Detonation!

[UK Rich]

Guys, I've been throwing an old 200t together over the weekend. It's made up from spares. Something old, something new, something borrowed and er, something melted...

What has caused this? It's always had fairly good fuel, and been tuned to suit. It must have been detting its t1ts off, no? Could have heard this...

Interestingly, it was still pulling 150psi with the base gasket (squish .042) and without 192psi (squish .024). So the rings are seemingly ok?

Shall I order a new piston? I've nailed it back together like this for now, it seems strong and I need something to get me up ad running.

Thanks in advance, Rich.

 

[GeeVee]

opcorn:

 

[Fish]

Buff the carbon off first. See what is under that.

 

[Fish]

Removing all of that will affect your compression ratio though...

 

[7sleeper]

Looks like someone used it for mixing cement. But who cares as long as it runs!

7

 

[zogger]

Running just a little bitty tad rich there methinks..

Glad to see you can get another climbing saw together though.

If I was over there I would help ya go pay a visit to your local skunks....

 

[UK Rich]

Looks like someone used it for mixing cement. But who cares as long as it runs!

7

Quite. It does seem to run and seems fairly well. I think a mouse has been living on the crown and been taking a chomp everytime it gets the midnight munchies.

There's not really too much carbon on the top, looks worse than it is. I cleaned it a bit, but then got bored...

 

[Andyshine77]

I will say yes pre ignition and heat erosion. Quite a few things can cause pre ignition, from carbon buildup/dirt ingestion, bad fuel, bad tuning, faulty ignition. Yes a new piston set is in order.

 

[Fish]

New piston, new sprocket, remove the carbon and inspect the cylinder, new air filter, probably a lot more. Clean it all up first. Bearings noisy??

 

[UK Rich]

Running just a little bitty tad rich there methinks..

Glad to see you can get another climbing saw together though.

If I was over there I would help ya go pay a visit to your local skunks....

Last time I ran it, (maybe 18months ago) I think I leaned it off a tad. Wouldn't it running rich cause more bore wash than det? It didn't sound lean and the plug doesn't seem to have got hot etc.

The cylinder looks fairly well, no scoring. Combustion chamber is a little pitted, nothing to write home about.

I'll change this photo of my Mrs, to one of the jug..!

 

[fearofpavement]

Detonation is the "explosion" of the fuel air mixture instead of the steady flame front it is supposed to have. It can be damaging to an engine. More so to an air cooled engine. This is the "pinging" or "spark knock" you hear on a car engine. On a chainsaw, I don't think it would be detectable.

Pre-ignition is caused by "something" causing the fuel/air mixture to ignite prior to the "spark".

This usually leads to detonation and the combination of the two can do very severe damage to an engine, especially air-cooled ones. eg, holes through pistons, etc.

The two most common causes of detonation are too low of octane fuel or spark timing incorrect.

The most common cause of pre-ignition is something "glowing" in the combustion chamber. This could be a piece of carbon, part of a spark plug, etc. (on valved engines, the culprit is often a feathered exhaust valve edge.)

I personally wouldn't recommend running that piston in the condition it's in. The price of a piston isn't worth the risk of further damage to an expensive engine.

 

[UK Rich]

Detonation is the "explosion" of the fuel air mixture instead of the steady flame front it is supposed to have. It can be damaging to an engine. More so to an air cooled engine. This is the "pinging" or "spark knock" you hear on a car engine. On a chainsaw, I don't think it would be detectable.

Pre-ignition is caused by "something" causing the fuel/air mixture to ignite prior to the "spark".

This usually leads to detonation and the combination of the two can do very severe damage to an engine, especially air-cooled ones. eg, holes through pistons, etc.

The two most common causes of detonation are too low of octane fuel or spark timing incorrect.

The most common cause of pre-ignition is something "glowing" in the combustion chamber. This could be a piece of carbon, part of a spark plug, etc. (on valved engines, the culprit is often a feathered exhaust valve edge.)

I personally wouldn't recommend running that piston in the condition it's in. The price of a piston isn't worth the risk of further damage to an expensive engine.

Thanks. I'm guessing this didn't happen in the last half a tank, so it must have been an on going thing? It has probably been running like that for quite a while, so surely it'll last a little while longer. At least till the new piston turns up.

I dont have a great deal of choice at the mo.

 

[Fish]

Running an engine with a lot of carbon will cause this. The lot of carbon thing needs to be addressed, as that was caused by an overly rich
condition, either clogged air filter, leaky welch plug or needle, etc.

The buildup of carbon raises the compression ratio, which causes this problem as well as the others mentioned. That is why newer, higher
compression ratio engines need a high octane fuel. So getting rid of the carbon will help things.

If you are thinking of just slapping a new piston in that old cylinder with out decarboning it, then you need to re-read this stuff....

 

[UK Rich]

If you are thinking of just slapping a new piston in that old cylinder with out decarboning it, then you need to re-read this stuff....

Nope, not going to just chuck a new piston in. Wanted to find out why it's done this? I think it's been running too rich for too long, probably before I even ran it, (I leaned it off when I first used it). Carbon has built up on the crown, got hot and pre-ignited. This sound about right?

Cleaning the saw up, new filters all round, de-carbon, new piston and fresh fuel. Should have it sorted?

The A/Vs are gone and it needs a new sproket, but it's gotta be the best part of 10years old. Bearings feel fine.

 

[LegDeLimber]

curious what a peek in the exhaust port and a look into the inlet side of the muffler shows.

 

[UK Rich]

curious what a peek in the exhaust port and a look into the inlet side of the muffler shows.

Just dirty and oily.

Still, we shall see what happens. It's going into service tomorrow.

 

[Joe Kidd]

new sprocket

That one is down right ugly.

 

[polkat]

i only saw this jusg recently but my dealer had a piston just like that pic;
and they told me that was from water, the water wouldnt pass threw and kept on hammering that piston. the question is how long that saw sat with fuel in it and what type of gas did you buy??

 

[CTYank]

Heat also plays into detonation. From what little I can see of the cyl fins, they look pretty dirty. Not good.

 

[CTYank]

i only saw this jusg recently but my dealer had a piston just like that pic;
and they told me that was from water, the water wouldnt pass threw and kept on hammering that piston. the question is how long that saw sat with fuel in it and what type of gas did you buy??

With the engine running, that supposedly happened? Don't waste your time listening to people like that. Idiots.