Boat stalls at low speed / idle after time on lake?

[DreadPirateRoberts]

Our late 70s Sea Ray Weekender has a mercruiser inboard/outboard - for as old as it is, boat has relatively low hours and typically runs great.  However, in the last couple of weeks the boat has stalled in the channel after we've spent time at high speeds on the lake.  I've researched the topic, but can't find anything specific to this scenario.  Current hypothesis includes:

• Cap, rotor and/or spark plug wires? 
• Vaporlock
• Carbon build-up on plugs
• Fuel / air mixture screws on carb

Anyone have a more concrete idea of what might be going on here?  Any ideas are appreciated - I have to use the boat to chase a giant, a sword-wielding Spaniard and a cocky Sicilian to the Cliffs of Insanity later in August, so I need the boat in top working order.

[Missdemeanor]

Change cap and rotor first... Easy fix. I replace mine every year...

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[pvelyk]

Had similar problem earlier this year with new to me boat.  It started up fine but would stall at very low speeds, most often on the way in and trying to dock.  As Missdemeanor says the cap and the rotor a great place to start and for me was the issue.  He helped me change it and we could see a large amount of corrosion.  Boat has run great since then.

Edited July 29, 2015 by pvelyk

[rbonnell]

Did you check the points

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[Gill-T]

Cap and rotor is a cheap fix so start there first as others have suggested.  Might be plugs and wires.  Might be the shift interrupter switch.  Might be the carb.

[Has Been]

Complete Tune up & Wires first IMO . No ethanol gas., Also Fuel /water separator filter

[DreadPirateRoberts]

Looks like a cap and rotor are the first area to explore - thanks! 

[thumbburn]

Might as well do the points while our in there..... I did mine this year.... Plugs wires cap rotor points and condenser and a new coil for just over a hundred bucks..... Night and day difference...... Carb job would be nice too at the same time

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[lakebound88]

It's always nice to do one at a time. So you know what was the cause.... Take this from a technician, as being a parts changer is not a diagnosis. I've seen guys throw hundred of bucks at once on an engine when it was only a 3 dollar gasket.

Cap and rotor is always a great start. The low idle needs a hot spark, without it, you get a surge in that baby. I change mine yearly like matt said.

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