How bad is this?

[pjmcburney]

My L hasn't been running too well lately...


After I changed the old, cruddy (but working well) carbs over to the new refurbished ones - although it could be coincidence, I noticed the bike had developed a noticeable rattle whilst running, mostly when the motor was loaded and mostly at lower revs.

I went through a few things in my head and tried changing out a few parts for others I have in my collection e.g. starter clutch, I couldn't think of a more plausible area to look than the pistons and rings themselves.

Now, this is my first and only GT, so I'm not too sure what noises they can develop over time, but this rattly noise is pretty loud, and didn't manifest itself when I thought it should for piston wear rattle...

I pulled the head today and this is what I found:









Is this detonation damage or water damage or what? I used a late model (thinner) head-gasket and I run the bike mostly on regular unleaded.

This head had nice smooth, bead-blasted combustion chambers when I fitted it about 15K klms ago.
Is this something I should be worried about?

Here's the tops of the pistons as a comparison:







I can move each piston in it's respective bore a bit and I can get each one to 'slap' inside the bore if I give it a tap with my fingers... is this a drama?


Thanks
Paul

[tz375]

That's corrosion not detonation. Don't worry about it. It is odd though and makes me wonder what happened when the head was blasted clean. Did it just hide the faults or are you feeding it a diet of Bundy?

That center head is interesting. Are those really radial ridges or is it an optical illusion?

Pistons look fine but it does sound like pre-ignition. Try a couple of degrees less timing and see if that helps. Or you could try raising the needle a notch and see if teh richer mixture cools it enough to stop the problem.

[pjmcburney]

Hey Richard,


Thanks for the reply. What the heck has caused the corrosion? (and no, it wasn't Bundy - I prefer actually drinking that )

Looks pretty weird huh? They are radial ridges on the centre combustion chamber, I can't for the life of me imagine how it got like that.

I forgot to mention that the picture was taken after I cleaned the head up - each chamber had a consistent carbon coating on it prior to stripping.

Question now is, can I reuse it?
I have another head in better nick I can use, I was going to polish the chambers before fitting it - does that help?


Cheers
Paul

[chainsaw]

I am not an expert by any means of measurement but could this be from some kind of gasoline additive reacting with the metals in the head? Not that YOU are adding anything in your gas, I mean from the modern day formulations. Who knows what the heck we are burning anymore.

[pjmcburney]

Well, I swapped heads and the rattle is gone... phew.


The head I've swapped to has much deeper (larger) combustion chambers and was the head I got with the L originally.
The head I used when I rebuilt the engine was either a later head from an M, or had been decked/shaved.

It must have been pre-ignition causing the rattle because of a higher than usual compression ratio, especially when the chambers and pistons were coated with carbon.
Good thing I caught it really - I'd rather replace heads and head gaskets than pistons.

In fact now that the engine is burning all of it's fuel instead of exploding it, the old girl pulls like a train.


Thanks for the feedback guys
Cheers
Paul

[tz375]

Paul,

I was too busy looking at the pit marks to notice that the head was "missing" the ridges around each chamber. On a stock head the chamber is recessed about 1.4mm. On that head there is no recess. That means the head was shaved by at least 1.4mm.

On 93 octane or better it should have been OK but on regular OZ gas it wouldn't work.

BTW, US gas is (RON+MON)/2 but Australian is rated on the higher of the two. So a US gas of 93 might be say (95+91)/2 where the Aussie gas would be rated as 95 Octane.

So 100 Octane Shell is actually more like 98 on the US scale, and 91AUS is closer to 89US.