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Old 04-01-2019, 06:26 PM   #1
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City: Port Saint Lucie
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Fuel vent Mainship 400

I recently purchased a 2003 Mainship 400. When we purchased the boat there was duct tape covering the fuel vents located on each side below the fill tubes....The tape was removed and all was good. When we started to fill the port tank there was an immediate "overfill" event causing diesel to flow from the vent tubes and out around the fuel nozzle. To make this long story short we found that if we slowed the flow from the fill nozzle to a slower flow mode the spill stopped although the vent tube did continue to drip very slowly. Same story on the starboard side. So here's the question......is this normal on a Mainship? Has anyone modified this vent to allow a high flow nozzle to work or is it just the way it is?
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Old 04-02-2019, 09:18 AM   #2
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This is not a Make or Model issue.

The through-hull vents can become clogged. Sometimes it's corrosion, sometimes its mud wasps. Sometimes there's simply a kink in the vent line.

When the vent is clogged, the symptom is slow filling and 'burping' of fuel through the fill.

The fact that it's coming out of the vent is unusual. Check, clear, fix repair the vent and vent line first.
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Old 04-02-2019, 09:38 AM   #3
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There may be a low spot in the vent lines that allows fuel to collect and if filling quickly can get pushed out the vent.
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Old 04-02-2019, 11:00 AM   #4
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Quote:
Originally Posted by kchace View Post
There may be a low spot in the vent lines that allows fuel to collect and if filling quickly can get pushed out the vent.
Yes, I was thinking about this some more and recall a friend who had a loop in a water vent line installed upside-down. (Literally, loop Down) It acted like a trap once it got filled, which was basically every time the tank is filled.
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Old 04-02-2019, 11:07 AM   #5
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Thank you guys. There is a access opening just below each deck fuel fill. I will open them and investigate further.
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Old 04-02-2019, 11:23 AM   #6
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Originally Posted by kchace View Post
There may be a low spot in the vent lines that allows fuel to collect and if filling quickly can get pushed out the vent.
This. Both vent lines on our 430 had a downward slope before going up to the vent. There was fuel stuck in the vent lines. I relocated the vents as far up as I could and made sure the lines sloped down to the tanks. Problem solved.
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Old 04-02-2019, 11:49 AM   #7
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Thank you guys. There is a access opening just below each deck fuel fill. I will open them and investigate further.
I hope you have better luck than I did.
My deck plates provide access to fill & vent area but I found all the hose clamps installed so accessing them was impossible. I guess they positioned them for ease of access before deck & hull were joined... gotta love assembly guys that won't consider future access needs.
I was able to blow them clear and it stopped the dribble.
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