Seek the Leak
Q & A — October 2003
By Capt. Ken Kreisler
Seek the Leak
| Poor outboard performance,
cleaning isinglass, and caring for line.
I am having trouble
starting my tender’s 40-hp, two-stroke outboard as well as noticing
poor performance at low speed and idle. I’ve checked the fuel and
ignition systems as well as done a compression test. Any suggestions?
W.P., via e-mail
As you can see in the left diagram, the downward-traveling piston first uncovers the exhaust port (A), allowing exhaust gases from the previous combustion cycle to leave the cylinder. Shortly thereafter, it uncovers the intake port (B), allowing a fresh air-fuel charge to enter. In the right diagram, as the piston moves up, it covers first the intake (B) and then the exhaust (A) port, compressing the charge and simultaneously creating a low-pressure area in the crankcase, drawing in more fuel and air through the carburetor. After combustion, the piston is forced down, uncovering first the exhaust, then the intake port, starting a new combustion cycle.
A leak in the crankcase means a diluted fuel-air charge will be drawn in during the intake stroke, creating a lean mixture in the combustion chamber and possibly producing the symptoms you described. The same problem can be caused by failure of the lower piston ring, allowing pressure to escape the crankcase via the combustion chamber. A compression test typically evaluates only the upper ring and not the lower ring, and even though the engine may run fine at higher speeds, a classic indicator of lower ring failure is the engine’s inability to idle at its recommended speed.
Check for crankcase leakage by looking for fuel residue seeping from the crankcase parting lines, the upper and lower crankcase seals, and the reed valves and intake manifolds. If you suspect there’s crankcase leakage, rub some oil on the suspect area; if there’s a leak, the oil will get sucked in at its location.
Leakage via the lower piston ring is more difficult to detect, so it’s best to let a reputable mechanic handle it by first evaluating the situation with a fuel pressure/vacuum gauge. As the engine is cranked over, a repeating pressure/vacuum cycle should be observed on the gauge, with the pressure reading being considerably higher than that of the vacuum reading. A test of all cylinders should read basically the same.
This article originally appeared in the September 2003 issue of Power & Motoryacht magazine.