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Surging after 20 min. run time

L

looseplay

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I have a 1996, 24ft. stingray cuddy cabin. It has a 5.7 mercruiser with a 2 barrell carb. The engine runs and revs fine with no load. Once a load is placed on it will run perfect 15-20 min then start to surge. I can back it down to 2000rpms and it will limp home just fine. I have rebuilt carb, had a valve job done, replaced coil and dist. Compression is good in all 8 cylinders. A while back I placed a electric fuel pump on it. Could this be a fuel starvation or just bad fuel? Any suggestions. Thank You
 
describe what you mean by 'surging'. reving up and down or choking or bucking. double check gaskets on the carb and intake. does it seem like low vacumm ?
 
When it starts acting up the engine will drop from 4000 or higher rpm"s and choke down unless I let up on throttle. Once I back off it will go fine around 2000 rpm. If I try to throttle it wants to stall out, thus my thinking not getting enough gas. New gaskets on carb and intake and torqued intake to speaks.
 
maybe check the fuel vent line, maybe a bug made a nest and is restricting it or somtimes they shift and wont let the tank breath enough?
 
when it starts to 'surge',take the cap off the fuel tank,see if theres any air sucked in.also when stopped,take fuel line off carbie,and put into clean container and activate electric pump,and let it run for 30 seconds,andd see if you get a steady stream of fuel.measure the amount and time taken,and post back
 
I hope to be able to clean fuel lines and filters this weekend and take it out on water. I will also be taking a spare tank of gas to rule out bad gas. Thanks
 
I put a marine mechincal advance dist. On it. I thought maybe the coil but after letting it sit all day on the water once i had to limp back home
 
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before I clean a tank I like to hook up a separate test tank, ( the 6 gal. outboard motor tank) to verify its needed. Then when I have to I pull the cover off the fuel gauge sender, remove sender, put in a pump and empty tank and flush out a few times with fresh gas, stuff on bottom I like to pump into a few glass jars just to see what critters and water is in the tank bottom.
 
my boat has an 18 gal metal tank that I can unbolt and take out of the boat. Makes it easier to clean. I realize most people can't get their tanks out so I would go with gl115's advice. Definitely run your boat using another fuel tank to make sure that's your problem.
 
Well just got back in from a test run, at 15 min mark boat started acting up again. Voltage to coil was good and it was not hot, carb was getting gas, so pump was working. Switched over to external tank and ran just fine. So where do I start to remove and clean the anti-siphon valve and clean tank out. I have a very small space to work with where the fuel lines/vent hose goes into tank at. thanks
 
how old is the gas and is it treated? almost sounds like an obstruction in the vent line? is it a separate vent or run in the fuel line, I would check it,might be partially obstructed or moved and is filling with fuel. I am not a boat mechanic so hopefully a real one will chime in, but I would add some seafoam to the tank, go out and run it, take the external tank just in case, when the motor starts to die open the fill fill and see if you hear AIR rushing in, if you do that would verify the vent line, find the source of the partial clog. once the tank with sea foam is run out change fuel filter.
 
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1/4 tank was about a year old, then at begining of season I filled up- 65 gallons of non-ethonal. Did just that yesterday when it started and no air heard so I am pretty sure it is siphon valve or just water in gas. I did look inside carb when it was acting up and both jets were still pumping which leads me to lean towards bad gas.
 
I hope the problem is the anti-siphon valve. I would also do what gl115 said:
...put in a pump and...pump into a few glass jars just to see what critters and water is in the tank bottom.
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Then you will know what is in the tank. Chiefalen has a good method of doing that job. Just make sure it is a marine rated if you use an electric pump. No drill pumps or shop vacuums.

If the jars show that there is water in the fuel it can be pumped out/off the bottom when it settles. Adding a good dispersant to the fuel will take care of a small amount of remaining water. Continue to change fuel/water separator filters several times until the tank is cleaned. Based upon experience do not use an "octane booster" in your fuel tank. It will foul the plugs and cause a buildup of crap/carbon on the piston tops.
 
It sounds like your fuel tank breather may be restricted, such that your fuel pump is pulling a vaccuum.
I would check that first.
If it was bad gas or a restricted syphon valve, you would expect it to choke out all the time you are pulling big power.

Rod
 
...do you agree it is a fuel related issue of some sort...
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That's about all that is left; some sort of fuel issue from the pump back. Do what Rod just stated or what gl115 said earlier; "maybe check the fuel vent line, maybe a bug made a nest and is restricting it or somtimes they shift and wont let the tank breath enough?" Also look for a low spot in the fuel vent line that could hold fuel and block ventilation. The tank siphon hose should also be pulled and inspected.
 
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