replace exhaust valve after holed piston?

[DimitriT]

It looks like material blew into the cylinder when the piston holed and some got sucked into the exhaust. To make matters worse, some also got caught in the exhaust valve seat and appears to have roughed up the valve. I cleaned the valve off with a wire wheel but the exhaust valve has some slight impressions along the seating surface. The surface looks wavy when I turn it under some light. The seat looks good. The intake valve has a few shiny spots where it looks like it pressed some aluminum particles, but the surface looks good.

Would some valve lapping be enough to clean up the exhaust valve or do I need a new valve?

Also, anyone know of a machine shop in our area that can weld and repair a spark plug hole.

[Dave8338]

I would, especially on the exhaust side, they have a tendency to "burn off" if left alone. But that also depends on how bad it is. If it is towards the stem and not around the outter edge, you may get away without having to change it. You'll have to be the judge unless you want to post a pic or two. :?

[SqDancerLynn1]

I have some used valves 80-81 GS750 if you need one should be same as 82. 83 is the redisigned

[DimitriT]

Thanks Lynn,

this is for the 550. I found a pair of NOS valves on ebay for not big money.
New valves for the blown piston and new rings all around should bring some life back to the old girl.

I also just pulled the oil pan. It was full of aluminum "sand" and a bunch of sludge.

I still can't figure out why the piston holed. I was running WOT when it holed. I have stock main jets. Maybe I need to use higher octane gas (I pump regular) or retard the timing a little bit.

[DimitriT]

I would, especially on the exhaust side, they have a tendency to "burn off" if left alone. But that also depends on how bad it is. If it is towards the stem and not around the outter edge, you may get away without having to change it. You'll have to be the judge unless you want to post a pic or two. :?

I'd post a pic but its hard to really see it. It's not pitted or discolored, just full of tiny indentations.

[salty_monk]

Is your airbox leaking & do you have a filter in it??

[DimitriT]

Is your airbox leaking & do you have a filter in it??

Yep, that's the next thing to check. I have a filter in the airbox and no leaks as far as I know but I plan on pulling the intake rubbers to check how well they're sealing.

[Nessism]

Hole in piston usually means detonation.

[DimitriT]

Hole in piston usually means detonation.

Yea, so why is it detonating?

[Guest]

Your ignition is retarted. ;-)

[Nessism]

Yea, so why is it detonating?

A few ideas are: octane rating of the gas is too low, the timing is too far advanced, the engine is being lugged while hot, and the mixture is too lean. Detonation has a characteristic sound - sounds like rocks in the engine. So the question, was your bike making noise like this? Regarding running lean, if you have modified the exhaust/intake system without rejetting the bike is most likely too lean.