l3301 mild backfire on starting

[marksmand]

The last 2 times I started my l3301 I got a bang(backfire?). This only happens when
the tractor had not been used for several days. Otherwise it runs and operates perfectly.
It's not especially hot or cold here lately(50's 60's) but it's very damp. 2014 model with 630HRS
that has been well maintained. Any thoughts?

 

[RayIN]

Possibilities;
tight valves
carboned-up valves
incorrect pump timing
contaminated fuel
leaking injector