Marsden wrote: ↑Tue May 30, 2023 10:38 pm
Hi thanks for the quick reply.
Are the jaws represented by the top end of the spring and the base?
Welcome to the club, Marsden.
Yes, measure the spring between the cut ends at the base and the loop at the top.
The bike was running corectly but has little compression ( at the end of a run the bike stalled and has since been difficult to start) and I have the impression of air being forced OUT of the inlet valve, despite having lapped the valves and decoking.
Are the pushrods free to turn at TDC on the compression stroke?
After lapping the valves did you invert the head and fill with petrol to check for leakage?
Did you anneal the (new or old) cyl head gasket before fitting it?
When the cyl head gets hot the valve seat insert can loosen in the head and turn, spoiling the seal. When it cools it will become secure as if there hadn't been a problem.
A rough check for compression loss......
Operate kickstart and note the compression.
Put half an oeufcup of engine oil in the bougie hole.
Replace plug and note compression.
If compression improves it's likely to be a worn bore or broken piston ring problem.
If it doesn't it's likely to be a valve sealing issue.
Are you sure the compression seems to be leaking from the inlet valve rather than the exhaust or cyl head gasket?
At first glance I doubt that it's a weak valve spring problem.
Come back with more info which may help diagnosis.
Hairpin springs gap.jpg