explosive start to season

[pisces]

Loud backfire on starting engine for first time this season
sufficient to blow fumes out of manifold!
Effect now intermittent. Compression normal, new plugs,
no obvious other problems. Suspect sticking valve? Have
lubricated upper cylinder through plug holes but cautious
about restarting again because of possible damage. Help!

[Don Moyer]

Problems that occur during spring start-up are usually related to something that could reasonably have happened to an engine while it was sitting idle over the winter.

My first guess would be that the contacts of your points have a small layer of corrosion that formed during winter. A layer of corrosion will usually prevent starting completely, but it may be possible that, with lesser amounts of corrosion, the points will work occasionally (at least enough to start), but not well enough to continue running.

The engine would see this condition the same as if someone were rapidly turning the ignition switch on and off during the start. Since you would normally have the choke pulled at this point in the starting sequence, you could be drawing a very rich mixture into the engine which would set up the possibility of a backfire or two.

Bottom line, I would run a piece of cardboard between the points and try to start again.

Don

[pisces]

solved - but no wiser

Went back through all steps of a tune up
and repeated. Problem has now gone and was obviously
an error I made over the winter.