Unusual carbon build-up?

[Jimmbuzz]

I picked up a parts gs750t for the motor, and I was going to do a top-end refresh as it has 52k miles on it. Took the heads off and this is what I see:

It looks to have been rebuilt at some point, as part of the pistons are very clean and the rings look relatively new... What would cause the carbon to build up like this? The spots are pretty thick and flaky and come off with ease. Any ideas? Rods appear to be ok, so I was going to clean it up and go to a machine shop to get the pistons, new rings and cylinders measured and honed.

[fastbysuzuki]

With that amount of carbon it looks like the motor has been burning more oil than petrol

[Jimmbuzz]

Most likely. Leaky valve seals?

[Nessism]

By the look of the crankcase it appears the engine was not well taken care of. I wouldn't worry per say, but it goes without saying that the bores and pistons should be measured to judge wear, and the head should be rebuilt.

[GregT]

Two points come to mind looking at the pic.Firstly, yes, probably bad valve guide seals.Secondly, the squish areas on the pistons front and back having so little carbon on suggests the block has been decked at some point.I'd have suggested that it may have been built with no base gasket - but it's visible.When you do reassemble it, check the head to piston clearance. If it's less than .040in you may need two base gaskets.

[Jimmbuzz]

I'll be sure to check. I have another block and set of pistons so whichever is more in range I'll probably end up using.

[bwringer]

Some carbon buildup is perfectly normal, expected, healthy, and righteous. These air-cooled engines do get more carbon buildup than you may be used to seeing on modern engines, especially water-cooled stuff.

That said, it is somewhat self-limiting, and softer, thicker deposits can indicate a problem. Hard to tell in a photo what's happening, but I've certainly seen far worse, cleaned it up, and found that the parts were fine. Oil from old hardened valve stem seals can create a heck of a mess, and of course that will be solved in the normal course of re-installing the top end when you replace the seals.

Measure the metal and see whatchya got.