It's Good to Vent

Maintenance Q & A — July 2004
By Capt. Ken Kreisler

It’s Good to Vent
A possible cause for fuel starvation, diagnosing a spark plug deposit, and more.
 More of this Feature
• Fuel starvation, and more
• Fuel filter plugging, and more
• PMY Tries... Yacht Brite Buff Magic

 Related Resources
• Maintenance Q&A Index

I have a 41-footer with twin gasoline stern drives. Lately I’ve noticed decreased engine power and surging at higher speeds on one engine. Everything seems to check out, but the problem reoccurs. Any suggestions? T.B., via e-mail
Assuming you have twin fuel tanks and that only one engine is drawing from one tank, look at the fuel tank vent serving the misbehaving engine. Proper venting is required for fuel to flow from the tank to the engine, and if there is even a small obstruction in the vent, the fuel pump may not be able to pull enough fuel from the tank to supply the engine, especially at higher speeds. The result can be decreased power or surging and, if the restriction is considerable, stalling at lower speeds or even failure to start.

A typical vent system for an integral fuel tank, as illustrated here, includes a hose that leads from the top of the tank to a fitting mounted on the outside of the hull. Having a small orifice, this fitting is notoriously prone to blockage by debris, especially insect nests.

To determine if this is the source of your problem, loosen the fill cap enough to allow the tank to vent, and then run the engine. If all appears to be normal, a clogged vent is most likely the culprit. To clear the obstruction, remove the hose from the vent and push any debris through to the outside. Failure to do so can cause the debris to be sucked into the fuel tank and, eventually, into your engine, causing more problems.

During a recent tune-up on my tender’s two-stroke outboard, I noticed a deposit wedged between the electrode and the spark plug electrode. What can be causing this? P.G., via e-mail
This condition is known as core bridging and is caused by excessive carbon adhering to the electrode. Possible causes include the use of improper oil, immediate high-speed operation after prolonged trolling, and an improper fuel-oil ratio.

Next page > Fuel filter plugging, and more > Page 1, 2, 3

This article originally appeared in the June 2004 issue of Power & Motoryacht magazine.

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