More of this Feature • Part 1: Exhaust Bellows • Part 2: Fuel-Bleed System, and More
Related Resources • Maintenance Q&A Index

I've got a 90-hp two-stroke outboard that I am told might have a problem with its fuel-bleed recirculation system. Can you explain how this works and what can happen if it fails? B.A., via e-mail
This system is designed to collect fuel and oil from the low spots of the individual crankcase areas, since two-stroke intake systems do not completely transfer all of the fuel sent through the crankcase to the combustion chamber. This is especially important during low-speed operation. The bleed system pumps the fuel/oil to the intake ports or intake manifold, where it is transferred to the combustion chamber and burned.

Many recirculation systems also collect the fuel and oil pooled in the lower crankcase bearing area and pump it to the upper crankcase bearing to ensure proper lubrication in this location. If this type of system fails, the engine can suffer an upper crankcase bearing failure. Regardless of the type of system, excessive fuel and oil will puddle in the crankcase and not reach the combustion chamber during low-speed operation, causing a lean mixture. When the engine accelerates, the puddles will be quickly drawn into the engine, causing a temporary rich mixture that will result in poor low-speed performance, poor acceleration, spark plug fouling, stalling or spitting at idle, and excessive smoke on acceleration.

What is the difference between intercooling and aftercooling? S.C., via e-mail
In modern engines, none. In some older two-stroke Detroit Diesels, the intercooler was located between the intake manifold and the blower, where the aftercooler was beneath (downstream of) the blower. In all other engines the terms are used interchangeably and refer to the process of cooling the engine intake air after it leaves the turbocharger but before it enters the cylinders. Because it allows more air to enter the cylinders, aftercooling/intercooling increases power and fuel efficiency and lowers NO2 emissions.

Need help with a maintenance problem? Write to Maintenance Q & A, Power & Motoryacht, 260 Madison Ave., 8th Fl., New York, NY 10016. Fax: (917) 256-2282. e-mail:ken_kreisler@primediamags.com. No phone calls, please.

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