Hi,
I have little experience with 4-stroke/cycle engines but have recently acquired a Ys91ac secondhand. It has less than a gallon thru it but had been laid up for over a year. I ran it as directed in manufacturers sheet and it ran sweetly (with a slight pressure leak from air-chamber).

Problem is the plug had previously been cross-threaded and I did not realise this and made the problem worse, panicing and trying to fix the problem only made it worse.

Anyway, I've ordered a new head (rather than try to mess with a helicoil) and should have it in around 10days - WHAT IS THE PROCEDURE FOR REPLACING THE HEAD ? I've already popped off the head to check for any shards of metal from the stripping plug inside the chamber - this seems quite straight-forward, but I dare not touch the valves or tappets until I get some advice.

How do I remove the valves, and refit them, do they require any action to re-seat in the new head ? Whats the exact procedure for setting the gaps on the rocker arms ? Is there any special advice on refitting the head to the lower casing ? does the piston ring have to be rotated to a certain position ?

I've also ordered a set or replacement gaskets.

Thanks for your help, hope you have the patience to help me !