He quickly shut off the gasoline and went back to the motor.  Now there are so many things that may happen to a gasoline engine that it would be difficult to name them all offhand, and Tom, who had not had very much experience, was at a loss to find what had stopped his machinery.  He tried the spark and found that by touching the wire to the top of the cylinder, when the proper connection was, made, that he had a hot, “fat one.”  The compression seemed all right and the supply pipe from the gasoline tank was in perfect order.  Still the motor would not go.  No explosion resulted when he turned the flywheel over, not even when he primed the cylinder by putting a little gasoline in through the cocks on the cylinder heads.

“That’s funny,” he remarked to himself as he rested from his labors and contemplated the “dead” motor.  “First time it has gone back on me.”  The boat was drifting down the lake, and, at the sound of another motor craft approaching, Tom looked up.  He saw the red streak, containing Andy Foger and his cronies.  They had observed the young inventor’s plight.

“Want a tow?” sneered Andy.

“What’ll you take for your second-hand boat that won’t run?” asked Pete Bailey.

“Better get out of the way or you might be run down,” added Sam Snedecker.

Tom was too angry and chagrined to reply, and the red streak swept on.

“I’ll make her go, if it takes all night!” declared Tom energetically.  Once more he tried to start the motor.  It coughed and sighed, as if in protest, but would not explode.  Then Tom cried:  “The spark plug!  That’s where the trouble is, I’ll wager.  Why didn’t I think of it before?”

It was the work of but a minute to unscrew the spark plugs from the tops of the cylinders.  He found that both had such accumulations of carbon on them that no spark could ever have reached the mixture of gasoline and air.

“I’ll put new ones in,” he decided, for he carried a few spare plugs for emergencies.  Inside of five minutes, with the new plugs in place, the motor was running better than before.

“Now for home!” cried Tom, “and if I meet Andy Foger I’ll race him this time.”

But the red streak was not in sight, and, a little later, Tom had run the arrow into the boathouse, locked the door and was on his way up to the mansion.

“I suppose Mrs. Baggert and Garret will be surprised to see me,” he remarked.  “Maybe they’ll think we don’t trust them, by coming back in this fashion to see that everything is safe.  But then, I suppose, dad is naturally nervous about some of his valuable machinery and inventions.  I think I’ll find everything all right, though.”

As Tom went up the main path and swung off to a side one, which was a short cut to the house, he saw in the dusk, for it was now early evening, a movement in the bushes that lined the walk.