Unexpected valve/sparkplug clearance issues.

[lancejack]

Hello everyone, has anyone came across this problem?~I have a completely rebuilt engine 57 G9 with G11 top end and electronic ign.., took it out for a ride (a bit more vibes than expected but running ok no smoke etc;) and after 15 miles the engine cut out, put some petrol in tank oops and started first kick, got another ten miles then engine started to vibrate very badly and cut out but it did restart/run ok after a few mins..and got me home. So after a top end strip found the offside piston had seized (original Hepolites fitted) nearside perfect. I bought new ones from club gapped and fitted them after a rebore/hone I left more expansion gap than usual for safety. Went for another test run and the same thing happened, stripdown showed offside piston again seized. All settings~ timing carb etc; spot on so no clues,, until I looked at the offside plug and found the heel of electrode had been brightly chamfered almost halfway through as though by a file! The inlet valve head had been hitting the plug electrode. If the plug had tightened up in a slightly different position it would have cleared the valve (N5''s fitted and protruding correctly in heads) the nearside was ok because it had tightened electrode away from valve. So as it happened twice with different pistons same side I can only assume this either caused the valve to stick and/or cause pre-ignition The heads used have the later but standard larger inlets...I've never known this to happen during many years of riding So shall see what happens after another rebuild using smaller valves earlier heads. Definitely something to watch for methinks...

[Duncan]

I have experienced the valve hitting the electrode in a 650, in my case it closed the electrode so no sparks, could your inlet valve be slightly bent because of the impact and not sealing effectively causing the buke to run lean?

[lancejack]

Hello Duncan. I checked and reground all valves during the rebuild,all valves straight. there was play in guides but not enough to renew them. BUT during/after the seizures I found the play had increased allowing the contact..even with perfect valve /guides the clearence between open valve head and plug is only 4 to 6 thou.. (the plug hole threads are actually tapped tight against the seat insert) I think you may be right about causing lean hot running so when I rebuild it I will look at adding extra washer to spark plugs or using shorter reach plugs, not ideal but safer perhaps