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Just bought an 83 Nitro 17'er with an 85 XR-2 150 HP Merc. The boat sat in the owner's garage for three years without running (fuel drained and winterised). Compression is good (125-135 on all six) and on the muffs, it starts right up and idles smoothly. On the water, it starts but misfires at idle (pop/chuff out the exhaust) and will die frequently, especially after pulling back to idle after a run across the lake. If I use 1400 rpm as a no-wake speed it runs great; pull it back to 1200 and it will die. Pull it back into neutral and it will sometimes run fine at 1000 rpm and sometimes die. It will always restart, and the bulb stays firm at all times. Checked all six plugs and they are firing, none looks fouled, and as I say, it runs great above 1400 rpm. I also pulled all six slow speed jets and checked to see that they were unobstructed. I do have both a Clymers and a Seloc manual. I'm thinking maybe carb synchronisation or fuel pump diaphragm (too rich at idle?) Big question is, why does it run perfectly on the hose? Less back pressure? Really hard to fix when it seems fine in the driveway and only misbehaves on the lake (one hour away). Any help, suggestions, etc, are greatly appreciated.
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You hit the nail on the head when you mentioned it sat for 3yrs. Chances are the carbs need to come off and cleaned internally.They are more than likely varnished up and need a bath and air shot through them. You should also replace the water pump, after years of sitting out of water the impeller is probably stiff and cracked. -
What you are describing is a lean POP. This happens when an insufficient fuel charge is drawn into the combustion chamber. Rather than burn, it just flashes. If the engine is idling slow enough it can at times even cause it to stall.
Lean POP = not enough fuel.
NOT an over rich condition.
Pull the carbs off and clean them up.
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