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sailhog 07-29-2007 08:33 AM

location of fuel tank vent
My A-4 is having trouble running at high rpm, and I'm pretty sure it's from being starved of fuel. I've gone through the sailnet search engine, and I'm thinking that the problem may be the result of an occlusion in the fuel vent. It runs fine for a half an hour or so, and then it loses rpm and finally dies. I restart, but as soon as I throttle up, it dies again. Runs fine, however, at idle speed. I thought it was possibly water in the fuel, but have addressed that side of the equation. Of course it could be a spent fuel pump. I installed a clear plastic fuel filter downstream of the pump, and there's only a tiny bit of fuel at the bottom. But I'm thinking this could be the result of an occluded vent. Soliciting for suggestions. Final question: Where, exactly is the vent?

rewell6 07-29-2007 09:28 AM

Sounds like the fuel pump. Does the fuel filter ever fill up? If not the pump is most likely the culprit.

We have had quite a problem with dirt dobbers around our place this year, so it is possible it is the vent. It should be toward the back, somewhere it won't get a lot of water splashing on it, but on the outside of the hull, not in the cockpit or topside. You should be able to smell it out.

sailhog 07-29-2007 10:20 AM

Thanks for the response. I'm on my way to the dock now...

Boasun 07-29-2007 11:07 AM

If it is the fuel vent and you have cleaned it out, be sure that you have replaced the 30 mesh screen for it. This screen will prevent flames from entering the vent line (#1 priority) and keep out the vermin (#2 priority).
And on your water tank vents use 16 mesh screen to keep out the vermin a #1 priority on this tank.
Also that the vent is of the same size (or Larger) as the fill line and isn't crimp/kinked anywhere along the vent line.

RichH 07-29-2007 12:08 PM

When was the last time you re-torqued the head studs????

If you evaluate and find its NOT an occluded vent, do a compression test *after the engine reaches full operating temperature*. Your signs and symptoms to me would indicate (at worst) a head gasket that is beginning to 'blow' (usually between #3 & #4). The indication here would be that under full load the engine wants to stall but under 'no load' (trans in neutral) you can reach max rpm.

Alternatively if you have *any* dirt on the emulsion tube in the carburator (an approximately 1" long tube with closed end and a few 'holes' drilled into the side of the tube) you can also result with erratic performance similar to that which you report. The emulsion tube is responsible for 'metering' the correct air/fuel mixture in the carburator. Any rust that develops in the fuel delivery system usually gets 'caught' on/in the emulsion tube. Many times when you change the 'fuel filter' you can break small particles of rust free which then get 'collected' on the small holes of the emulsion tube.

Usually the tank vent line will terminate in a small 'clamshell' that incorporates a fine monel or stainless screen (spark arrestor) and is mounted somewhere near the stern of the boat .... spiders LOVE to build their nests in these clamshells and the spider nests in the vent externals will block the vent.

hope this helps.

sailhog 07-29-2007 12:51 PM

I've never re-torqued the head studs. The engine was rebuilt by Moyer in 2002, so it would seem a little premature for something of that order to be occuring -- or am I wrong here? One thing I have noticed is that the clear fuel filter does indicate only a tiny bit of fuel when she quits on me, which leads me to believe it's a fuel starvation matter. Thanks again for your help.

Valiente 07-29-2007 01:21 PM

You retorque every season to 35 ft./lbs. on a cold engine and you do it in the order indicated in the Seacraft Yellow Book of Atomic 4 Maintenance Obscurities.

As for the vent, the odds are excellent that it's occluded. You find it by looking at the tank top: the 3/8" to 1/2" hose leading up from the tank that ISN'T the fuel fill (usually 1 1/2" wire-wound fire-resistant hose) and by following it to the deck edge or coaming where there is usually a little chromed vent cap. If you undo the vent line from both ends and blow it out, you'll likely evict three types of spiders and their little webby graves.

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