86 honda big red, broken intake valve

[nick c]

Hi this my first post. A friend of mine for 54 years that passed away one year ago bought this 3W new in 86. In 2009 it stopped running and he couldn't determine what was wrong. He called me and I took it to my shop and I removed the spark plug and shined a flash lite down the hole and I could see the valve mushroom laying on top of piston. I pulled the head and bought a new intake valve, 2 outer springs, long chain guide, seals and all the gaskets. I timed it according to the book, it ran fine. I was given the 3W a few months ago and the same thing happened, but this time it damaged the piston, exhaust valve and the 2 seats. Could someone explain why?
thanks nick c

[350for350]

Is the valve guide possibly extremely worn?

[TheRealFatShady]

Your timing chain may be stretched, or tensioner not functioning properly. It shouldn't jump time and the fact that it's happened more than once indicates something along those lines.

[Rob Canadian]

Once the initial problem was fixed. How long did the engine run before the second thing happened?

Do you have any pics?

Give as much info as possible. If you know.

[nick c]

Thanks for the replies.
the 2 valve guides are snug. it was 7 1/2 yrs between break downs.
The tensioner was all the way out when I removed it but I think they are made to run out with no pressure on them. the timing chain is probably 30 yrs old.
Could some one tell me where to but the 2 valve seats?
thanks nick c