'77 gs750. Coils, valves and leaks... oh my!

[erch]

Here's a better pic of that piston:

 

[Nessism]

Now that the head is off I suggest you take your time and go though all the parts. A wire wheel in a drill motor works well to get the carbon out of the combustion chamber. Next, get your hands on a valve spring compressor and take out the valves. Number each valve and bucket so they can go back into their origional hole. The valve stem seals should be replaced, they are cheap parts which will lead to oil burning if they are leaking - which is likely considering their age. To clean the valves further soak them in carb dip. Since you are going this far I also suggest you pull off the cylinder and replace the rings. The pistons can be soaked in carb dip to remove the carbon. A machine shop can hone the cylinder to break the glaze before installing the new rings, you also might want to have them check out the head and cylinder sealing surfaces to make sure they are properly flat - if they are warped you will get leaks.

Sorry if this is more than you bargined for but this is the proper way to fix the engine.

 

[tkent02]

Are you sure about that #3 exhaust valve though? It looks pretty fried to me.

The part that would be burnt is the sealing face where it touches the seat, it can't be seen with the valve in the head, unless it is very burnt. Those look fine, but since you have it off the engine, pull them out to check.

As a test of their sealing, install a spark plug, put some fuel in the head, sit and watch, if it leaks out quickly there is a problem, if it stays there the valves are sealing.

All of this would have been known had you adjusted the valve clearances and done a compression test before pulling it all apart.

I would still like to see a focused picture of the piston tops to see what those deposits are.

 

[erch]

Nah, once I ruled out the carbs I was pretty sure I was going off the deep end.

Took me about two hours but the piston heads are clean now. I'll get the cylinder heads done tomorrow.

You still sure there's nothing wrong with that #3 exhaust valve? My understanding is you can't adjust the valves on these engines so once they start to wear it's just best to replace them.

 

[erch]

All of this would have been known had you adjusted the valve clearances and done a compression test before pulling it all apart.

I would still like to see a focused picture of the piston tops to see what those deposits are.

I didn't think I'd be going this far in when I took the cam cover off, my bad for not running the compression test first =/

As for the deposits, on the pistons themselves it appeared to just be carbon. On #4 it was about 1mm thick and patchy whereas the other cylinders it was more like a film over the piston. It's all gone now thanks to a can of solvent, a brass brush and a couple beers.

 

[tkent02]

Nah, once I ruled out the carbs I was pretty sure I was going off the deep end.

Took me about two hours but the piston heads are clean now. I'll get the cylinder heads done tomorrow.

You still sure there's nothing wrong with that #3 exhaust valve? My understanding is you can't adjust the valves on these engines so once they start to wear it's just best to replace them.

You didn't rule anything out, you just took the head off. You still don't know why it was running poorly.

What did you clean off the pistons? Just some harmless carbon or was it tiny deposits of aluminum blobs? It was hard to tell in those blurry pics.

No, you have to pull the valves out and look, or see if they seal some other way such as a compression check or the gasoline trick. Since the head is already off, pull the valves and take a look.

The valves are easy to adjust, once every 4,000 miles is recommended. If they are not adjusted, they tend to wear tighter, eventually they won't seal very well.
This can lead to a burnt valve, especially if the engine is lean. The valves do not need replacing unless they are burnt due to neglect.

It's very easy to keep these engines running well.

As for the deposits, on the pistons themselves it appeared to just be carbon. On #4 it was about 1mm thick and patchy whereas the other cylinders it was more like a film over the piston.

That is all completely normal, and harmless.

 

[erch]

You didn't rule anything out, you just took the head off. You still don't know why it was running poorly.

Sorry, I meant I'm fairly certain I ruled them out before this little escapade.

What did you clean off the pistons? Just some harmless carbon or was it tiny deposits of aluminum blobs? It was hard to tell in those blurry pics.

Fairly certain it wasn't aluminum, didn't see anything shiny in it, just black soot.

The valves are easy to adjust, once every 4,000 miles is recommended. If they are not adjusted, they tend to wear tighter, eventually they won't seal very well.
This can lead to a burnt valve, especially if the engine is lean. The valves do not need replacing unless they are burnt due to neglect.

How do you adjust them on a gs? I didn't see any tightening nuts under the tappets.