Tapping tappets

Ian Smith felt that the 1993 Suzuki GN250 he’d bought was much nosier from the top end than a 250 and 400 he’d had in the past. Finding both inlet and exhaust tappet clearance at zero, he opened them up – but the noise remained and he wonders what damage may have been caused by running them tight.

The answer is that a tight valve will leak combustion gas and eventually burn out so it won’t be able to seal – even with the correct clearance. But that is more likely to cause bad starting/idling than noise. A long time ago, a friend asked me how to adjust his noisy Triumph tappets and I told him what engine position to check and set them. He called back to say I must be wrong because at that position there was no clearance. ‘In that case, they’re tight not loose,’ I told him and although he sounded doubtful on the phone, he slackened the settings and was surprised to find the noise went away. I think the reason is that cams incorporate ‘quietening ramps’ that begin and end the valve opening gently, taking up any clearance so the valve doesn’t get bashed open too fiercely by the cam lobe.

That would explain why tight tappets are noisy... but, unfortunately, not why Ian’s are still noisy after adjustment. My concern was that the cams or followers were worn and a previous owner had already tried to cure the noise by closing up the clearance. So I suggested Ian checked the oil for metallic content – once the surface hardening goes, the metal wears very quickly. Luckily it wasn’t needed; he took the bike back to the selling dealer who reset the tappets to the settings in the manual and the bike now runs and sounds fine.