pulled in through the intake port.

 

[RobbyB]

You might want to check if the head was 'shaved'. It's fashion for kids to find a local machinist to shave the head and ruin a perfectly fine engine. Been there, done that... :( . Often it doesn't show until you break out the caliper.

Thanks, I will check this. A compression read might indicate a smallar combustion chamber. This would probable make it run hotter. Also, I was told by another fellow to check the timing as well, he said if it is advanced it will run warmer, which it is doing.

 

[whenfoxforks-ruled]

Rereading, detonation would take out the exhaust side, not the intake. The head work, if it appears to have an about 1/8th" lip, where it tapers into the dome, thats the squish band. I have seen fools with tools totally remove it. It may run, if the piston does not hit the head. The squish clearance is crucial for proper tuning. The 1/8th" is estimated. The clearance is in Eric's books, and should be in the shop manual.

 

[RobbyB]

Rereading, detonation would take out the exhaust side, not the intake. The head work, if it appears to have an about 1/8th" lip, where it tapers into the dome, thats the squish band. I have seen fools with tools totally remove it. It may run, if the piston does not hit the head. The squish clearance is crucial for proper tuning. The 1/8th" is estimated. The clearance is in Eric's books, and should be in the shop manual.

On the head I have here the squish band is approx 22thou. I was told that it needs to be at least 50thou or there will be problems running pump fuel. I noticed that when kicking over other 125s by hand that this bike seems to have more compression. Could it be that the head has been planed? What exactly should the factory squish band measure? Could this be what is making it run warmer. I never did check the compression, which I will do after I put it back together.

 

[whenfoxforks-ruled]

If its 22 and supposed to be 50, I would send the head to get fixed or replaced. I sure hope no more garage work was done to the cylinder and cases!

 

[Uchytil]

Wow, you still playing with that tiddler...sometimes Eric has extra parts on hand, ie. heads/jugs. You could send your stuff to him and have it done right.

 

[RobbyB]

I've been up and running for a couple of weeks now with no problems. Here were the problems.
- Timing was a slight bit advanced. I turned this back to stock.
- Main jet was stock so I went up two sizes for the temp and for sand riding. This works well. I went from 420 to 440. I adjusted the needle one notch richer as well.
- Head was planed at some point, I corrected this somewhat with a thicker base gasket.
- I didn't do anything to find the source for the damage to the piston due to the foreign object. I rinsed the bottom end out several times into white cloth over a pail. There was abolutely nothing that came out. So I didn't bother with pulling the case apart.
- The piston was salvagable so I re-used it. However, I don't like the Namura piston, to loose a fit. Next one will be the Weisco or Yamaha, they seem to have a much better fit.
Thanks for all your assistance with these problems. We're going to upgrade to the yz250 next season. My son is to heavy for the 125, especially in the sand. The yz125 is an impressive performance machine I must say.

 

[whenfoxforks-ruled]

Good to hear! And ditto on the funky piston. As bad as buying the cheaper Chinese engine bearings. You get what you pay for. Generally when something is recommended around here, its pretty much gospel for good reasons. Why go through all the problems again for a few dollars sake, next time it could damage more, and the you could get hurt thing. Some micky mouse stuff gets by in other applications, or do they? Vintage Bob