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Old 08-25-2007
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Join Date: Dec 1969
Posts: 4
Default 40 hp 3 cylinder bucking and sputter

I have an older 40hp that I purchased with a boat. the motor starts and idles fine. When I apply throttle the motor runs very rough and sputters. It also bucks very hard. I have removed and cleaned the carbs, bypassed the fuel filter completely (O-ring can be an issue). All coils are developing at least 30 kv as per my spark tester at idle. The timing at idle is ~12 deg btdc. It jumps like crazy when the motor is under load and bucking and is not readable! Here is the weird part last year it did the same thing except when you put it into a starboard turn, it would then pick up and run fine until you turned to port, and then it would run really rough again. I have gone over every wire, connection and fuel line looking for a culprit, no luck. Now this year it just runs rough and no power at all, yet it idles fine and you can run for hours at very low rpm. I have sprayed carb cleaner, run fuel conditioner, and done about as much as I know how to do. I will check compression next, should have done it long ago.

I have read fuel pumps are a problem. How do you check them.

Thanks in advance for any assistance you can bring to this vexing problem
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Old 08-26-2007
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Join Date: Dec 2009
Location: Lake Placid, Fl
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If this is remote control model there may be a internal short(in white wire or on some the yellow wire) in main harness causing bthe ignition to break down when yoy turn to port. If you can get another key switch and main harness try it to see if solves problem.
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Old 08-28-2007
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Join Date: Dec 1969
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I started the motor and put the motor on fast idle, this is where the problems manifest themselves, and was playing with the harness. I inadvertantly pulled the main harness plug completely out of the connector on th emotor. No change whatsoever, the motor kept on running as poorly as ever. So I dug deeper but havent found bad wires yet. Still looking and if there are any other suggestions sure appreciate your assistance.
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Old 08-28-2007
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Join Date: Dec 2009
Location: W.P.B. FLA
Posts: 2,375
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run it at night...looking for any errant spark.

what effect does choking have ? or pumping ball while acting up ?

have you done a comp test ?

try another tank, hose and ball to the motor filter.

pull each plug wire at a time while running on hose, listen for a change.

sounds like youre not hitting on all 3 or maybe you need to reroute your fuel line.
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