Top End Rebuild

jdm9123

78 xs650SE
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Hey fellas. So I noticed I had a pretty bad seeping head gasket so I dropped the motor today and tore down the top end to see what was up. I have a lot of carbon build up on the tops of the pistons and the valves. The cylinders themselves don't look bad. Besides the gasket being bad and seeping oil into the combustion chamber what are some other symptoms of this bad build up? The motor ran pretty well but it would spit out black smoke from time to time and the left cylinder was always running lean and popping around 3500 RPM's. Could the valves also not be seating properly or the rings bad? Any thoughts would help me out...here are a few photos to help with your diagnosis.

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I'm no expert, but the sides of the pistons look like they've gotten a lot of combustion past the rings. At minimum while you have it apart you should hone and replace the rings.

That being said, unless you're doing it yourself, the cost of honing 2 cylinders isn't much less than boring/honing 2 cylinders. So for the additional cost of 2 pistons and the bore/hone vs hone, you could have a completely fresh top end. Probably $150 extra vs new rings ($60) and honing ($60 total?). So $120 for new rings and honing vs $270 for new pistons/rings/bore/hone.

Might want to measure the cylinder walls to see if they're in spec or not, but again, to measure them properly you're likely paying a machine shop to run their gauges down the cylinder to ensure it's consistent etc., and the cost of that is probably not much less than just having them bore it out 0.5mm to get a nice fresh cylinder.

Not sure what else you have into the bike and if it's worth spending money on the engine. Of course you'll also want to do a valve job while you're at it and put fresh seals on, check the guides etc.
 
The pistons do have blow-by. This is obvious and that is most likely just due to the piston rings being worn out. Very seldom the cross hatch could be done improperly allowing the rings not to seat in properly. The carbon build up is most likely just the oil consumption over the past few thousand miles since the engine was eating oil. It also could indicate a rich condition but I am most likely ruling that out. I just didn't know if anyone else had any other ideas on what else could cause this? And also the pale color on the right side intake and exhaust valves. I am unfamiliar with that condition.
 
Honing doesn't cost that much, certainly not $60. It only takes seconds to do. I do my own now but when I had one done years ago, it was only like $5. I'd think it might be about $20 now.

The tan color of the valves on the right cylinder is what I would consider normal, certainly better than that black and oily left one.
 
Yes, looks like head gasket passing oil.

Here's a couple of pics of my engine from a few years back. Not a pretty site! Mine was using large amounts of oil and puffing black smoke. I did a top end overhaul and engine now runs like a new engine.

Don't guess at what components need replacing. Read the Yamaha Service manual found at places such as www.biker.net. If you don't have the tools, pay an engine machine shop to measure the pistons/cylinders/valve guides/valves etc. The measurements will dictate what needs machining/replacement.
 

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