socarco,
let's hope that the spare engine isn't primed to pull the same trick when you try to start it.
If there's any doubt about its condition, it's well worth removing the plugs and blowing out any loose deposits _before_ trying to get it going.

Some folk have reported that it was only after the engine had given a few coughs and splutters, so that it was on the brink of bursting into life, when it suddenly locked up.
Presumably, those few bursts of activity in the cylinders were enough to dislodge any layers of carbon which were loose but hadn't yet fallen off...

ken.

as this seems to be a hot topic with cars coming out of hybernation....a hoover works wonders, you have to jury rig some adaprer to take some thick tubing/old nylon fuel pipe plus demijhohn bung or lots of tape

and no any residual fuel does not result in a fireball you have to suck up lots of petrol for that sort of fun

Sean