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Engine Stalls after long troll

My father has a 16 ft. Crestliner boat (vintage late 60's?) with a 50 hp Merc (not sure of age, 4 cyl., dual carbs, blue trim on cowling - early 80's?). Anyway, we use this boat up in Quebec Canada fishing. Don't have a problem with the motor, except when we are trolling. It's an amazing motor, it can troll down to very low (had to make cable adjustment) rpm, which is what we need to catch the fish. However, here comes the problem. Tank is in front of boat (vented 18 gallon) with long run of fuel line to back of boat where we run an in-line fuel separator filter, then to line with prime bulb and bayonet. When running on built-in tank, after a short time, the engine will begin to falter and stall. When one of us runs for the prime bulb, we can save it if we get there in time. Sometimes advancing the throttle will maintain running, although probably only on a couple cylinders until someone gets to the prime bulb. We do not have this effect at above troll speed throttle or on our 6 gallon portable tank - only happens on built in tank after long >15 minute troll runs.

This is my guess: the diaphram pump does not produce enough suction to maintain primed conditions at very low rpm. We change fuel lines and bulb about every 2-3 years thinking that we were helping. I think that we are just operating outside the range of the pump with that great of a restriction (lines, filters, etc.). So, this is my idea: put a small low pressure electrical pump in back of boat near in line fuel separator to keep diaphram primed during trolling periods - which could be 8 hours long! But I have worries: what is too much primer pump pressure - don't want to hurt diaphram or flood carbs. What about under full steam? Will pump allow enough fuel to get to engine? I'm not that versed in these pumps, and probably wouldn't know where to find one. I could design a board where I could mount the separator and the pump if I could just find the right pump.

Well, there you have it. If you can help me with my quest, I sure would appreciate it. I appreciate all that you might be able to do to help.

Thanks, Ray

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Topic - Engine Stalls after long troll - WalleyeTroller 00:58:13 12/22/03 (0)

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