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Fuel Delivery and Exhaust Blockage


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Howdy Crew - Two questions today:

I am in the final stages of installing a new Mercruiser 350 (ski 260) long block in my 1987 Malibu Skier.

1. When I hook my MightyVac up to the fuel line (right where it connects to the mechanical fuel pump) and prime the fuel system. I get about 60-80% fuel in coming down the clear plastic MightyVac line . . . not 100% fuel. I can hear back at the tank, a sound like a straw sucking . . I have 5 gallons of fresh fuel in the tank (just put it in). I would have expected the MightyVac to prime the system and have a solid stream of fuel being pulled through the system. . . . is what I am experiencing normal? Any additional thoughts?

Only other thing I can add, is that if I pressurize the tank through the breather, no fuel will come out of the fuel line . . . which leads me to believe there must be a check valve in the tank that keeps fuel from being forced to the engine. Any additional comments here would be appreciated. I am 100% certain that the from the fuel pump leading back to the tank is 100% clear (I removed it and blew air through it, no resistance. I am also 100% certain that the breather line is 100% clear, as I have removed it from the tank and checked both the tank breather outlet (clear) as well as the line leading to the outside of the boat (clear).

2. Now the more interesting one. I had about 15 minutes left today, before leaving my shop. So I hooked up the fuel line to the mechanical pump, squirted some fuel in the carb and was going to see if I could get the motor to kick off (dry) before heading home. Cranked it over, it coughed and sputtered a bit. Cranked over a couple times with a couple squirts of fuel . . same result. Decided to call it a day and do the full water hose in the drive way routine tomorrow. . . I hear this hissing sound . . thought it might be the gas tank (give my puzzlement inf #1 above) and then there is a BAM! and the rear lid (covering the exhaust / driveshaft outlet) kicks up about 4 inches. I lift the lid and the port side fiberglass exhaust tube has a 12" slit in it. Blew the darn thing apart. I go to the back of the boat and the rubber exhaust exit / flapper was blow out against the wall of the shop. I looked up both exhaust tubes with flash light, nothing obstructing. I run a tape measure up there all the way past the blow out, clear as a bell. Only thing I can come up with is that the flapper was fused / glued shut some how (boat hasn't ran in over 2 years as I am the guy piecing it back together after the previous owner let it freeze solid). . . . very strange. Now of course I need to order a replacement tube before taking it out to the water. . . . still plan on doing the drive way test tomorrow (blew out upwards, so water should still run out of boat).

Thinking about #2 a little more, if my guess about the flapper being fused is correct, I am glad I attempted to dry fire it, as if I had it on the hose, water could have filled all the way back to the exhaust ports and into the motor!

I appreciate any guidance / help you folks can provide. Once I have this project concluded, I plan to list the hours it took and the bill of materials purchased. Should be pretty entertaining.

Thanks again.

Kevin

Lake Stevens, WA

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If the boat overheats the exhaust tubes can collapse from the inside and cause an obstruction, may be why they blew apart.

Flapper could have fused shut, but I doubt enough to blow a hose.

-Chris

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If the boat overheats the exhaust tubes can collapse from the inside and cause an obstruction, may be why they blew apart.

Flapper could have fused shut, but I doubt enough to blow a hose.

-Chris

Exhaust tubes are fiberglass, they were structurally sound (thin wall actually so nothing to really collapse). Rubber components are also in decent shape. Flapper was blown out of the tube (was put in with some sort of sealant) so that was my only guess (fused flapper). Seems very strange. Gray sealant that you see in the picture is where I had previously sealed a very slight crack (looked like the tube was kneeled on at one point, or maybe the lid came in contact with the tube. Just a little high temp RTV . . . but all for not now.

Welcome thoughts on the fuel delivery . . . . don't want to put any more fuel in the tank, until I am determined I do not need to remove the tank to change up pick up tube or something.

2m81ibk.jpg

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perhaps the raw fuel that accumulated in the exhaust was ignited?

Maybe . . but I heard a noise (sorta of a hiss) like pressure had slowly built up . .. then a BAM. Exhaust manifolds are brand new.

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Maybe . . but I heard a noise (sorta of a hiss) like pressure had slowly built up . .. then a BAM. Exhaust manifolds are brand new.

Do NOT SQUIRT FUEL IN TO A CARB! That with improper static ignition timing could have been much worse! Water to the engine would've atleast helped push that bomb out of the exhaust tube or may have prevented it from igniting.

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Do NOT SQUIRT FUEL IN TO A CARB! That with improper static ignition timing could have been much worse! Water to the engine would've atleast helped push that bomb out of the exhaust tube or may have prevented it from igniting.

Or the water could have back up into my exhaust ports and hydro-locked the new motor. . . sounds like you view of the fact pattern is something ignited, instead of pressure being built up.

What is your recommendation on priming the fuel system? Do you have any thoughts on my puzzling fuel challenge I posed in #1 of my first post?

I appreciate your thoughts.

Kevin

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One other note . . I was NOT cranking the motor over when the BAM occurred. I had time to climb out of the boat, and start to clean my hands. I would guest 10-15 seconds from the time I stopped cranking, to the BAM.

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One other note . . I was NOT cranking the motor over when the BAM occurred. I had time to climb out of the boat, and start to clean my hands. I would guest 10-15 seconds from the time I stopped cranking, to the BAM.

anything else come out?

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Nope. No scorch marks on the fiberglass and only a small amount of exhaust when looking into the exhaust from the flapper end.

Also, one more note on priming the carb with a couple squirts of fuel . . . I struggle to think how this is any different that pumping the throttle and getting the accelerator pump to squirt fuel down the primaries. . . but I am more than open to correction in my thinking.

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Update:

1. Fuel. Yes the outlet form the tank has a check valve and it was the check valve that I heard when using the MightyVac. I fired the boat up today and set the timing. Why the MightyVac show a few air bubbles when priming the system? Who knows. Squirted some fuel in the primaries of the carb, kicked right off, set timing. Time for break in . . . as soon as I solve one last (?) mystery which I will post in a new topic.

2. Exhaust. I am writing it off as a plugged/fused flapper. Hard to believe, but I don't have any other explanation that make sense given the delay of the BAM, and lack of any scorch indications (don't think it was fuel). I patched the damage w/some think aluminum sheet and some 3M 5200.

Thread closed . . . I think.

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