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post #1 of 3 Old 04-30-2007 Thread Starter
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A Fuel's Tale

Here’s a fuel supply problem story that may serve as a heads up for others. Sorry in advance for the length of this post….

Last season our engine started losing RPMs randomly, often recovering after a short time only to have the revs drop again. This occurred quite randomly, only at cruising revs, at no set intervals and at no time did the engine ever stop or quit completely. Still it was unnerving and any time we had to motor for an extended period we were uneasy.

Obvious cause: air in fuel, so we bled the system and things improved for a bit (and indeed some air was found at the high point of the fuel system on occasion – but not every time) We put it to a possible leaking fitting as the entire system in under suction pressure so no fuel leaks would be seen in any case. Of course all fittings were checked, tightened, and in some cases changed, all to no avail.

Over the winter I gave this some thought, and decided to get rid of an inline squeeze – bulb primer in case that was the source of the leak. Though it seemed to work, it never really felt “full”. In any event the theory was to minimize the number of potential leaks by minimizing the connections.

We bought a Racor diesel filter c/w a built in primer pump to eliminate the squeeze bulb. Installed it, replaced the line between the shutoff and the filter just in case, used high quality fittings where we could and ran the engine confident that wherever the leak was, we had addressed the problem.

No big surprise to you readers, I suppose, that the problem had not gone away. GRRRRRR!

The previous season we had had a run of filter pluggages, (the boat was then new to us) necessitating pumping out the tank and cleaning it of particulate. After that we had (and still have had) no problems with filter service life

At any rate, now we’re heading back into the tank to look for reasons for this problem. The fuel looked good, no visible sediment or problem in the bottom of the tank. As I was inspecting things I was able to look at the internal connection of the pick-up tube – I was suspecting a leak or crack in the pickup line inside the tank by now.

What I did find was a sharp kink in the plastic pickup tube just at the internal connection. There did not seem to be a crack, but the tube was kinked severely enough to form a restriction to high flow. I replaced the pickup with solid tubing. I’m happy to say, after getting out for the first time this season this past weekend, that the problem does indeed seem to have disappeared. (Touch on wood)

Anyhow, in hindsight I think this was self inflicted. I remember swinging that pickup line out of my way when mucking out the tank, I likely put the kink in it then but it went unnoticed. (I needed a mirror to spot it) I think now that at higher RPM the kink was starving the system, and it behaved similarly to air in the fuel. The system would, after a spell, pull enough vacuum to overcome the restriction for a short time, but in the end it would starve again… But on the other hand, in fact there may well have been a small crack at the kink that admitted a little air under vacuum conditions too. Whatever, it seems to be OK now.

Maybe Don Street, the Pardey's and other engine-less people got it right!!
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post #2 of 3 Old 04-30-2007
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Good tale Faster...one more thing to add to the checklist of possible causes!
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post #3 of 3 Old 05-01-2007
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Had the same issue once on my Atomic 4 that turned out to be a partially occluded tank vent (spider debris) that would take some time to manifest as the relatively weak (2 PSI) pull of the mechanical fuel pump could overcome the vacuum.

The spider parts and webbing residue were forming an effective joker valve after certain RPMs had been reached. At low revs, the engine would keep on going.

Thankfully, a wise fellow at dock loaned me a dental pick and gave me the suggestion to "check the vent line".
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