Whilst adjusting the valve rocker clearances is outside the normal annual service schedule, only needing to be done once every two years, I’m doing all these jobs in order to completely baseline the service schedule.
Adjusting the clearances is a relatively simple job but prior knowledge of a few things makes it much easier.
The rocker covers contain oil so it’s best to do this whilst you’re changing the oil anyway. It’s not much oil but it will drain onto the chassis member and then spread out making it hard to catch in a pan on the floor. After finding this out the hard way whilst doing the first one, I cut the bottom off an empty milk bottle and put that on the chassis to catch the oil
Use a 12mm spanner to undo the retaining nut and remove the jelly-moulds allowing the oil to drain.
With the jelly-mould off you can access the valve rockers easily. Rotate the engine using the starting handle until one valve is open (fully depressed). Then check the corresponding valve on the opposite side of the engine. There should be a slight amount of play in the rocker which will allow you to insert a 0.25mm feeler gauge into the gap between the rocker and the top of the valve stem. (Note that 0.25mm is larger than most manuals recommend but this is to cope with newer bio-ethanol additives in fuel.)
Slacken off the 10mm lock-nut and use a flat head screwdriver on the adjuster until the feeler gauge will only just move in the gap.
Keeping the adjuster screw in place with the screwdriver, tighten the lock nut using the spanner ensuring the feeler gauge still drags slightly. This job ideally requires you to possess a third hand.
Repeat this process for all four valves and then go round and re-check to ensure they are all still correct. If it needs replacing, use a new rubber seal on the jelly-moulds and re-fit.
If you didn’t do this during an oil change you’ll now need to top-up the oil to replace the small amount lost when the jelly-moulds were opened.