Detonation

[Deleted member 12676]

Fury, base gasket leaks allow a lean mixture which contributes to detonation.

Fabian, a picture sometimes is not a thousand words but trust me, this was detonation. He said the edges of the piston were eroded away although we see that eroded aluminum resting on top of the piston which means it can be mistaken for something else such as chewed up bearings. But look closely at the picture to see that the residue is much finer than chewed up bearings.

 

[FurryOnTheInside]

I got that, thanks Jaguar. But I was thrown at first by the oily look and the rest of the thread was a bit much to take in all at once, for me.
Was the lean condition confirmed? Was detonation confirmed? Was (another means of) pre-ignition disproved?
I'm only asking because I'm keen to learn and I have no idea how I would tell the difference if I opened and engine to inspect and was confronted by this. At the moment I would just have to guess.

 

[keatonx]

the AX100 uses oil injection. I just tried to lower the amount of oil it uses but it comes stock set on "minimum".
Keep in mind too that I do a fair amount of putting around at minimum throttle in town which induces a rich condition in 2 strokes. It was worse before I increased the slide cutaway.
HS, have you ever heard of the do it yourself system that corrects for that? Connecting a tube from the intake between carb and reed valve, and running it to the float? Put a aquarium valve adjuster on the breather tube of the float. Put the same type adjuster on the first tube? I did it as a gas saving device on my Honda car and it worked really good. But in this case it is to eliminate the rich low speed cruising mixture that causes that oh-so-irritating rat-a-tat-tat sound of eratic combustion.
I am going to put it on my to-do list.

Wait, to the float or the float bowl?

wouldn't that cause it to just suck gas from the float bowl making it even richer?

 

[HeadSmess]

correct me if im wrong, but i believe the idea is that...at full throttle, jet correctly, use the main. at idle/very low speed...rather than use the emulsion tube and jet, allow some air to bypass through the bowl... it picks up vapor, not liquid that has to then be vaporised.

bit of tweaking between the idle speed and the bypassed air to get it in, leave the idle circuit on the lean side. iunno. ive heard of bubbling air through somehow, i forget the details...


but that is a perfect picture of detonation. a perfect ring of peppering in the squish, while conrod bits would show most damage near the ports, and by the time enough material got up there to do such damage, the engine would have locked solid. also note theres no skirt damage, and the very small "pores" over the entire piston. near the thumb, the crown has eroded away enough to start exposing the ring.


makes me recall that once upon a time i did this to a nitro RC car...shame i threw that engine out now

 

[keatonx]

correct me if im wrong, but i believe the idea is that...at full throttle, jet correctly, use the main. at idle/very low speed...rather than use the emulsion tube and jet, allow some air to bypass through the bowl... it picks up vapor, not liquid that has to then be vaporised.

bit of tweaking between the idle speed and the bypassed air to get it in, leave the idle circuit on the lean side. iunno. ive heard of bubbling air through somehow, i forget the details...

I read that's the elusive "100mpg carburetor"; just bubbling the air through the gas. I guess they change the mixture (richen it) by putting resistance before the "carb" so that there's more vacuum and the gas evaporates faster. Seen lots of vids of mowers running like this

 

[Deleted member 12676]

Fury, if your piston ever looks like this then look for an air leak, retard the ignition a bit (if your CDI is adjustable), make sure the jetting isn't too lean (richen the mixture and see how it runs), and check that your cranking pressure isn't more than 140psi.

 

[FurryOnTheInside]

Fury, if your piston ever looks like this then look for an air leak, retard the ignition a bit (if your CDI is adjustable), make sure the jetting isn't too lean (richen the mixture and see how it runs), and check that your cranking pressure isn't more than 140psi.

Thanks jaguar, will do. Oh it will be adjustable and properly retarded, not going to build without one of your CDIs.
You've given me another question to ask now.. Can the cranking pressure go up? I'll build with it set to a reasonable pressure but I wasn't aware anything could then raise it except perhaps for loss of volume through carbon build up.

 

[Deleted member 12676]

raising pressure:
1) lathe off metal from the head mating surface
2) planing cylinder and head mating surfaces and then using copper coat instead of a gasket
3) use the Honda piston I recommend on my site which seals better due to the ring ends being closer

lowering pressure:
1) using your rotary tool to raise the exhaust port (for higher peak RPM)

 

[FurryOnTheInside]

raising pressure:
1) lathe off metal from the head mating surface
2) planing cylinder and head mating surfaces and then using copper coat instead of a gasket
3) use the Honda piston I recommend on my site which seals better due to the ring ends being closer

lowering pressure:
1) using your rotary tool to raise the exhaust port (for higher peak RPM)

I haven't been able to find out from others the CR of the engines sold here, but I can see the logic in your approach to altering pressure. When I actually get my hands on the engine I'll check compression and think about which route to take.
I think when I get around to playing with replacement pistons (WELL into the future) I'd be going with the Hoca Minarelli and all the expensive non-DIY mods that go with that.

Actually my question was about UNintentional increases to the pressure... i.e. during use. I was just unsure how to read what you said about checking the pressure isn't over 140psi.. but I guess from your response you were just referring to during the build.
I know I will need to check the pressure during the build, and then later at times if I think there may be a head gasket leak (if it isn't obvious from oil stains on the cylinder/head).

 

[Fabian]

It appears that i was wrong in my assessment of debris from connecting rod bearing failure getting trapped in the squish band area. Consensus says that it is detonation related damage.