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Old 08-29-2018, 07:21 PM
DVW70 DVW70 is offline
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Boat: Mastecraft, 190, 1987, 351
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boat starts idles smooth - 45 minutes later rough and missing

Hi - been working through some issues - many thanks to all the help - I know why the chap sold it.....LOL

what I've done so far and the results

had the 4160 carb rebuilt by a marine mechanic
replaced the spark plugs
replaced the thermostat - the old one appeared fine, but operated inconsistently - new one is perfect at 150 consistently
replaced the coil
wires/cap/rotor are new this year
aftermarket electronic points

boat starts and idles smooth as the first day. choke opens perfectly
boat sounds like it will idle all day - can balance a nickle on the intake

45 minutes (and I mean 45 minutes nearly on the minute!!) of pulling the kids, starts to idle rough, then finally starts to miss and cleans out then misses, then misses off throttle and continues in this fashion.

I am at a loss - it is definitely heat related - once at this temperature something is acting up.

what is odd is it runs so smooth for the first 45 minutes, no hiccups, seriously smooth! pulls from idle to wot.
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Old 08-29-2018, 08:22 PM
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Table Rocker Table Rocker is offline
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Have you checked your fuel tank vent? If it is blocked, the tank will pull a vacuum and starve for fuel. As it sits, air will creep in and it will run fine for a while, starting the process over.
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Old 08-29-2018, 09:29 PM
DVW70 DVW70 is offline
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Boat: Mastecraft, 190, 1987, 351
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Last time I over filled the tank and gas shot out the overflow - and all over me... and tonight I left the boat in the water for 3 hours, ate dinner and engine still running so rough that it stalled twice getting it onto the trailer.
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Old 08-29-2018, 11:08 PM
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Table Rocker Table Rocker is offline
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Quote:
Originally Posted by DVW70 View Post
Last time I over filled the tank and gas shot out the overflow - and all over me... and tonight I left the boat in the water for 3 hours, ate dinner and engine still running so rough that it stalled twice getting it onto the trailer.
It's not the vent then.
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  #5  
Old 08-30-2018, 07:19 AM
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Psgwake Psgwake is offline
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Boat: 2007 X45 496 crusader
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Just thinking out loud...
Vapor lock?
Have you checked fuel pressure during this condition?
Coil ground?
Fuel line gets warm and collapse?
If you add fuel to the airflow during this condition does it clean up?(Disclaimer:I am not condoning this in any way)
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Old 08-30-2018, 09:26 AM
DVW70 DVW70 is offline
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Quote:
Originally Posted by DVW70 View Post
Hi - been working through some issues - many thanks to all the help - I know why the chap sold it.....LOL

what I've done so far and the results

had the 4160 carb rebuilt by a marine mechanic
replaced the spark plugs
replaced the thermostat - the old one appeared fine, but operated inconsistently - new one is perfect at 150 consistently
replaced the coil
wires/cap/rotor are new this year
aftermarket electronic points

boat starts and idles smooth as the first day. choke opens perfectly
boat sounds like it will idle all day - can balance a nickle on the intake

45 minutes (and I mean 45 minutes nearly on the minute!!) of pulling the kids, starts to idle rough, then finally starts to miss and cleans out then misses, then misses off throttle and continues in this fashion.

I am at a loss - it is definitely heat related - once at this temperature something is acting up.

what is odd is it runs so smooth for the first 45 minutes, no hiccups, seriously smooth! pulls from idle to wot.
so I noticed - when the engine is idling the most rough, and I spray car cleaner at the base the engine smooths out. I have not tried the carb cleaner when the boat is started cold and idles smoothly.

I would assume if the engine RPM increases with spray when the engine is cold and idling perfectly, and the RPM increases with spray when the engine is hot/warm, and idling rough, then it may Not be the carb.

then any thoughts - I installed a new coil, plugs and the cap/rotor/wires were changed this spring.

I just installed a brand new coil from SKIDIM - No change

Once hot and the magic 45 minutes of running happens - the engine will misfire, clear and maybe misfire as I motor along at 12mph-25mph and then maybe clear or continue to miss at wot

I would think an electrical issue would continue to missfire

AND I pulled the old plugs and they "looked" damp, but the electrode was clean, not soot and looked to be firing correctly.

could the quality of spark be effected by temperature?
could the coil not be producing enough voltage to the cap?

maybe when the complete Holley rebuild kit comes in September 5 - do it myself like I should have before I spent 350 for it to be done by the mobile marine mechanic??
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Old 08-30-2018, 10:08 AM
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Table Rocker Table Rocker is offline
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It sounds like you could have a vacuum leak.
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  #8  
Old 08-30-2018, 01:11 PM
curver900 curver900 is offline
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^^^^ this... and you know where it is.. when you spray and it smooths out you found the place to look
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  #9  
Old 08-30-2018, 05:48 PM
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thatsmrmastercraft thatsmrmastercraft is offline
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Likely the coil or possible the electronic ignition. I assume you cleaned all the connections and have battery voltage to the coil with the key on/engine off; and while running.

I would wire in your digital voltmeter to monitor the coil positive to check for some crazy voltage spikes.
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  #10  
Old 08-30-2018, 06:14 PM
Cumminscj Cumminscj is offline
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Check the voltage to the distributor and coil when this is happening. Wires heat up, resistance goes up, voltage goes down and bad things happen. I had a boat that would do this but not quite as bad before I found it. On cold starts it would throttle up great. As it got hotter and the wiring heated up it would start to hesitate when the throttle was thrown down.

As others have suggested I would also throw a vacuum gauge on it and check for leaks.
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