Intake manifold Leakage.

Q We recently encountered an engine-misfiring mystery in an gasoline-engine tractor. We are aware that three basic requirements--fuel, compression, and ignition--must be met if an engine is to operate properly. We checked the fuel flow, cylinder compression, and ignition system and found all of them to be okay. However, when we start the engine, it briefly runs properly, but soon starts misfiring. Any thoughts on possible causes?

A It's not enough for the three basic requirements to be present only when the engine is "at rest." They must also be present when the engine is operating. You checked fuel flow and the ignition-system performance with the engine operating both during normal operation and during the misfiring times, and concluded that there wasn't much that could change in the compression situation between the engine performance conditions. However, there's more to the fuel aspect than just flow of fuel to the carburetor. The flow of fuel--in vapor form--must have the correct ratio of fuel-to-air for proper combustion to occur.

Although you have torqued the intake-manifold studs to the recommended value, there may be a seldom-encountered and easily-overlooked cause--a tiny, almost unnoticeable crack in the intake manifold. Air sucked in through this crack can dilute the fuel-air mixture enough that it becomes too lean for proper combustion, causing the engine to misfire. It sounds as though the crack opens just enough to cause the misfiring when the engine warms to operating temperature. If you can find the suspected crack, try squiring engine oil on it. If the oil is sucked in, you will have located the likely cause of your problem. Either repair the crack by brazing, or by other appropriate method, or replace the faulty intake manifold.