Piston examination Detonation?

[Zmod]

Hello! I ran into quite a strange trouble today. My piston did loose a bit at its top and i wonder what could be the cause? At the sides its very clean no signs of scuffing against the wall (i.e. lean mixture).

The piston is micoloured on the underside but i think thats normal? Like its brown. But no ashes just quite brown.

I run ethanol if that does any difference with a rich mixture. But still can get clean on full throttle after a while so i hardly doubt that its a too lean mixture.

I do not use any octane boster or anything like that i run on std. compression for a teamspeed 2.

BTW My spark plug was full of aluminium pieces and the cylidner head too. But it was very easy to pick away! I just wiped it off with some paper!

 

[Jasper V]

It looks like a very lean mixture.

 

[Zmod]

It looks like a very lean mixture.

Do you think so? Cause it 4 strokes at full throttle and the plug reading have always shown dark black color. I wonder if it could be when i Put full throttle on long straights that it runs out of fuel doesnt get enogh to the carb.

 

[Jaspah]

If it runs out of fuel on a long straight, than that is obviously the problem.
Maybe your petrol-filter isn't clean anymore.. something like that.

But don't let it run out of fuel anymore, that kills your piston

 

[tiba_karotsu]

looks like pre-ignition or detonation to me, even though ethanol has a high octane rating you may have to use a fuel with more octane to prevent that from happening again.

i have never ever experienced the problems you had, i'm very confident of the fuel i use.

 

[StepVino]

It's interesting that the center of the problem seems
to be the ring locating pin. Could the problem have
something to do with the ring gap?

 

[adas]

Do you think so? Cause it 4 strokes at full throttle and the plug reading have always shown dark black color. I wonder if it could be when i Put full throttle on long straights that it runs out of fuel doesnt get enogh to the carb.

I have seen a bike run lean and four stroke at the same time. Example is that it will four stroke below your pipe power range and you think it is running rich. Then when you come on the pipe it leans out and heat builds up and you burn your piston like you did. Did you modify your pipe at all, such as the stinger longer or smaller diameter? Better to have a cheap EGT and avoid this. frank

 

[Zmod]

ANOTHER THING! Worth mentioning.
Around the ring gap where there was a strange thing. The side of the piston is gone you can se too much of the piston ring. It's like it burned away the edge of the squishband!

Jaspah: I think that could be the problem but i'm not sure. Cause i use a Standard fuel tank from a Yamaha Jog R with a Vaccum. And i got like a 60cm of fuel hose before it gets to the carb. And a very Weak vaccum signal. I'll be taking away the vaccum to see whats happening.

Stepvino: I also thought about that! But can't see what the problem would be. Cause im very careful and the ring gap was just by the book. 0,15 mm. And one thing that i saw was that the ring melted to its place the ring sat just where it should be and there was a nice ring gap. Probably 0,15mm.

Tiba: Yea it surely does. I might blend some acetone in the future too help the ethanol even more.

adas: Yeah could be that too. When it stops to 4strouke it might run łean cause the enginge rpm has reached the right rpm. I'll make the carburattion fatter for the new piston. The pipe is a totally standard Yasuni C21!

 

[tiba_karotsu]

remember detonation also makes the engine run hotter hence the melted aluminum on the center of the crown & just above the locating pin of the piston ring.

i've had pistons that looked very much like yours, if high octane gasoline isn't available you can just increase the squish clearance to lower compression ratio to prevent detonation.

 

[MONZAV8]

did you re-jet when you went to ethanol? While it has a much higher octane, it has a 15% lower BTU content, so you have to jet up, otherwise you are running lean. I tried ethanol for a while with good results, but I had to jet up about 20% to keep it from running lean. I went back to gas since my MPG went in the toilet with the ethanol (was already bad enough at 35mpg, 14.9km/l).

Also, I dont know if you noticed, but most of the 2t lubes dont like to mix well with the alcohol. I ended up using a castor oil based lube that actually mixed at a alc/gas mix higher than 50%...but that shit was expensive! (racing Kart stuff)

Looks like the locating pin was the source of the detonation BTW. Must have been nice and hot

 

[StepVino]

ANOTHER THING! Worth mentioning.
Around the ring gap where there was a strange thing. The side of the piston is gone you can se too much of the piston ring. It's like it burned away the edge of the squishband!

Jaspah: I think that could be the problem but i'm not sure. Cause i use a Standard fuel tank from a Yamaha Jog R with a Vaccum. And i got like a 60cm of fuel hose before it gets to the carb. And a very Weak vaccum signal. I'll be taking away the vaccum to see whats happening.

Stepvino: I also thought about that! But can't see what the problem would be. Cause im very careful and the ring gap was just by the book. 0,15 mm. And one thing that i saw was that the ring melted to its place the ring sat just where it should be and there was a nice ring gap. Probably 0,15mm.

Tiba: Yea it surely does. I might blend some acetone in the future too help the ethanol even more.

adas: Yeah could be that too. When it stops to 4strouke it might run łean cause the enginge rpm has reached the right rpm. I'll make the carburattion fatter for the new piston. The pipe is a totally standard Yasuni C21!

I've been doing some Google research on this. Can't seem to come
up with anything but vague references to the effects of oil reducing
octane rating. Can't help speculating that maybe you have a "localized"
reduction of the fuel octane, right at the ring gap. Possibly because
that's where some oil collects... mixes with the fuel just right to
lower the octane enough for detonation at that location.

If you are using a generous oil mixture already, might be worth a try
lowering the ratio a hair, see if that helps, or maybe trying a
different oil?

Another solution might be (excuse the complete speculation). to
raise the squish band a few 10th of mm, just over the ring gap.
Maybe in the form of a V, with the open end towards center of
combustion chamber.

 

[ruud.]

i dont think its the main problem, but did you also mount the proper sparkplug?

 

[Zmod]

ruud: Champion N2C (been using the sparkplug for some time now). Equals about a NGK B9ES.

stepvino: I run a 4.2% procent mixture of castrol r40.

monzav8: I drained the whole system in alcohol before going to ethanol. I also drill my own jets now. Cause the jets i buy are just too small. The oil i don't think is a problem! It mixes well with the ethanol.

Thanks for all answers anyway !

 

[Jasper V]

Did you check your gaskets/sealings? Maybe there is a leak where the engine sucks air into the engine.

 

[Zmod]

Did you check your gaskets/sealings? Maybe there is a leak where the engine sucks air into the engine.

All gaskets looks fine at the cylinder. But i haven't checked the intake sealings.

But could be a problem with that. (but hardly think its from the intake side)

Cause where the cylinder connects with the block oil is sippering ut via the piston bolt and then out in the water system. And if oil/ethanol can get out ofcourse theres a possibilty that a small amount of water can get in the cylinder and so on. But i hardly doubt that! Anyway now i'm using sealing on tube above and under the bottom gasket. So i don't think i'll have that problem anymore. Soon im ready to fire it up again for testing!

 

[Zmod]

I solved the problem! It was one of the oil seal who had given up. Strangely though after the piston change. I was unable to go past 100 km/h it would just moooee. Like it wanted more fuel.

Now i changed both oil seal. And now its back up running. So it was probably the oil seals fault.

//Daniel